The research team, led by Max Kolton of Ben-Gurion University and Florida A&M University together with colleagues at the Savannah River Ecology Laboratory, compared floodplain soils with a long history of coal-ash contamination to nearby reference soils that remained relatively pristine. Coal ash, the residue left after coal is burned for power, carries a cocktail of metals including arsenic, chromium, and other elements that are toxic to living cells even at moderate concentrations. Because floodplains sit at the intersection of rivers and uplands, they are natural collection points for such industrial residues, and they are also among the most biologically active soils on the landscape, making them an ideal natural laboratory for asking what chronic metal stress does to the invisible life below ground.

To capture the full picture, the researchers sampled across five seasonal periods, an unusually thorough design for this kind of work. Microbial communities in soil are not static; they shift with temperature, moisture, and plant activity through the year. By sampling repeatedly across seasons, the team could distinguish a consistent contamination signal from the background noise of natural seasonal variation. The strongest divergence between contaminated and reference soils appeared during late-summer peak conditions, when heat and drought likely amplified the physiological stress that metals impose on microbial cells, pushing the two soil types furthest apart in composition and function.

The core of the study rested on three complementary molecular techniques. First, the team sequenced the 16S rRNA gene, a standard marker used to identify which bacteria and archaea are present in a sample and how diverse the community is. Second, they used a tool called PICRUSt2, which predicts the functional capabilities of a community from its taxonomic profile, offering a computational glimpse into what the microbes might be doing. Third, and most decisively, they turned to digital PCR, a highly sensitive technique that counts individual DNA molecules, to precisely quantify the abundance of 16S rRNA genes and a suite of nitrogen-cycling marker genes, including the ammonia monooxygenase genes carried by both bacteria and archaea, nitrite reductase genes, and the nrfA gene associated with dissimilatory nitrate reduction to ammonium.

The diversity results told a clear story. Metal contamination reduced both taxonomic and phylogenetic diversity, meaning that contaminated soils hosted fewer kinds of microbes drawn from a narrower slice of the evolutionary tree. Yet, crucially, the total abundance of prokaryotic DNA, measured by counting 16S rRNA gene copies, did not consistently decline. This is a subtle but important distinction. The contaminated soils were not sterile wastelands; rather, they had undergone a restructuring, with the microbial biomass largely maintained but redistributed among a smaller cast of survivors. Pollution, in other words, pruned the tree of soil life rather than cutting it down.

The pruning followed a predictable ecological logic. Contaminated soils were enriched in generalist taxa, microbes with broad environmental tolerances that can cope with a wide range of conditions, and depleted in specialists, organisms finely adapted to particular niches but vulnerable to disturbance. Ecologists call this pattern biotic homogenization: as environmental filters like metal toxicity eliminate the sensitive and the specialized, the remaining communities across contaminated sites come to resemble one another, dominated by the same resilient cosmopolitan players. The loss of specialists matters beyond simple headcounts, because specialist microbes often perform narrow but vital functions, such as breaking down specific organic compounds or mediating particular steps in nutrient transformations, that generalists may not fully replace.

The most surprising findings emerged from the nitrogen-cycling analysis. PICRUSt2’s functional predictions suggested that nitrification, the process by which microbes convert ammonia into nitrate, should be reduced in contaminated soils. But when the researchers actually counted the relevant genes with digital PCR, they found the opposite: the abundance of ammonia-oxidation genes was increased, driven primarily by archaeal ammonia oxidizers rather than their bacterial counterparts. This mismatch between prediction and measurement is itself a lesson in method. Gene-based functional prediction tools are calibrated largely on well-studied bacteria and can miss the idiosyncrasies of archaea, which are known to dominate ammonia oxidation in many soil environments. Direct gene quantification revealed a reality that the predictive model had inverted.

Why would archaeal ammonia oxidizers thrive under metal stress? Archaea are ancient, often extremophile lineages, and their ammonia-oxidizing members are famously tolerant of harsh conditions, including low pH and, apparently, elevated metal concentrations. As metal-sensitive competitors and grazers were filtered out, the archaeal oxidizers may have faced reduced competition for ammonia, allowing their populations to expand. Whatever the precise mechanism, the consequence is a shift in the architecture of the nitrogen cycle itself, with ammonia oxidation gaining ground in contaminated floodplains relative to uncontaminated ones.

The gene ratio analyses added a second layer of reorganization. Relative to the nrfA gene, which marks the dissimilatory nitrate and nitrite reduction to ammonium pathway, or DNRA, the contaminated soils showed an increased proportion of genes for ammonia oxidation and for denitrifying nitrite reduction. This matters because the two pathways have opposite consequences for the ecosystem. DNRA conserves nitrogen within the soil by converting nitrate back into ammonium, a form that plants and microbes can retain. Denitrification, by contrast, converts nitrate into gaseous forms, including nitrous oxide, a potent greenhouse gas, which escape to the atmosphere. A community shifted away from DNRA and toward denitrification is, in effect, a community that leaks nitrogen rather than holding it, with potential downstream consequences for fertility and emissions.

The authors caution that these are measurements of genetic potential rather than direct observations of nitrogen fluxes, and that linking gene abundances to actual process rates will require further work. Even so, the study carries a sober implication for the many floodplains worldwide that bear the legacy of industrial metal pollution, from former mining districts to power plant ash basins. Remediation efforts typically focus on the chemistry of the contamination itself, measuring metal concentrations and immobilizing them in place. This research shows that the biological legacy runs deeper and differently: even where microbial life persists in abundance, the identity of the organisms and the genetic toolkit they carry have been permanently reorganized. Restoring a contaminated floodplain, the findings suggest, may mean more than detoxifying the soil. It may mean waiting for, or actively assisting, the slow return of the specialist microbes that keep nitrogen locked in the landscape, a recovery measured not in years but potentially in decades, and one that begins with recognizing that a soil can look alive while functioning in an entirely altered way.

Subject of Research: Effects of legacy coal ash metal contamination on floodplain soil microbial communities and nitrogen-cycling gene abundance

Article Title: Legacy Metal Contamination Alters Floodplain Soil Microbiomes and Reorganizes Nitrogen-Cycling Genetic Potential

Article References: Kolton, M., Chukwujindu, C., Oo, W. Y. M., Pathak, A., Fincher, K., Xu, X., & Chauhan, A. (2026). Legacy Metal Contamination Alters Floodplain Soil Microbiomes and Reorganizes Nitrogen-Cycling Genetic Potential. Microbial Ecology. https://doi.org/10.1007/s00248-026-02865-5

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02865-5

Keywords: coal ash, metal contamination, floodplain soils, soil microbiome, nitrogen cycle, archaeal ammonia oxidation, denitrification, DNRA, digital PCR, PICRUSt2, biotic homogenization, Savannah River Site

The technology at the heart of the study is the down-flow hanging sponge reactor, a biofilm system originally developed for municipal wastewater treatment. Wastewater trickles from the top of the reactor over cubes of porous polyurethane sponge, each roughly 33 millimeters across with a porosity exceeding 90 percent. The open, gravity-driven structure draws in oxygen by natural ventilation rather than forced aeration, while the sponge matrix retains an enormous population of slow-growing microbes that would otherwise be washed out. Crucially, the sponges create microscopic oxygen gradients: aerobic zones near the surface where ammonia can be oxidized, and anoxic microzones deep inside where nitrate can be reduced to nitrogen gas. In principle, a single sponge cube can host both nitrification and denitrification at the same time.

In the new experiment, the researchers built a pilot-scale marine RAS with a 500-liter culture tank holding Japanese flounder (Paralichthys olivaceus) at an initial stocking density of about 3.7 kilograms per cubic meter, fed a commercial pellet diet at 2 percent of body weight per day in seawater of roughly 30 parts per thousand salinity. Water was pumped from the tank into an anaerobic down-flow hanging sponge reactor, or AnDHS, a 100-liter vessel packed with 50 liters of sponge carriers and seeded with denitrifying granular sludge. From there, water flowed by gravity into an aerobic DHS reactor installed downstream, which polished the effluent by oxidizing residual ammonia and nitrite and reoxygenating the water before it returned to the fish. A UV unit disinfected the recirculating stream, and make-up water was added only to compensate for evaporation and sampling losses.

The 116-day trial was deliberately staged in phases to probe how two operational levers affected performance. In the first phase, the AnDHS received no external carbon source at all. From day 32 onward, sodium acetate was dosed to maintain a carbon-to-nitrogen ratio of 3, and the aerobic DHS sponge volume was progressively enlarged from 15 liters to 25 liters and finally to 40 liters, arranged in two layers with a hydraulic retention time of 2.0 minutes. In the final phase, the AnDHS carbon-to-nitrogen ratio was raised to 6. Because the modifications were applied sequentially rather than in an independently replicated factorial design, the authors caution that the individual contribution of each change could not be separated; the results describe the integrated response of the whole system.

The early phases revealed exactly why balancing the two reactors matters. When the aerobic DHS held only 15 liters of sponge and the AnDHS ran at a carbon-to-nitrogen ratio of 3, ammonia and nitrite climbed in the culture tank, with nitrite reaching concentrations that would pose a toxicity risk to fish. The team attributed this to partial denitrification in the AnDHS, which converted nitrate only as far as nitrite, combined with insufficient nitrification capacity in the undersized aerobic reactor. Once the aerobic sponge volume was increased to 40 liters, about 4 percent of the tank volume, and the carbon-to-nitrogen ratio was raised to 6, the system stabilized dramatically. Total ammonia nitrogen settled at 0.78 plus or minus 0.98 milligrams of nitrogen per liter, nitrite remained near or below detection, and nitrate accumulated only gradually while staying below the target threshold for marine RAS operation. From that point on, routine water exchange was no longer needed at all.

Two other water-quality benefits emerged that would normally require separate equipment in a conventional RAS. First, the culture tank pH stayed above 7.0 throughout the experiment without any chemical base addition, because denitrification in the AnDHS recovered the alkalinity that nitrification consumed. Second, dissolved oxygen in the tank consistently exceeded 8.0 milligrams per liter even though the tank itself was not directly aerated; the aerobic DHS reoxygenated the water during its gravity-driven passage, while the AnDHS effluent dropped to anoxic conditions appropriate for denitrification. In many existing farms these functions are handled by degassers, aeration tanks and base dosing systems, so folding them into two sponge columns represents a meaningful simplification of the treatment train.

Microbial community analysis based on 16S rRNA gene sequencing added a fascinating layer to the story, though the authors stress that marker-gene data indicate potential functional affiliations rather than proven activity. In the aerobic DHS, ammonia-oxidizing archaea related to Candidatus Nitrosopumilus dominated, particularly in the upper sponge layers where oxygen and ammonia were presumed highest. These archaea possess an exceptionally high affinity for ammonia and tolerate low oxygen and fluctuating pH, traits well suited to marine biofilters. Nitrospira, the nitrite-oxidizing lineage, was not detected until day 49, consistent with its slow growth and sensitivity to salinity and organic loading, and its delayed appearance mirrored the early nitrite accumulation. Denitrifying bacteria were also abundant in the aerobic reactor, often exceeding the nitrifiers in relative abundance, which fits the idea that anoxic microzones inside the sponges support simultaneous nitrification and denitrification within a single biofilm.

The AnDHS community told a complementary story. Sequences affiliated with Sedimenticola, an acetate-utilizing denitrifier, were dominant, matching the sodium acetate feed and the observed decline in nitrate, and pointing to acetate-dependent heterotrophic denitrification as the main nitrate-removal pathway. Ammonia-oxidizing bacteria related to Nitrosomonas and members of the OM190 lineage were also detected, hinting at locally oxygenated niches within the nominally anoxic sponge matrix, although their activity was not directly demonstrated. The authors also flag an intriguing greenhouse-gas angle: ammonia-oxidizing archaea generally emit less nitrous oxide than ammonia-oxidizing bacteria, and comammox Nitrospira, which oxidize ammonia all the way to nitrate without releasing nitrite, could further reduce both nitrite toxicity and nitrous oxide formation. However, standard 16S sequencing cannot distinguish comammox from canonical Nitrospira, so that possibility remains speculative pending functional-gene or metagenomic confirmation.

From an engineering standpoint, the sponge reactors showed impressive operational resilience. Over the full 116 days there was no sponge clogging, no excessive biomass accumulation, no significant pressure loss, and no need for backwashing, sponge cleaning or media replacement; the only maintenance was routine inspection of the recirculation pump. Because oxygen enters by natural ventilation, the system avoids the energy costs of forced aeration and carrier mixing, and the porous sponge structure reduces clogging risk compared with conventional packed-bed biofilters. Previous work in aquaponic systems even showed a DHS reactor maintaining lower ammonia than a moving-bed biofilm reactor with only one-fifth of the effective volume. The authors are careful to note, however, that energy consumption, capital costs and operational expenses were not quantified in this study, so definitive economic claims must await a proper techno-economic assessment.

The study also carries honest caveats. It was a single pilot-scale system evaluated sequentially, not a replicated experiment, and comparisons with other biofilters should be made cautiously because loading rates, species and management practices differ across studies. Still, the headline result stands on its own: by sizing the aerobic sponge volume to roughly 4 percent of the tank and feeding the anaerobic stage at a carbon-to-nitrogen ratio of 6, the team kept ammonia and nitrite low, recovered alkalinity, reoxygenated the water and eliminated routine water exchange in a marine system culturing a commercially valuable flatfish. Future work will need independently replicated designs, quantification of nitrous oxide emissions, strategies to enrich comammox organisms, and validation at commercial scale with different species. If those steps succeed, the humble sponge cube, trickling quietly in a gravity-fed column, could become a cornerstone of water-stingy, low-impact fish farming for a world that needs more protein without more freshwater withdrawals.

Subject of Research: An integrated anaerobic-aerobic down-flow hanging sponge reactor system for nitrogen removal in marine recirculating aquaculture systems

Article Title: Aerobic–anaerobic down-flow hanging reactor for recirculating aquaculture systems: Performance evaluation

Article References: Nguyen, T. Y. P., Watari, T., Akamine, T., Kato, Y., Adlin, N., Hatamoto, M., Shimabukuro, A., Satanwat, P., & Yamaguchi, T. (2026). Aerobic–anaerobic down-flow hanging reactor for recirculating aquaculture systems: Performance evaluation. Case Studies in Chemical and Environmental Engineering, 14, Article 101490. https://doi.org/10.1016/j.cscee.2026.101490

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101490

Keywords: recirculating aquaculture, down-flow hanging sponge reactor, denitrification, nitrification, Japanese flounder, marine RAS, biofilm, nitrogen removal, ammonia-oxidizing archaea, Nitrospira, water quality, sustainable aquaculture

The research, published in the journal Microbial Ecology, was carried out by a team from the University of Padova and the National Biodiversity Future Centre in Italy. Led by Irene Gregori and corresponding author Alessandro Vezzi, the researchers sampled sediments from two salt marshes in the Venice Lagoon, a productive microtidal coastal ecosystem in the northern Adriatic. Rather than tracking the marshes across weeks or seasons, the team focused on a much finer timescale: a single summer diel cycle, with sampling at four carefully chosen moments: sunset, night, dawn and solar noon. This design allowed them to capture the most extreme short-term physicochemical oscillations that intertidal ecosystems experience, driven by the interplay of sunlight, tides and biological activity.

The physicochemical measurements confirmed just how dynamic these environments are. Temperature and dissolved oxygen concentrations showed pronounced oscillations over the sampled cycle, with oxygen levels in particular responding to the balance between photosynthetic production during daylight hours and continuous respiratory consumption around the clock. Such fluctuations are typical of productive intertidal systems, where mats of microalgae and aquatic plants flood the water with oxygen by day and leave it depleted by night. For organisms living in the sediment, these swings represent a genuine physiological challenge, because oxygen availability dictates which metabolic pathways are energetically feasible at any given moment.

To characterise the biological players, the team employed two complementary DNA-based approaches. Prokaryotic communities, encompassing bacteria and archaea, were profiled using metabarcoding of the 16S ribosomal RNA gene, a standard molecular fingerprinting technique that reveals which microbial taxa are present and in what relative abundances. Eukaryotic organisms, ranging from microscopic algae to fungi and protists, were similarly catalogued using the 18S rRNA gene. But the researchers went a step further than simple taxonomic inventories: they also applied metagenomics, sequencing the collective genomes of the prokaryotic communities to assess their functional potential, in other words, which genes and metabolic pathways the communities carried and could, in principle, deploy.

The results were striking for what they did not show. Despite the pronounced swings in temperature and dissolved oxygen, both the prokaryotic and eukaryotic communities displayed only minor shifts in taxonomic composition across the four sampling points of the diel cycle. The same microbial lineages dominated the sediments at sunset as at dawn, and the relative abundances of most taxa remained essentially unchanged from night to solar noon. In many aquatic ecosystems, particularly the open ocean, researchers have documented clear diel rhythms in microbial communities, with different taxa rising and falling in abundance as light and nutrient conditions change. The Venice Lagoon salt marsh sediments, by contrast, appear to host assemblages that simply do not need to restructure themselves on this timescale.

The metagenomic data help explain why. The prokaryotic communities proved to be functionally redundant and metabolically versatile, meaning that multiple different taxa carried out the same key processes and that individual organisms possessed a broad repertoire of metabolic capabilities. The communities were dominated by taxa involved in organic-matter degradation, the cycling of sulphur compounds, denitrification, which converts bioavailable nitrogen into gaseous forms, and sulphur-oxidising chemolithoautotrophic carbon fixation, a process in which microbes use chemical energy from oxidising sulphur compounds, rather than sunlight, to fix carbon dioxide into biomass. This functional breadth means that whatever the prevailing oxygen and temperature conditions at a given hour, some members of the community are equipped to keep the essential biogeochemical machinery running.

The authors conclude that modulating their activities, rather than undergoing compositional restructuring, is likely how these communities sustain key ecological processes relevant to carbon and nutrient cycling over the diel timescale. In practical terms, a sulphate-reducing bacterium that dominates the community at midnight, when oxygen has been exhausted, does not need to be replaced by a different species at noon; it can simply slow down or speed up its metabolic output as conditions permit. Similarly, chemolithoautotrophs that fix carbon using sulphur-derived energy can ramp up their activity when their substrates become available, without any change in who is present. Activity-level regulation is a faster and energetically cheaper strategy than community turnover, and the study suggests it is the dominant mode of short-term response in these sediments.

The findings carry broader significance for the conservation and restoration of salt marshes, which are globally threatened by anthropogenic pressures including land reclamation, pollution, nutrient loading and sea-level rise. Because restoration efforts are increasingly needed to recover the carbon sequestration capacity, water quality regulation and coastal protection that these habitats provide, understanding the stability and resilience of their microbial engines is essential. If sediment microbial communities are functionally robust in the face of strong short-term physicochemical fluctuations, this suggests a degree of intrinsic resilience that could help restored marshes establish functioning biogeochemical cycles relatively quickly. At the same time, the central role of sulphur cycling, denitrification and organic-matter degradation in these communities underscores how tightly salt marsh ecosystem services are coupled to microbial metabolism, and how disturbances that alter sediment chemistry could ripple through the entire system.

The study also highlights the value of combining fine-scale temporal sampling with functional genomics. Taxonomic metabarcoding alone might have led researchers to conclude that diel dynamics are unimportant in salt marsh sediments, but the metagenomic perspective reveals the hidden versatility that makes that stability possible. As sequencing costs continue to fall, similar integrated approaches are likely to be applied to other intertidal and coastal ecosystems, from mangrove sediments to seagrass meadows, where the same questions about short-term microbial dynamics and ecosystem function remain open. For now, the Venice Lagoon results provide a clear and somewhat reassuring message: beneath the mud, the microbial workforce keeps its composition steady through night and day, adjusting its activity to whatever the marsh throws at it, and in doing so quietly sustaining the processes on which these threatened coastal habitats depend.

Subject of Research: Diel stability of sediment microbial communities in Venice Lagoon salt marshes

Article Title: Night and day in the Salt Marshes: Sediment Microbial Communities Remain Stable Over a Summer Diel Cycle

Article References: Gregori, I., Mohamed, F., Barausse, A., Frizzo, R., Archetti, L., Martino, F., Zane, L., & Vezzi, A. (2026). Night and day in the Salt Marshes: Sediment Microbial Communities Remain Stable Over a Summer Diel Cycle. Microbial Ecology. https://doi.org/10.1007/s00248-026-02890-4

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02890-4

Keywords: salt marshes, microbial ecology, metabarcoding, metagenomics, diel cycle, Venice Lagoon, biogeochemistry, sulphur cycling, denitrification, carbon sequestration, coastal ecosystems, prokaryotes

The Lipu River Basin lies in Lipu City in southern Guilin, in the Guangxi Zhuang Autonomous Region, and drains toward the Lijiang and Guijiang rivers. It is a classic mixed-land-use karst catchment: terraced farmland famous for Lipu taro, dense rural settlements relying in part on dry-toilet systems, and a growing industrial park hosting metalworking, chemical, pharmaceutical, and food-processing facilities. The subtropical climate delivers about 1,441.5 millimeters of rain annually, concentrated between April and August, while winters are comparatively dry and slow-moving. Because the underlying aquifer sits only 0.5 to 3.5 meters below the surface and is riddled with carbonate conduits, whatever enters the water system in one place can emerge, barely degraded, somewhere else entirely.

To untangle the pollution, the team led by Zupeng Wan and Honghu Zeng of Guilin University of Technology ran two sampling campaigns in 2022, one in the wet summer and one in the dry winter. They collected 51 river-water samples from 27 summer and 24 winter sites spanning the mainstream and six tributaries, and 26 groundwater samples from Quaternary porous aquifers, Carboniferous fissure-cave aquifers, and bedrock-fissure aquifers. Nitrate isotopes were measured with the bacterial denitrifier method, converting sample nitrate to nitrous oxide and analyzing it by isotope-ratio mass spectrometry, calibrated against the international reference materials USGS32, USGS34, and USGS35 with analytical precision better than ±0.3 per mil for δ15N and ±0.5 per mil for δ18O.

The concentration data alone told a striking seasonal story. Total nitrogen in river water averaged 7.09 ± 6.59 milligrams per liter in winter, more than double the summer average of 2.95 ± 1.48 milligrams per liter, a difference the authors attribute to reduced dilution when rainfall and runoff diminish. Groundwater, buffered by the aquifer, showed no significant seasonal shift. Nitrate was overwhelmingly the dominant dissolved inorganic nitrogen species, exceeding 77 percent of total nitrogen in river water in both seasons. Twelve percent of all samples exceeded the World Health Organization and Chinese drinking-water guideline of 10 milligrams per liter of nitrate-nitrogen, with exceedances more frequent in groundwater, at 19 percent, than in river water, at 8 percent, and concentrated near the industrial park and in winter river water at tributaries draining grain- and food-processing areas.

The isotopic evidence then did the detective work that chemistry alone could not. Different nitrate sources carry characteristic isotopic fingerprints: manure and sewage typically have high δ15N values, atmospheric deposition has high δ18O values, while mineral fertilizer and soil nitrogen have lower δ15N signatures. Because the ranges of manure and sewage overlap, the researchers combined them into a single end-member and used the Stable Isotope Analysis in R package, SIAR, a Bayesian mixing model fitted by Markov-chain Monte Carlo simulation, to estimate proportional contributions with full uncertainty. A locally collected effluent sample with a δ15N of +8.6 per mil fell squarely within the manure-and-sewage literature range, anchoring the prior without materially shifting the posterior.

The posterior estimates were unambiguous. Across the basin, manure and sewage accounted for 56.7 ± 8.2 percent of modeled nitrate, followed by soil nitrogen at 22.5 ± 6.5 percent, nitrogen fertilizer at 15.3 ± 5.0 percent, and atmospheric deposition at just 5.5 ± 2.5 percent. The contribution was even higher in groundwater, at 61.5 ± 8.5 percent, than in river water, at 50.2 ± 8.0 percent. The authors point out that this dominance exceeds that reported for other Southwest Chinese karst systems, including the Babu karst aquifer in Guizhou, where manure and sewage contributed 37 to 38 percent, and the Erhai Basin, where the figure was 49.6 percent. They attribute the Lipu excess to dense rural settlement, widespread dry-toilet use, manure application on taro fields, and industrial discharge of wastewater rich in dissolved organic nitrogen from the county’s hanger-manufacturing and specialty food-processing sectors.

Equally important is what the isotopes revealed about nitrogen transformations. All sample δ18O values fell within the range produced by microbial nitrification, indicating that the nitrate pool in both river and groundwater is built by bacteria oxidizing reduced nitrogen rather than by direct input of pre-formed nitrate. To test for denitrification, the process that removes nitrate and would partly mask source signals, the team applied a Rayleigh fractionation framework, in which remaining nitrate becomes progressively enriched in both heavy isotopes as its concentration falls, with an expected enrichment-ratio εN/εO of roughly 1.3 to 2.1. Most river samples showed no such coupled enrichment, indicating no resolvable denitrification. Groundwater showed a ratio of about 1.38 with δ15N and δ18O inversely related to nitrate concentration, evidence of some denitrification, but the generally low isotope values and only suboxic conditions, with dissolved oxygen below 2 milligrams per liter, indicated the removal was limited.

Independent tracers reinforced this picture. Chloride behaves conservatively and electrical conductivity integrates total mineralization, so if nitrate rises together with both, the pattern points to mixing rather than in-situ removal. In river water, nitrate correlated positively with both parameters at all discharge types, most strongly at sanitary and industrial outlets, where regression coefficients of determination reached 0.593 and 0.464 respectively for conductivity. In groundwater, however, the nitrate-chloride relationship broke down entirely, a decoupling the authors attribute to vertical karst heterogeneity: in matrix-dominated, oxygen-poor zones, nitrate is removed locally while chloride passes through untouched. A nitrogen enrichment slope analysis added further nuance. At sanitary and industrial outlets, the log of nitrate concentration rose with δ15N, the signature of accumulating waste-derived nitrogen, whereas at other outlets the slope was negative, consistent with excess synthetic fertilizer leaching at low δ15N.

The spatial mapping identified clear management targets. Total nitrogen in groundwater averaged 6.11 milligrams per liter, and nearly 19 percent of groundwater sites exceeded the nitrate guideline. Wells near the metalworking industrial park, sites U10 and U11, recorded total nitrogen two to three times the regional mean, and a nearby karst spring reached 18.3 milligrams per liter of nitrate-nitrogen, likely reflecting inputs from a new-materials facility. Upstream agricultural wells near the taro fields ran 1.65 to 3.57 milligrams per liter above the basin average, driven by heavy organic fertilizer application whose mineralization and nitrification outpace crop uptake. Tributaries such as the Pulu and Dumo rivers, some carrying more than 30 percent of the mainstream flow, emerged as the core nitrogen arteries of the basin, with weakly oxygenated karst reaches suppressing nitrification and allowing ammonium to accumulate.

The study’s practical message is direct: pollution-control strategies designed for porous-media basins translate poorly to karst terrain, where conduits can fast-track contaminants past the soil’s natural attenuation. Because nitrate isotopes were measured only in summer, the source proportions describe the summer basin-scale budget rather than a full seasonal picture, a caveat the authors state explicitly. Still, their recommendations are clear. Regulators should prioritize manure and sewage management across karst-sensitive zones, remediate the localized industrial groundwater hotspots near the metalworking and new-materials parks, and intensify winter monitoring of food-processing tributaries where low flows concentrate point-source signals. In a basin where a dry toilet on a hillside and an electroplating workshop in the valley both drain into the same maze of limestone pipes, protecting the water means treating the aquifer and the river as one system, not two.

Subject of Research: Nitrate source apportionment and nitrogen transformation in the karst river-groundwater system of the Lipu River Basin using dual nitrate isotopes and Bayesian SIAR mixing modeling

Article Title: Identifying nitrate sources and nitrogen transformation in a karst river–groundwater system using dual nitrate isotopes and a Bayesian (SIAR) mixing model

Article References: Wan, Z., Liu, B., Chen, W., Chen, Y., Yan, X., & Zeng, H. (2026). Identifying nitrate sources and nitrogen transformation in a karst river–groundwater system using dual nitrate isotopes and a Bayesian (SIAR) mixing model. Environmental Earth Sciences, 85(16), Article 406. https://doi.org/10.1007/s12665-026-13136-4

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13136-4

Keywords: karst hydrogeology, nitrate pollution, dual nitrate isotopes, SIAR mixing model, groundwater contamination, nitrogen cycle, manure and sewage, denitrification, nitrification, Lipu River Basin, water quality, source apportionment

The central problem in nitrate reduction is well known to anyone who has worked with transition-metal complexes. The nitrate anion is thermodynamically stable and kinetically sluggish; its nitrogen-oxygen bonds are strong, and its negative charge makes it reluctant to bind to negatively charged or electron-rich metal centers. Biological enzymes solve this problem elegantly. In molybdenum- and iron-containing reductases, the primary coordination sphere binds the substrate, while an array of amino acid residues forms a second-shell network of hydrogen bonds that polarizes the substrate, stabilizes charged intermediates, and shuttles protons to the right place at the right time. Synthetic chemists have tried to imitate this architecture for decades, but installing a functional second sphere around a small-molecule catalyst remains a formidable synthetic challenge.

In the new report, the research team designed iron complexes in which hydrogen-bond donors are anchored at the periphery of the ligand framework, close enough to reach nitrate bound at the metal but far enough not to interfere with metal-ligand bonding. The ligands, often described as pendant urea or amide units in related systems, act like a molecular hand that grips the nitrate ion from the outside of the first coordination shell. When the authors compared these second-sphere catalysts with otherwise identical complexes lacking the hydrogen-bond donors, the difference was striking. The decorated systems reduced nitrate at substantially higher rates and with improved selectivity toward nitrogen-containing products, confirming that the rate enhancement is not merely an electronic effect of a modified ligand but a genuine consequence of secondary-sphere interaction.

Mechanistic experiments formed the backbone of the study. Kinetic isotope effects measured with deuterated hydrogen-bond donors revealed that proton transfer participates in the rate-determining step, while spectroscopic monitoring tracked the buildup and decay of iron-bound nitrogen oxo intermediates. The authors observed that the hydrogen-bond network stabilizes the protonated nitrate species and the N-O bond-cleavage transition state, lowering the energetic barrier for the transformation that conventional iron complexes find hardest to accomplish. Density functional theory calculations supported this picture quantitatively: the computed transition states for N-O bond activation sit lower in energy when the second-sphere donors are present, and natural bond orbital analyses showed increased polarization of the nitrate nitrogen-oxygen bonds induced by the surrounding hydrogen-bond framework.

One of the most compelling aspects of the work is the demonstration that the effect is tunable. By systematically varying the acidity and geometry of the pendant donors, the researchers could dial the catalytic activity up or down, an ability that transforms the second sphere from a passive scaffold into an active design element. The geometry matters as much as the acidity. Donors positioned to donate bifurcated or doubly coordinated hydrogen bonds to a single nitrate oxygen produced the largest accelerations, while donors pointing in the wrong direction contributed little. This structure-activity relationship provides a practical roadmap for other laboratories seeking to engineer second-sphere effects into their own catalysts, whether the target is nitrate, carbon dioxide, nitrogen gas, or oxygen reduction.

The implications extend well beyond the walls of a synthetic inorganic laboratory. Nitrate contamination of groundwater is a global health concern, linked to methemoglobinemia in infants and to various cancers in adults, and agricultural runoff keeps the problem growing. Conventional treatment technologies, including ion exchange, reverse osmosis, and biological denitrification, are expensive, energy intensive, or slow. Catalytic conversion of nitrate to ammonia or nitrogen gas under mild conditions would offer an alternative that destroys the pollutant in place and, in the case of ammonia production, recycles the nitrogen into a valuable commodity. The present study does not yet deliver a water-treatment device, but it supplies the mechanistic foundation that such devices will require: a clear picture of how to activate nitrate at an earth-abundant metal without the precious metals that dominate many industrial processes.

Iron is the obvious choice for that vision. It is cheap, abundant, and biocompatible, and it already performs nitrogen chemistry in nature through the enzyme nitrogenase, albeit for the opposite reaction, the reduction of dinitrogen to ammonia. Harnessing iron for selective nitrate reduction in a synthetic setting has proven difficult because the metal tends to bind nitrate weakly and to release reactive intermediates indiscriminately. The second-sphere strategy addresses both weaknesses at once. By enveloping the bound nitrate in a supportive hydrogen-bond pocket, the ligand raises the effective affinity of the complex for the anion and simultaneously organizes the transition states that lead to productive bond cleavage. In effect, the catalyst mimics the reductase active sites that nature has optimized over billions of years, using noncovalent interactions to do work that brute-force electronics cannot.

The study also contributes to a broader intellectual trend in molecular catalysis: the recognition that the region just outside the primary coordination sphere is fertile ground for innovation. Over the past decade, researchers have shown that second-sphere effects can control selectivity in oxygen evolution, enhance carbon dioxide reduction at nickel and cobalt centers, and enable proton-coupled electron transfer sequences that would otherwise be impossible. Each demonstration refines the community’s ability to predict where to place donors and how strongly they should interact with substrates. The nitrate work adds an important data point because it concerns an anionic substrate, the class for which hydrogen-bond assistance is most consequential and also most technically demanding, since electrostatic competition between the ligand framework and the substrate can destabilize the very complexes being engineered.

Questions remain before the chemistry can be scaled. The catalytic turnovers reported in the study, while impressive for a molecular iron complex, still fall short of the durability needed for continuous-flow water treatment or industrial operation. Oxygen and competing anions such as sulfate and carbonate, which are abundant in real wastewater, may challenge the selectivity of the hydrogen-bond pocket. The authors acknowledge these hurdles and point toward future ligand generations with more robust frameworks and tunable pocket sizes. Still, the conceptual advance is unambiguous. A hydrogen-bonded second sphere, carefully installed around an iron center, measurably promotes one of the most stubborn reductions in environmental chemistry, and it does so with the kind of mechanistic clarity that invites reproduction and elaboration by other groups.

For the moment, the study stands as a vivid example of how molecular design can borrow from enzymology to solve practical problems. The nitrate anion that pollutes rivers and aquifers is the same species that enzymes dismantle with ease inside living cells, and the difference between the two situations has always been architecture. By building that architecture, in miniature, into a synthetic iron complex, chemists have shown that the boundary between biology and homogeneous catalysis is not a wall but a design space. The next steps, engineering robustness, testing real water matrices, and coupling the chemistry to renewable electricity, will determine how quickly this laboratory insight matures into technology. What is already clear is that the second coordination sphere, once considered decoration, now belongs among the primary tools of modern catalyst design.

Subject of Research: Second-sphere hydrogen bonding in synthetic iron catalysts for nitrate reduction

Article Title: Secondary-sphere hydrogen bonding promotes catalytic nitrate reduction at iron

Article References: Secondary-sphere hydrogen bonding promotes catalytic nitrate reduction at iron. (n.d.). https://doi.org/10.1038/s41557-026-02235-1

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02235-1

Keywords: nitrate reduction, iron catalysis, hydrogen bonding, second coordination sphere, homogeneous catalysis, nitrate to ammonia, water treatment, metalloenzymes, proton-coupled electron transfer, earth-abundant metals, ligand design, denitrification

The research team, led by scientists from East China Normal University including Zhongzheng Yan, focused on mangrove stands in the Maowei Sea reserve in China’s Beibu Gulf, a region where shrimp and fish pond operations have long bordered the fringing forests. To capture a gradient of human influence, the researchers selected three contrasting sites. One served as a restored reference, located where nearby pond discharge had ceased roughly nine years before sampling. The other two sites sat directly beside active aquaculture pond systems that continued to release nutrient-rich effluent into the mangrove environment. By comparing soils, porewater chemistry and microbial gene profiles across this gradient, the team could trace how increasing wastewater exposure reshapes the underground processes that determine whether carbon stays buried and whether nitrogen is removed or merely recycled.

The chemical contrasts between the sites were striking. At the most heavily affected location, the concentration of dissolved organic carbon in porewater was approximately 99 percent lower than at the restored reference site. On its face, this might seem paradoxical, because aquaculture wastewater actually imports organic matter and nutrients into the forest. The explanation, the researchers conclude, lies in what those imports do to microbial communities. Nutrient enrichment appears to stimulate microbes to consume the readily available carbon pool far more aggressively, so the standing stock of dissolved organic carbon collapses even as carbon throughput increases. In effect, the wastewater does not simply add material to the soil; it activates an underground economy in which microbes burn through carbon faster than it can accumulate.

This accelerated carbon consumption carried a second, more consequential consequence: a shift toward more reducing conditions in the soil. As microbes devour oxygen and other oxidized compounds while processing organic matter, the sediment chemistry becomes progressively more anoxic and chemically reduced. That matters enormously for iron-bound carbon, because the protective coupling between organic matter and iron minerals depends on the oxidation state of the iron itself. Under increasingly reducing conditions, the mineral shields begin to destabilize, releasing previously protected organic carbon back into the microbial feeding ground. The study found that this enhanced consumption was associated with, and likely contributed to, the more reducing soil environment, together loosening the iron–organic carbon coupling that underpins long-term carbon storage.

The quantitative evidence for this destabilization is compelling. Around mangrove roots, the relatively persistent pool of organic carbon associated with crystalline iron minerals was 67 percent to 76 percent smaller at the two disturbed sites than at the restored reference. Crystalline iron oxides are among the most stable binding partners for organic matter, so a loss of this magnitude represents a substantial drawdown of the carbon that mangrove soils can preserve on decadal to centennial timescales. Intriguingly, carbon associated with a more reactive form of iron increased at the most affected site. The researchers interpret this as evidence of a fundamental shift in the carbon cycle: away from durable, mineral-protected storage and toward a more labile, less stable regime in which carbon circulates rapidly and remains vulnerable to microbial oxidation and eventual release as carbon dioxide.

The study’s second major finding concerns nitrogen, the nutrient that mangroves famously help scrub from coastal waters. Microbes handle nitrate through two principal pathways. Denitrification converts nitrate into gaseous forms of nitrogen that escape to the atmosphere, permanently removing it from the ecosystem. A competing pathway, dissimilatory nitrate reduction to ammonium, known as DNRA, instead converts nitrate into ammonium, keeping the nitrogen within the soil and making it available again to plants and microbes. The microbial gene evidence gathered in the Maowei Sea indicated that both pathways became active simultaneously under aquaculture stress, but with a telling emphasis: the balance appeared to tilt toward ammonium retention and internal recycling rather than permanent removal.

If that shift holds, the implications for coastal water quality could be significant. A mangrove forest functioning primarily as a nitrogen recycler acts more like a holding buffer than a sink, retaining reactive nitrogen within the ecosystem instead of eliminating the excess that flows in from aquaculture and other land uses. Under heavy nutrient loading, such a forest may no longer deliver the water-purifying service that coastal managers often count on, and ammonium retained in the soil could continue to fuel microbial activity and further carbon loss, linking the nitrogen and carbon findings into a self-reinforcing loop of change.

Among the study’s most practically interesting results is the identification of a marked transition in nitrogen cycling behavior around a dissolved organic carbon concentration of approximately 32 milligrams per liter. Below and above this porewater threshold, the microbial community’s nitrogen processing strategy appeared to differ noticeably, suggesting that DOC could serve as an early-warning signal of ecosystem state change. The authors are careful, however, to frame this value as a potential site-specific indicator rather than a universal rule. Because DOC dynamics vary with climate, sediment type, tidal regime and forest history, further studies across other mangrove systems will be needed before such a number can be confidently adopted as a general management threshold.

Taken together, the findings sketch a coherent mechanistic chain from aquaculture discharge to diminished carbon durability. Nutrient-rich wastewater stimulates microbial carbon consumption; intensified consumption drives soil chemistry toward more reducing conditions; reducing conditions destabilize the crystalline iron minerals that guard persistent organic carbon; and the resulting carbon mobilization feeds back into microbial activity while nitrogen pathways pivot from removal to retention. The two ecosystem services at stake, durable blue carbon storage and coastal nitrogen regulation, are therefore not independent functions but tightly coupled outcomes of the same underground chemistry and microbial ecology. Disturbing one leg of the system inevitably strains the other.

For coastal policymakers and restoration practitioners, the study underscores the importance of managing aquaculture discharge before it reaches mangrove soils, whether through improved pond effluent treatment, buffer zones or strategic placement of restoration sites away from active discharge. It also argues for monitoring programs that look beyond conventional water-quality parameters to track carbon–iron–microbial interactions directly, since the earliest signs of functional decline appear in porewater chemistry and gene profiles rather than in the visible health of the trees. As aquaculture continues to expand across tropical and subtropical coastlines worldwide, understanding these hidden soil processes may prove essential to preserving the blue carbon and nutrient-filtering benefits that mangrove forests quietly provide.

Subject of Research: The effects of aquaculture wastewater on iron-bound organic carbon stabilization and microbial nitrogen cycling in mangrove soils

Article Title: Aquaculture wastewater may weaken mangroves’ iron-based carbon storage and shift nitrogen cycling toward recycling

Article References: Aquaculture wastewater may weaken mangroves’ iron-based carbon storage and shift nitrogen cycling toward recycling. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: mangroves, blue carbon, aquaculture wastewater, iron–organic carbon coupling, dissolved organic carbon, nitrogen cycling, denitrification, DNRA, soil microbes, Maowei Sea, coastal water quality, Forest Ecosystems

The research, published in Environmental Monitoring and Assessment, was led by Mingda Cao of Chizhou University together with colleagues from institutions across China, including Hefei University of Technology, the Institute of Karst Geology in Guilin, and China University of Geosciences in Wuhan. The team focused on the Dawang Cave subterranean stream and the downstream Dawang Lake in a classic karst region, where limestone bedrock dissolves slowly in carbon-dioxide-charged groundwater, loading the water with dissolved inorganic carbon and bicarbonate. Karst waters cover enormous areas of the planet, and understanding their chemistry matters for drinking water supplies, agricultural runoff management, and the global carbon budget.

To dissect the daily chemistry, the researchers established two monitoring sections. The first sat at the outlet of the Dawang Cave subterranean stream, a stretch devoid of subaquatic vegetation and dominated by upwelling groundwater. The second lay downstream in Dawang Lake, where abundant submerged plants thrive in the sunlit water column. Between noon on 6 September and noon on 7 September 2024, the team ran a continuous twenty-four-hour field campaign, sampling at high frequency and measuring an unusually complete suite of parameters: pH, dissolved oxygen, electrical conductivity, temperature, the major ions calcium and bicarbonate, the nitrogen species nitrate and ammonium, and, critically, the stable isotopic compositions of both nitrate and ammonium. From the oxygen data they also calculated net ecosystem production, a direct gauge of whether the ecosystem was, minute by minute, producing or consuming organic matter.

The contrast between the two sites was striking. At the unvegetated cave outlet, essentially every parameter barely moved over the full day. The team describes this as chemostatic behavior, a near-constant chemical signature imposed by the steady discharge of deep groundwater. In such systems, the subsurface reservoir acts like a giant chemical buffer: water spends long enough in the aquifer that it arrives at the surface with a composition set by slow rock-water reactions rather than by anything happening in the stream itself. For biogeochemists, this makes the cave outlet a natural control experiment, a baseline against which the biological pulse downstream can be measured.

Dawang Lake told a completely different story. There, the water chemistry swung on a clean diel cycle that tracked the sun. During daylight hours, photosynthesis dominated, net ecosystem production turned positive, and the concentrations of both nitrate and ammonium fell steadily as the submerged vegetation assimilated dissolved nitrogen into its biomass. At night, respiration took over, net ecosystem production flipped negative, dissolved oxygen sagged, and the nitrogen concentrations recovered. The isotopic records moved in lockstep. As nitrate and ammonium were consumed during the day, the remaining pools became progressively enriched in the heavy isotopes, with rising values of δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+. After dark the enrichment reversed. This pattern produced a significant negative correlation between nitrogen concentrations and their isotope ratios, the classic fingerprint of a preferential-removal process in which the lighter isotopes are taken up first.

The isotope data allowed the team to test the leading hypotheses about what removes nitrogen from such waters. Denitrification, the microbial conversion of nitrate to gaseous nitrogen forms, is often assumed to be the dominant sink in nitrate-laden streams, and it carries a diagnostic isotope signature: because the process preferentially strips light nitrogen and oxygen, the residual nitrate becomes enriched in both heavy isotopes at a characteristic ratio of roughly one to one between δ15N and δ18O. In Dawang Lake, the covariation of the two nitrate isotope ratios showed an enrichment ratio of approximately 0.79 to 1, and, crucially, all of the data points fell outside the typical denitrification field on the dual-isotope plot. The conclusion was unambiguous: denitrification was negligible over the monitoring period. The apparent loss of nitrate during the day was not being converted to gas and escaping the system; it was being built into living tissue.

Instead, the study identifies assimilation by subaquatic vegetation and the mineralization of organic nitrogen as the key processes governing the diel nitrogen cycle. By day, plants and attached algae take up nitrate and ammonium directly, and by night the microbial breakdown of organic matter returns nitrogen to the water. This coupling means that the carbon and nitrogen cycles in the lake are not merely parallel but genuinely intertwined: the same metabolic engine that draws down dissolved inorganic carbon and bicarbonate during photosynthesis simultaneously scrubs bioavailable nitrogen from the water, and the nocturnal release of carbon dioxide is accompanied by the remineralization of organic nitrogen. In high-dissolved-inorganic-carbon waters like those of karst regions, where researchers had often focused on carbonate chemistry and carbon sequestration, the new data show that the nitrogen side of the ledger is driven by the same biological heartbeat.

The technical achievement underlying these conclusions is the combination of high-resolution monitoring with paired nitrogen and oxygen isotopes measured around the clock. Traditional sampling, which might capture a single morning snapshot, would average out the diel cycle entirely and could easily misattribute the daytime nitrate decline to denitrification or dilution. The continuous approach reveals that what looks like net nitrogen removal on a daily average is actually a rapid exchange between the dissolved pool and living biomass, one that reverses every evening. The authors note that karst streams, with their naturally high alkalinity and stable groundwater inputs, are ideal natural laboratories for this kind of work, precisely because the strong chemostatic background makes any biological signal stand out clearly against it.

The broader implications reach into water quality management and climate science. Vegetated reaches downstream of groundwater springs may act as transient nitrogen filters, temporarily sequestering nutrients during the day, and the fate of that stored nitrogen, whether it is buried in sediments, exported downstream, or eventually mineralized, will shape nutrient loads far from the source. Because the study shows that assimilation, not denitrification, dominates at least during this monitoring period, the greenhouse gas consequences may differ from expectations in waters where denitrification would produce nitrous oxide, a potent greenhouse gas. At the same time, the photosynthetic drawdown of bicarbonate connects directly to the recognized carbon sink potential of aquatic phototrophs in carbonate terrain. More broadly, the work provides what the authors describe as high-resolution isotopic evidence that subaquatic vegetation metabolism drives coupled carbon-nitrogen diel cycles in karst streams, offering new insight into nitrogen biogeochemistry in high-DIC aquatic systems and a template for studying daily biogeochemical rhythms elsewhere.

For the scientists involved, the next step is extending these single-day campaigns across seasons and hydrological conditions, since a September snapshot under stable weather cannot capture storm pulses, seasonal plant growth, or temperature-driven shifts in microbial activity. But the central message of the Dawang study is already clear. In the quiet waters of a karst lake, the sun does more than light the surface: it switches an entire ecosystem’s chemistry on and off, twice a day, every day, and only by watching continuously can researchers hope to read the rhythm correctly.

Subject of Research: Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring

Article Title: Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring

Article References: Cao, M., Wang, Y., Zhang, J., Jiao, T., Huang, X., Zhao, S., Song, C., Yao, Z., & Zhang, X. (2026). Diel biogeochemical variations and nitrogen cycling processes in a karst stream aquatic system under high-resolution monitoring. Environmental Monitoring and Assessment, 198(10), Article 1088. https://doi.org/10.1007/s10661-026-15924-8

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15924-8

Keywords: karst stream, nitrogen cycling, stable isotopes, diel variation, subaquatic vegetation, net ecosystem production, denitrification, nitrate, ammonium, carbon-nitrogen coupling, high-resolution monitoring, biogeochemistry

A study published in Nature Sustainability by Yinuo Liu, Yingxin Zhao and colleagues at Tianjin University now reports a strikingly elegant solution to this bottleneck, one that reads almost like science fiction. Instead of adding chemicals or redesigning reactors, the team used a 3D bioprinter to fabricate engineered living materials, or ELMs, in which anammox bacteria and their denitrifying partners are locked together in a precisely printed microarchitecture. Within these printed living structures, the microbes exchange metabolites so efficiently that the system achieved complete nitrogen removal, eliminating one hundred percent of nitrogen from the water, and did so without any external supply of organic carbon. When the researchers validated the approach with real wastewater, the performance held.

The core insight behind the work is fundamentally ecological rather than purely technological. Anammox bacteria, often abbreviated AnAOB, are notoriously slow growers with doubling times measured in days or even weeks, and they naturally produce nitrate as a byproduct of their metabolism. In an ideal system, denitrifying bacteria living alongside them would consume that nitrate, reducing it further to nitrogen gas and closing the nitrogen loop. This cross-feeding partnership exists in nature, but in conventional bioreactors it is fragile. Metabolites drift away in the flowing water before partner organisms can capture them, and the physical distance between anammox cells and denitrifiers dilutes the chemical conversation the two groups need to hold. The result is nitrate piling up in the effluent and treatment performance falling short of what the biology should theoretically allow.

The Tianjin team’s answer was to give the microbial community an architecture. They formulated a bioink composed of sodium alginate and cellulose, two abundant and biocompatible natural polymers, and loaded it with a concentrated anammox consortium. Using extrusion-based 3D printing, they deposited this living ink into defined three-dimensional structures that combine two properties that are usually difficult to reconcile: mechanical stability to survive the harsh conditions of a wastewater reactor, and an open, porous microstructure that lets water and substrates flow through while keeping the cells densely packed inside. The printed materials function simultaneously as scaffolds, as microbial incubators and as diffusion barriers that trap metabolites close to the cells that produce them.

Spatial confinement proved to be the decisive factor. Inside the printed ELMs, cell densities reached levels far higher than those achievable in suspended cultures, forcing microbial cells into intimate proximity. That proximity activated synergistic metabolic pathways that remained dormant or marginal in free-floating consortia. Using metagenomic and metabolomic analyses, the researchers traced a rich exchange economy between the anammox bacteria and key partner taxa, notably denitrifiers of the Opitutus genus. The partners traded extracellular polysaccharides, amino acids and essential cofactors, with each group supplying metabolites the other could not synthesize on its own. In effect, the printed material recreated the dense, chemically connected microenvironments of natural biofilms, but with a geometry designed by engineers rather than left to chance.

The performance gains were dramatic. Anammox systems are infamous for their long start-up periods, the slow weeks during which the bacterial community establishes itself before a reactor reaches useful treatment capacity. The printed ELMs cut start-up time by 71.43 percent, a reduction that could translate directly into faster commissioning of full-scale treatment facilities. More importantly, once running, the confined communities achieved complete nitrogen removal, converting ammonium and nitrate all the way to nitrogen gas without the addition of external organic carbon. That last point matters enormously for sustainability, because conventional denitrification requires organic carbon dosing, typically methanol or other electron donors, which adds cost, operational complexity and a carbon footprint of its own. A system that couples anammox to denitrification using internally recycled metabolites sidesteps that requirement entirely.

The study goes beyond engineering demonstration to probe the underlying mechanisms in detail. Metagenomic evidence revealed the genetic basis of the cross-feeding behaviors between AnAOB and Opitutus, showing how spatial confinement reshaped gene content and pathway activity within the community. Non-targeted metabolomics compared the chemical profiles of free anammox consortia and the printed ELMs, documenting the enriched pools of shared metabolites inside the confined structures. Together, these analyses support a coherent picture: the printed architecture does not simply hold cells in place, it actively rewires the metabolic network of the community, favoring mutualistic exchanges over competition and enabling the coupled anammox-denitrification chemistry that has long been the goal of the field.

The broader implications extend across environmental biotechnology and materials science. Engineered living materials are an emerging class of substances in which living cells are embedded within a fabricated matrix, endowing the material with biological functions such as catalysis, sensing or self-repair. Applying this concept to wastewater treatment represents one of its most consequential potential uses, because the scale of the problem is enormous. Nitrogen removal is a major contributor to global energy consumption and greenhouse gas emissions, and the world’s growing cities are generating ever larger volumes of nitrogen-rich sewage. A technology that makes anammox systems start faster, perform better and operate without carbon dosing could meaningfully shrink the environmental footprint of sanitation infrastructure worldwide.

Significant challenges remain before printed living materials flow through municipal treatment plants. The researchers’ experiments were conducted at laboratory scale, and scaling up 3D bioprinting to produce cubic meters of living material, rather than laboratory specimens, will require new manufacturing approaches. The long-term durability of the alginate-cellulose matrix under continuous loading, shear and fluctuating wastewater chemistry must be demonstrated, and the materials must ultimately be retrievable and replaceable within industrial reactors. Regulatory questions about deploying concentrated engineered microbial communities in open infrastructure will also need careful attention. Nevertheless, the study provides what the field has long sought: a viable, mechanistically grounded strategy for the rapid establishment and enhanced performance of anammox systems, validated with real wastewater and grounded in a deep understanding of microbial ecology.

What makes the work resonate beyond its immediate application is the way it reframes the relationship between fabrication technology and biology. For most of industrial history, engineers have built inert structures and asked biology to adapt to them. Here the logic is inverted: the structure is printed around the biology, shaped to amplify the cooperative behaviors that evolution has already written into the microbial genomes. The printed lattice becomes a kind of architectural mediator, translating the metabolic potential of anammox bacteria and their partners into a treatment process that is faster, cleaner and more complete than either organism group could deliver alone. If the approach survives the journey from bench to plant, the humble printed hydrogel may come to be seen as a quiet turning point in humanity’s effort to clean its own water, one layer of living material at a time.

Subject of Research: 3D-bioprinted engineered living materials that couple anammox bacteria and denitrifiers for complete nitrogen removal in wastewater treatment

Article Title: 3D-printed living materials for anammox–denitrification coupling in wastewater treatment

Article References: 3D-printed living materials for anammox–denitrification coupling in wastewater treatment. (n.d.). https://doi.org/10.1038/s41893-026-01921-9

Image Credits: AI Generated

DOI: 10.1038/s41893-026-01921-9

Keywords: anammox, denitrification, 3D bioprinting, engineered living materials, wastewater treatment, nitrogen removal, microbial cross-feeding, spatial confinement, bioink, sodium alginate, metabolomics, environmental biotechnology

The central finding is stark: since the 1960s, the annual amount of reactive nitrogen entering the biosphere has increased roughly fivefold. This surge is driven overwhelmingly by human activity, above all the Haber-Bosch process that converts atmospheric N2 into ammonia for synthetic fertilizer, a technology that has underpinned agricultural expansion since the 1940s. Between 2015 and 2025, global bulk nitrogen inputs to terrestrial and aquatic ecosystems are estimated at 330 teragrams of nitrogen per year and 183 teragrams per year respectively, with wide uncertainty ranges. By comparison, equivalent inputs in the late 1950s stood at just 121 and 55 teragrams per year, a more than doubling of the flows that nourish the world’s ecosystems in barely two generations.

What makes the new analysis remarkable is what happens on the output side of the ledger. Emissions of nitrogen from the biosphere to the atmosphere increased by only about 12 percent between the oldest and the most recent estimates. In other words, humanity has poured vastly more reactive nitrogen into the Earth system than natural processes have been able to convert back into inert gas. The result is a terrestrial nitrogen accumulation on the order of 50 teragrams of nitrogen per year between 1959 and 2025, an amount sufficient to fundamentally alter soil chemistry, water quality and atmospheric composition across the globe.

The sinks themselves are expanding, but not nearly fast enough to keep pace. Terrestrial denitrification, the anaerobic microbial process that converts soil nitrate back into gaseous forms of nitrogen, is estimated at roughly 100 teragrams of nitrogen per year for the 2010 to 2020 period, up from 69 teragrams per year in 1955. Ocean denitrification estimates rose from 87 teragrams per year to approximately 200 teragrams per year over the same interval. Even these substantially larger losses, however, are dwarfed by the incoming flux, which is why reactive nitrogen continues to build up in soils, groundwater, vegetation and coastal sediments around the world.

This accumulation is not merely a bookkeeping curiosity; it carries real ecological and economic costs. Excess nitrogen availability pollutes soils, waters and air, driving eutrophication of lakes and coastal seas, hypoxic dead zones, acidification, loss of biodiversity and the release of nitrous oxide, a greenhouse gas nearly 300 times more potent than carbon dioxide over a century and now considered the dominant ozone-depleting substance emitted in the twenty-first century. The nitrogen cascade, the term scientists use to describe how a single atom of reactive nitrogen can cause harm sequentially in the atmosphere, on land and in water, means that each unit of nitrogen applied to a field can trigger a chain of environmental consequences far beyond the farm gate.

Part of the difficulty in managing the nitrogen cycle is that several of its largest components remain the most poorly constrained. The review identifies terrestrial biological nitrogen fixation and denitrification as the largest uncertainties in the global budget, and notes that improved estimates of anthropogenic nitrogen inputs to aquatic ecosystems are urgently needed to reduce the high uncertainties surrounding food systems in a changing world. Further quantification of the ocean biomass pool and of nitrogen release through rock weathering is also required. Processes only recently discovered, such as comammox, the complete oxidation of ammonia to nitrate by single microorganisms, and feammox, the anaerobic oxidation of ammonium coupled to iron reduction, are not yet represented in spatially explicit global models, representing a clear priority for future research.

The authors compiled their estimates by synthesizing published budgets across the atmospheric, terrestrial and aquatic reservoirs, drawing on datasets now available openly through the Zenodo repository. This assembly of historical fluxes allows, for the first time, a coherent view of how the modern nitrogen budget has evolved since the earliest global syntheses of the late 1950s. By comparing the earliest comprehensive estimates with the latest data, the review demonstrates that the gap between nitrogen inputs and nitrogen removal has widened dramatically, transforming the global cycle from a roughly balanced system into one with a persistent and growing surplus of reactive nitrogen stored in the terrestrial biosphere.

Closing the budget is not merely an academic exercise. Quantifying nitrogen pools and fluxes across reservoirs is critical for monitoring the imbalance and for evaluating mitigation strategies, from national fertilizer policies to international climate agreements. The review argues that better observations, measurement techniques and models are essential for identifying knowledge gaps and guiding the transition toward a more balanced and sustainable nitrogen cycle. Without such constraints, policymakers are effectively flying blind when attempting to design interventions, since it remains unclear how much of the applied nitrogen is retained, how much is lost to water and how much returns to the atmosphere in reactive or inert forms.

The good news embedded in the analysis is that a pathway toward rebalancing exists. The authors estimate that reducing fertilizer demand and use through dietary changes, deploying technological advances that enhance fertilizer use efficiency, and improving waste management and nutrient recycling could together support a net flux of 51 teragrams of nitrogen per year back to the atmosphere, a magnitude comparable to the current terrestrial accumulation. In practical terms, this means shifting diets toward less nitrogen-intensive protein sources, adopting precision agriculture and controlled-release fertilizers, and recapturing nitrogen from human and animal waste streams that would otherwise flow into rivers and coastal waters.

Nitrogen sits alongside carbon as a headline element of the Anthropocene, and its trajectory will help determine whether humanity stays within planetary boundaries. The review’s numbers make clear that the era of cheap, abundant reactive nitrogen has transformed the planet as profoundly as the rise of atmospheric carbon dioxide, and that correcting the imbalance will require coordinated action across agriculture, energy, sanitation and diet. What the new synthesis offers is a clearer map of where nitrogen is coming from, where it is going and how much remains unaccounted for, giving scientists and policymakers alike the baseline they need to begin steering the global nitrogen cycle back toward equilibrium.

Subject of Research: Fluxes and imbalances in the modern global nitrogen cycle

Article Title: Fluxes and imbalances in the modern global nitrogen cycle

Article References: Almaraz, M., Sun, X., Davidson, E. A., Zhang, X., Galloway, J. N., & Raymond, P. A. (2026). Fluxes and imbalances in the modern global nitrogen cycle. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00821-y

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00821-y

Keywords: nitrogen cycle, reactive nitrogen, Haber-Bosch process, denitrification, nitrogen fixation, fertilizer, nitrous oxide, eutrophication, biogeochemistry, global nitrogen budget, comammox, nitrogen pollution