The colony occupies the left side of the cave’s sulfidic stream in a permanently dark passage beginning about 50 metres from the entrance. The entrance is in Greece and the deeper sections extend into Albania. The stream maintains a year-round temperature of 26 °C and carries hydrogen sulfide concentrations as high as 65 mg l⁻¹, while the cave atmosphere can reach 14 ppm H₂S. White biofilms containing filamentous sulfur-oxidizing bacteria, including Beggiatoa and Thiothrix, cover the stream sediments.

Researchers combined field surveys, microscopy, DNA barcoding, stable-isotope analysis and preliminary 16S metabarcoding. For population estimates, they placed 15-by-15-centimetre quadrants at 30 randomly selected wall locations and counted funnel-shaped T. domestica webs in photographs. T. domestica had a mean estimated density of 652 ± 149 individuals per square metre, with local values ranging from 178 to 2,756. P. vagans averaged 823 ± 556 individuals per square metre, with a range of 133 to 2,200. The counting method may overestimate abundance because abandoned or unoccupied funnel webs are difficult to distinguish from occupied webs.

The web coincided with a dense swarm of the non-biting midge Tanytarsus albisutus above the stream. Resting midges were estimated at 45,500 per square metre, and their observed distribution across roughly half the colony’s area produced an extrapolation of 2,414,440 flies. The authors considered abundant chironomid prey to be the most likely driver of T. domestica colonization, although that explanation was presented as a hypothesis rather than directly tested experimentally.

Carbon and nitrogen stable-isotope measurements indicated a food web associated with material produced inside the cave. Adult T. albisutus and the agelenid and linyphiid spiders inhabiting the colony had δ¹⁵N and δ¹³C values ranging from −2‰ to −10‰, compared with values of 1‰ to 4‰ in conspecific or ecologically similar surface-dwelling taxa. Together with observations of microbial biofilms and abundant terrestrial invertebrates, these measurements support the interpretation that sulfur-oxidizing chemoautotrophs form an important basal resource in the cave food web. The study did not directly trace the transfer of carbon from microbes to midges and spiders, however, and the proposed pathway through chironomids to the colony predators remains inferential.

Morphological examination of genital structures and analysis of mitochondrial cytochrome c oxidase sequences identified T. domestica as the web-forming species and P. vagans as its colony associate. Three additional species—Metellina merianae, Lepthyphantes magnesiae and Kryptonesticus eremita—occurred elsewhere in the cave, while a small, blind and depigmented Cataleptoneta species was reported from deeper sections. All 32 sequenced T. domestica individuals shared a haplotype one mutation from a common haplotype represented across North America, Europe, Turkey, Pakistan and New Zealand. The three sequenced P. vagans individuals shared a haplotype three mutations from a widespread haplotype, while the two K. eremita individuals had a distinct haplotype. The study interpreted these patterns as evidence consistent with limited exchange between some cave and surface populations, but it did not directly measure gene flow.

Reproduction also varied among sampling periods. The researchers opened 86 T. domestica egg sacs collected in June 2024, October 2024 and March 2025. Clutches averaged 16.15 ± 0.62 eggs, with a range of 6–33, and egg size averaged 0.53 ± 0.05 mm, ranging from 0.4 to 0.6 mm. An analysis of variance detected differences among months. June clutches averaged 26 eggs, significantly more than October clutches at 14.5 and March clutches at 13, while October and March did not differ significantly. The authors did not determine why cave clutches may contain fewer eggs than the first cocoons reported for surface-dwelling females.

Preliminary microbiome comparisons found lower bacterial diversity in T. domestica from the cave than in one surface individual collected near Pixaria Cave. The cave spiders had a mean Shannon diversity index of 0.82 ± 0.25, compared with 2.99 for the surface specimen, and many reads came from intracellular symbionts including Mycoplasmopsis, Mycoplasma and Wolbachia. This comparison involved one surface individual and 29 analyzable cave individuals, and whole opisthosomas were used, so the results could not separate microbial contributions from different abdominal organs.

The study provides a detailed field characterization of a mixed T. domestica–P. vagans colony in a chemoautotrophic cave ecosystem, while also documenting reproduction, genetic variation and microbiome differences. Its principal limitations are the extrapolated population estimates, small microbiome comparison and inferential linkage between sulfur-oxidizing biofilms, chironomid prey and the spiders. The authors hypothesized that darkness may reduce the larger T. domestica’s visual detection of the smaller P. vagans, allowing the two species to coexist within the shared web, but no dedicated experiment tested that explanation.

Subject of Research: Field documentation and trophic characterization of a mixed-species spider colony in a sulfidic cave on the Albania–Greece border

Article Title: An extraordinary colonial spider community in Sulfur Cave (Albania/Greece) sustained by chemoautotrophy

Article References: Urák, I., Vrenozi, B., Głąbiak, Z., Lecoquierre, N., Eiberger, C., Maraun, M., Ştefan, A., Flot, J.-F., Brad, T., Dainelli, L., Sarbu, S. M., & Băncilă, R. I. (2025). An extraordinary colonial spider community in Sulfur Cave (Albania/Greece) sustained by chemoautotrophy. Subterranean Biology, 53, 155-177. https://doi.org/10.3897/subtbiol.53.162344

Image Credits: AI Generated

DOI: 10.3897/subtbiol.53.162344

Keywords: Sulfur Cave, spider colony, Tegenaria domestica, Prinerigone vagans, chemoautotrophy, sulfur-oxidizing bacteria, stable isotopes, Albania, Greece, subterranean ecology, extraordinary, colonial

The research team, led by Fernando Álvarez of the Universidad Nacional Autónoma de México and colleagues, compiled published carbon and nitrogen isotope data for Typhlatya shrimps from the Yucatan Peninsula and added new measurements from five cenotes. In total, the dataset covered three of the four species present in the region, Typhlatya mitchelli, T. pearsei, and T. dzilamensis, across thirteen cave systems. The fourth Mexican species, T. campecheae, is so rare that no ecological or isotopic data exist for it, so it was excluded from the analysis.

Stable isotope analysis works because the ratio of heavy to light isotopes in an animal’s tissue reflects the food it has consumed. Carbon isotope values, expressed as δ13C, distinguish between different basal food sources: decaying plant material and particulate organic matter typically produce values around -20 to -35 per mil, while biomass from methanotrophic bacteria, which oxidize methane rising from anoxic sediments, yields far more negative values that can reach below -50 per mil. Nitrogen isotope values, δ15N, indicate trophic position, increasing roughly 3 to 4 per mil with each step up the food chain. By reading these signatures, researchers can reconstruct what an animal has been eating without observing it directly.

The results show that Typhlatya shrimps span almost the complete range of δ13C values found in the entire anchialine community, from -22.31 to -55.48 per mil, reflecting the diversity of microbial resources they exploit. The three species showed partially overlapping but distinct carbon ranges. Typhlatya mitchelli had the least negative values, from -22.31 to -39.65 per mil, consistent with feeding on decaying vegetation and bacterial mats dominated by nitrifying bacteria in shallower cave sections. Typhlatya pearsei showed the most negative values, reaching -55.48 per mil, indicating a diet rich in methanotrophic bacteria. Typhlatya dzilamensis fell in between, from -31.26 to -43.78 per mil, likely exploiting resources associated with the halocline, the sharp salinity gradient that traps imported organic material in deeper cave layers.

Statistical comparisons confirmed these differences. A Kruskal-Wallis test revealed significant variation among species, and Dunn’s multiple comparison showed that T. mitchelli differed significantly from T. pearsei and from unidentified Typhlatya individuals. The unidentified specimens from Cenote Bang, reported by Brankovits and colleagues in 2017, showed carbon values between -29.27 and -49.65 per mil, a range consistent with any of the three known species or potentially with a yet undescribed lineage suggested by prior molecular analyses.

To examine how the two dominant freshwater species partition resources, the team constructed convex hulls in δ13C-δ15N space. The hulls for T. mitchelli and T. pearsei overlapped by roughly 20 percent, with T. mitchelli sharing 23.05 percent of its area and T. pearsei sharing 17.62 percent. This limited overlap leaves ample margin for coexistence. Typhlatya pearsei showed broader carbon and nitrogen ranges and a larger total hull area than T. mitchelli, indicating a wider diversity of exploited resources. The overlap zone, at carbon values between -33 and -40 per mil, appears to mark a transition between the two primary feeding strategies.

Seasonal comparisons between the rainy and dry seasons showed no significant differences in carbon isotope values for either species across four cenotes. Paired Wilcoxon signed-rank tests conducted separately for each species and site revealed no seasonal shifts. This stability contrasts with expectations based on prior work showing an inverse relationship between methane concentration and rainfall in an inland cenote of the Ox Bel Ha system. The authors caution that their samples came from geographically distant cenotes collected in 2022 and 2023, whereas the earlier methane study examined a single site during 2015 and 2016, so larger standardized datasets will be needed to determine whether seasonal methane variation consistently affects shrimp isotopic signatures.

Geography mattered. Cenotes in the Ring of Cenotes, an arch of deep, short, unconnected passages around the city of Mérida, showed narrower and less negative carbon ranges for Typhlatya species, averaging -28.65 per mil, than those in the Caribbean Cave Area of Quintana Roo, which averaged -36.20 per mil. The Caribbean Cave Area contains shallower caves with tens to hundreds of kilometers of interconnected passages and strong lateral flows. In this region, organic matter from the overlying rainforest percolates through the rock matrix, generating small amounts of methane that are oxidized by methanotrophic bacteria near cave ceilings. These bacteria provide an additional basal resource that complements direct organic inputs, producing broader carbon ranges consistent with mixed reliance on particulate organic matter and methanotrophic biomass. This process depends on the integrity of the rainforest, which is crucial for maintaining the rich anchialine fauna in remote cave sections where surface organic matter is not transported.

The evolutionary context adds weight to these findings. The family Atyidae is an ancient group of primary freshwater shrimps that probably arose around the Permo-Triassic boundary roughly 250 million years ago. The clade inhabiting anchialine caves may have evolved in late Jurassic or early Cretaceous times, possibly occupying coastal areas of the ancient Tethys Sea. The contemporary distribution of Typhlatya in coastal areas of the Greater Caribbean, Mediterranean Sea, Ascension and Galapagos Islands, and Indian Ocean, along with related genera in Madagascar and Australia, supports this Tethyan origin hypothesis. Fifteen of forty-two atyid genera include cave-dwelling species, a success probably linked to their specialized filter-feeding and scraping mouthparts.

Support for trophic convergence comes from Australia, where Stygiocaris stylifera, a close relative of Typhlatya endemic to the Cape Range Peninsula and Barrow Island, displays isotope values indicative of partial reliance on chemoautotrophic bacterial biomass. This parallel evolution of bacterial-based feeding in distant atyid lineages suggests that the ability to exploit chemoautotrophic production may explain the persistence of anchialine systems as ecological refugia over geological timescales. The versatile feeding apparatus of atyids, with modified first and second pairs of pereopods adapted for scraping rather than cutting, likely facilitates this opportunistic strategy, setting them apart from epigean filter-feeding relatives.

The study’s authors propose that Typhlatya shrimps function as keystone species, introducing nutrients to the anchialine trophic web by taking advantage of microbial production from nitrifying bacteria acting on plant debris and methanotrophic bacteria consuming methane from the water column. Other anchialine species that could feed on bacterial mats, such as the thermosbaenacean Tulumella unidens, occur in numbers too low to play a comparable central role. By converting microbial biomass into animal tissue accessible to predators, Typhlatya shrimps connect primary producers and particulate organic matter with higher trophic levels, potentially facilitating the arrival of other species once they colonize a cave. The breadth of their isotopic niches underscores their trophic importance, the diversity of resources they exploit, and the influence of regional geomorphological differences. Continued study of these shrimps promises to further illuminate the functioning of this still poorly known ecosystem, one of the largest anchialine cave networks on Earth, which may exceed 1,700 kilometers in Quintana Roo alone.

Subject of Research: Stable isotope analysis of trophic niches of Typhlatya cave shrimps in Yucatan anchialine caves

Article Title: The trophic role of cave shrimps of the genus Typhlatya seen through stable isotope eyes

Article References: Durán, B., & Álvarez, F. (2026). The trophic role of cave shrimps of the genus Typhlatya seen through stable isotope eyes. Subterranean Biology, 55, 43-56. https://doi.org/10.3897/subtbiol.55.164068

Image Credits: AI Generated

DOI: 10.3897/subtbiol.55.164068

Keywords: Typhlatya, anchialine caves, stable isotopes, trophic ecology, Yucatan Peninsula, cenotes, methanotrophic bacteria, food webs, Subterranean Biology, isotopic niche, cave shrimps, chemoautotrophy

Ostracods are a class of small crustaceans encased in a calcified, hinged carapace that resembles a tiny clam shell. This durable shell has granted them one of the most complete fossil records of any arthropod group, stretching back hundreds of millions of years. While the majority of ostracod species inhabit marine environments and surface freshwaters, several lineages have independently invaded subterranean waters, colonizing alluvial aquifers, karstic fissures, and caves. Life underground imposes a characteristic set of evolutionary pressures: perpetual darkness, scarce and patchy food resources, and remarkably stable environmental conditions. In response, subterranean ostracods often lose their eyes and body pigment, elongate sensory hairs to better detect chemical and mechanical cues, and slow their metabolisms to a crawl, growing slowly, reproducing late, and producing few offspring. These adaptations make them fascinating models for studying how evolution sculpts organisms under energetic constraint, and useful indicators of groundwater quality and hydrological connectivity.

Sulfidic caves, however, raise the difficulty of subterranean life to another level entirely. Hydrogen sulfide, which can seep into caves from geothermal sources, the oxidation of sulfide-bearing minerals, or microbial sulfate reduction, is toxic to most animals even at moderate concentrations. Yet where sulfide-rich water meets oxygenated water or cave air, steep chemical gradients form, and these gradients become the engine of an entire ecosystem. Sulfur-oxidizing bacteria flourish along these boundaries, converting hydrogen sulfide into other sulfur compounds to harvest energy. Because these microbes can build organic matter from inorganic chemicals, a process called chemoautotrophy, they form the base of a food web that requires essentially no photosynthetic input. The famous Movile Cave in Romania and its hydrogen sulfide-rich waters, for example, host an entire endemic fauna sustained this way. Only a handful of non-marine ostracod species have ever been documented from such environments, which makes the Albanian find especially significant.

The new species was collected in the evocatively named Breath of the Dragon Cave, located in the Langarica Canyon of Albania. This extensive hypogene karst system comprises more than ten kilometres of passages, and its lower section holds a large thermal sulfidic stream along with several stagnant sulfidic lakes. The streambed is coated with a thick layer of bat guano, a rich external food resource for the cave’s stygobitic invertebrates, which include amphipods and both cyclopoid and harpacticoid copepods. Even more importantly for the new ostracod, chemoautotrophic sulfur-oxidizing bacteria abound in the stream, both as free-living bacterioplankton and as structured microbial biofilms covering the streambed. Researchers collected specimens during two surveys in April and October 2025 using a fine 180-micrometre hand net, preserving the animals in ethanol for later analysis.

Describing the species required an integrative taxonomic approach, combining meticulous morphological study with molecular sequencing. The specimens are strikingly small: males measure between 1.09 and 1.13 millimetres in carapace length, and females slightly less. Under light microscopy and scanning electron microscopy, the team documented the smooth, brownish valves bearing long, stiff setae, the intricate segmentation and setal patterns of the antennules, antennae, mouthparts, and three pairs of thoracic limbs, and the detailed architecture of the male and female reproductive structures, which in ostracods are often decisive for species identification. Notably, all collected individuals had decalcified, fragile shells, which were frequently damaged during dissection. The researchers attribute this decalcification to the acidic conditions typical of sulfidic environments, a chemical reality the animals apparently tolerate even at the cost of their own protective armour.

Molecular work on six specimens yielded mitochondrial COI sequences and nuclear 28S ribosomal DNA sequences, all belonging to single haplotypes for each marker. Phylogenetic analyses built with Bayesian inference placed Pseudocandona sulphurella firmly within the rostrata group of the genus Pseudocandona, alongside Western Palearctic relatives including Pseudocandona marchica from Norway, Pseudocandona movilaensis from Romania’s sulfidic Movile Cave, and Pseudocandona hartwigi from Poland. COI p-distances to the closest species, roughly 0.150 and 0.151, are consistent with clear species-level separation. Three independent species delimitation methods, the distance-based ASAP and the tree-based bPTP and GMYC approaches, all confirmed the new species as distinct. Intriguingly, the analyses also revealed that members of the genus Pseudocandona do not all form a single evolutionary lineage, pointing to the likely polyphyly of this taxonomically tangled genus and reinforcing calls for future revision.

That the new species belongs to the rostrata group is itself noteworthy. This clade, comprising more than a dozen known species, is otherwise dominated by animals living in ordinary surface waters, with only one previous stygobiont representative, Pseudocandona movilaensis of Movile Cave. The rostrata group is Palearctic in distribution, and only a few species have also been recorded in North America. Most members live in lakes, ponds, and streams, although several, including Pseudocandona stagnalis, Pseudocandona semicognita, and Pseudocandona tenuirostris, are known to tolerate acidic conditions, a possible pre-adaptation that may have eased the evolutionary transition into acidified sulfidic caves. With two strict stygobionts now known from sulfidic caves, the group appears to have independently produced cave-restricted sulfide specialists more than once, hinting at a recurring evolutionary pathway into these extreme habitats.

Perhaps the most visually arresting finding came from the scanning electron microscope: the valve surfaces and bodies of Pseudocandona sulphurella are extensively coated with bacteria, including small undetermined bacilli and filamentous, Thiothrix-like sulfur oxidizers. These bacteria convert hydrogen sulfide into other sulfur compounds to gain energy, and they are frequently found as epibionts on crustaceans in sulfidic habitats, from amphipods in Romanian groundwaters to ostracods at methane seeps in Lake Baikal. Such associations can benefit the bacteria, which gain transport and access to host-derived nutrients, and potentially the host as well, since the microbial coat may serve as a browsable food resource. Whether the relationship in Pseudocandona sulphurella is mutualistic, commensal, or something more complicated remains an open question, but it places the new species squarely within a recurring ecological theme of sulfidic ecosystems worldwide.

The discovery carries significance well beyond the description of a single animal. Albania’s living ostracod fauna has been studied mainly around the ancient, biodiverse lakes Prespa and Ohrid, which host exceptionally rich and endemic assemblages, but no stygobiont ostracod had ever been documented in the country until now. More broadly, the find underscores the value of sulfidic caves as natural laboratories for studying adaptation, speciation, and the ecology of chemoautotrophic communities, systems where life is fuelled by geochemistry rather than sunlight. Because subterranean species typically have small, localized ranges and narrow physiological tolerances, they are also acutely vulnerable to environmental change and groundwater disturbance. Each new species documented from these hidden ecosystems, whether a dragon-breath cave in Albania or a sealed sulfur cave in Romania, expands our understanding of where and how life can persist on Earth, and offers a preview of the kinds of chemically driven biology that astrobiologists suspect might operate on other worlds.

Subject of Research: Discovery and integrative taxonomic description of a new sulfide-tolerant cave-dwelling ostracod crustacean from Albania

Article Title: A new extremophile species of Pseudocandona ( Crustacea , Ostracoda ) from a sulfidic cave in Albania

Article References: Rossetti, G., Ștefan, A., Motoc, R., Sarbu, S. M., & Mazzini, I. (2026). A new extremophile species of Pseudocandona (Crustacea, Ostracoda) from a sulfidic cave in Albania. Subterranean Biology, 56, 137-159. https://doi.org/10.3897/subtbiol.56.188589

Image Credits: AI Generated

DOI: 10.3897/subtbiol.56.188589

Keywords: ostracod, extremophile, sulfidic cave, Albania, stygobiont, Pseudocandona, groundwater biodiversity, chemoautotrophy, integrative taxonomy, phylogenetics, epibiotic bacteria, karst

The scale of the underlying process is staggering. Photoautotrophs, the phytoplankton of the sunlit ocean, account for roughly half of the planet’s primary production, fixing carbon dioxide into organic molecules that sustain nearly all marine life. Alongside them, chemoautotrophic microbes fix dissolved inorganic carbon throughout the entire water column, from deep-sea hydrothermal vents to shallow coastal shallows, at an estimated global rate of one to three petagrams of carbon per year. Although that is roughly ten times less carbon than photosynthesis fixes, it still dwarfs the roughly 0.4 to 0.5 petagrams of organic carbon carried into the ocean by all of the world’s rivers combined. Yet the researchers were interested in a third, even more shadowy contributor: heterotrophic organisms, which are normally considered net emitters of carbon dioxide, but which carry carboxylase enzymes that stitch carbon dioxide directly into their own biomass.

These carboxylation reactions, known as anaplerotic processes, replenish the intermediates of the tricarboxylic acid cycle, the metabolic hub from which cells draw building blocks for amino acids and other essential molecules. Laboratory studies suggest that anaplerotic carbon dioxide incorporation can account for between one and eight percent of heterotrophic cell carbon, and in some light-exposed bacteria bearing light-driven proteorhodopsin pumps, dark inorganic carbon fixation can supply up to a third of cellular carbon biomass. Cumulative evidence from Arctic surface waters, the mesopelagic North Atlantic, and even the hadal depths of the Hellenic Trench has increasingly shown that heterotrophic microbes take up inorganic carbon far more substantially than their textbook reputation implies. What remained almost entirely unknown was how this flux behaves over time in shallow, seasonally variable temperate waters, where sunlight, temperature, and river-borne nutrients swing dramatically through the year.

To fill that gap, the team sampled a long-term ecological research station in the Gulf of Trieste, a shallow basin less than 25 meters deep that is flushed by freshwater from the Isonzo and Timavo rivers. Salinity there oscillates between 29 and 38.5 and temperatures between roughly 4 and 29 degrees Celsius, producing strong summer stratification and complete winter mixing. From October 2018 to April 2021, the researchers collected seawater at one meter and fifteen meters depth each month, measuring dark inorganic carbon fixation rates with radiolabeled sodium bicarbonate, heterotrophic production with tritiated leucine, and the composition of the microbial community through 16S rRNA gene amplicon sequencing. They also quantified copies of the archaeal amoA gene, which encodes a subunit of ammonia monooxygenase, the enzyme that ammonia-oxidizing archaea use to kick-start nitrification and, in doing so, to power their own chemoautotrophic carbon fixation.

The rate measurements revealed a striking and repeatable seasonality. Dark inorganic carbon uptake sank to winter minima of around 0.09 micrograms of carbon per liter per day in February and March, then climbed steadily through spring, plateauing at summer maxima in surface waters and peaking sharply in bottom waters between September and October. Occasional spikes reached more than 1.8 micrograms per liter per day, as in November 2019. Heterotrophic carbon production traced a near-identical arc, rising from winter lows near 2 micrograms per liter per day to summer highs averaging over 13 micrograms in surface samples. The two processes were strongly correlated, and a partial least squares regression model identified temperature and heterotrophic production as the strongest positive predictors of dark carbon uptake, with dissolved organic carbon and nitrogen also contributing positively and ammonia concentration adding a smaller positive signal.

The implication, the authors argue, is that the bulk of dark inorganic carbon fixation in these shallow waters is fueling heterotrophic metabolism rather than chemoautotrophic growth. The ratio of carbon fixation to heterotrophic production in the study ranged from about 0.1 to 0.5, squarely within the 0.1 to 0.8 range reported for heterotroph-dominated systems worldwide. Notably, particulate organic carbon, which peaks during spring phytoplankton blooms, was actually a negative predictor of dark carbon fixation. That decoupling makes sense biochemically: freshly produced, labile phytoplankton organic matter requires few carboxylation steps to be assimilated, whereas the more refractory compounds that accumulate in the dissolved pool by late summer demand far greater metabolic effort, forcing microbes to lean harder on anaplerotic carbon dioxide incorporation. Photoheterotrophy may compound the summer pattern, since several taxa that bloom alongside the summer fixation maximum, including the OM60/NOR5 clade and HIMB11, carry the genomic machinery for light-driven energy harvesting and anaplerotic carbon fixation simultaneously.

Winter told a very different story. Quantification of the archaeal amoA gene revealed recurrent abundance peaks every December and January, rising as high as 2.26 times ten to the seventh copies per liter, and closely mirroring blooms of the ammonia-oxidizing archaeon Candidatus Nitrosopumilus, which can make up as much as ten percent of the winter community. These archaea perform the first and rate-limiting step of nitrification, oxidizing ammonia to nitrite and using the liberated electrons to fix carbon dioxide autotrophically. Their gene abundance correlated tightly with nitrite concentrations, and each winter the study area accumulated a distinct nitrite maximum, the coastal analogue of the ocean’s well-known primary nitrite maximum. Short winter days likely favor the archaea, which are photoinhibited, while simultaneously suppressing the phytoplankton that would otherwise consume the nitrite.

By back-calculating carbon fixation from observed nitrite accumulation, assuming the carbon yield of pure-culture Nitrosopumilus strains, the team estimated that nitrification could account for between 7.9 and 22.5 percent of measured dark inorganic carbon fixation in surface and bottom waters, respectively, and on average about 13.3 percent of wintertime uptake overall, with a peak estimate reaching 34 percent in bottom samples during winter blooms. A parallel calculation using a global euphotic nitrification rate yielded strikingly similar contributions of roughly fifteen percent. The authors caution that these figures likely overestimate the true autotrophic share, because carbon yields measured in idealized cultures probably exceed those in nature. Still, both independent approaches converged on the same conclusion: chemoautotrophic ammonia oxidation is a non-negligible, seasonally concentrated contributor to carbon cycling in shallow temperate seas, potentially supplemented by winter-active SUP05 cluster bacteria capable of sulfur-based autotrophy.

The broader significance of the work lies in what it says about the marine carbon budget. If heterotrophic anaplerotic fixation dominates dark carbon uptake in productive, sunlit coastal waters, then global estimates of chemoautotrophic carbon fixation, and of the carbon basis for microbial food webs, may need careful re-examination in these environments. Heterotrophic bacteria, long cast simply as recyclers that respire organic carbon back to carbon dioxide, emerge as genuine, if inadvertent, participants in carbon fixation, with inorganic carbon behaving as a ready co-substrate for their core metabolism. Meanwhile, the recurring winter nitrite maximum documented in the Gulf of Trieste suggests that archaeal nitrification imposes a seasonal biogeochemical fingerprint even in waters barely twenty meters deep. As ocean warming continues to raise metabolic rates, the calculated temperature sensitivity of dark carbon fixation in this study, with a Q10 of about 2.5, hints that this hidden carbon flux could intensify in a warmer sea, reshaping how coastal ecosystems process both nitrogen and carbon.

Subject of Research: Dark dissolved inorganic carbon fixation by coastal microbial communities and the relative roles of heterotrophic anaplerosis and archaeal nitrification.

Article Title: High contribution of dark dissolved inorganic carbon uptake to microbial carbon cycling in a shallow Mediterranean basin

Article References: Manna, V., Balestra, C., Banchi, E., Fonti, V., Kralj, M., & Celussi, M. (2025). High contribution of dark dissolved inorganic carbon uptake to microbial carbon cycling in a shallow Mediterranean basin. Ocean Microbiology, 1(1), Article 2. https://doi.org/10.1186/s44375-025-00002-0

Image Credits: AI Generated

DOI: 10.1186/s44375-025-00002-0

Keywords: dark DIC fixation, ammonia oxidation, amoA, chemoautotrophy, heterotrophic CO2 fixation, anaplerosis, nitrification, time-series, Mediterranean Sea, 16S rRNA, microbial community dynamics, northern Adriatic Sea