The research, published in Nature Communications, turns to the Paleocene-Eocene Thermal Maximum, or PETM, a hyperthermal event about 55.9 million years ago that is widely regarded as the best deep-time analog for modern climate change. During the PETM, between 3,000 and 10,000 gigatons of isotopically light carbon were injected into the ocean-atmosphere system within roughly 10,000 years, producing a negative carbon isotope excursion of 3 to 5 per mil. Seawater temperatures climbed by 5 to 8 degrees Celsius, and the ocean suffered acidification, deoxygenation, and major turnovers among bottom-dwelling foraminifera and calcareous nannoplankton. Although the carbon release rate was about an order of magnitude slower than today’s fossil fuel emissions, the total amount of carbon released is comparable to what high-emission scenarios project for 2100. That makes the PETM a rare natural experiment in what happens to marine ecosystems under sustained greenhouse forcing.

The team, led by Xiuwen Zhou and Weiqi Yao of the Southern University of Science and Technology in Shenzhen, tackled a long-standing contradiction in the PETM literature. Marine barite accumulation rates, considered a reliable tracer of carbon export from the surface ocean, show an average threefold increase in export productivity during the event, likely driven by enhanced nutrient input from land. But evidence from benthic foraminifera and nannofossil assemblages suggests nutrient supply declined in the South Atlantic, Southern Ocean, and North Pacific after the PETM onset, and Earth system models indicate a warmer, more stratified ocean should have suppressed productivity in the tropics. To resolve the puzzle, the researchers needed an independent record of productivity higher up the food chain, and they found one in an unlikely place: fossilized fish teeth.

Ichthyoliths, the microscopic remains of fish teeth and shark dermal denticles, rain down onto the seafloor as fish naturally shed teeth or as predators excrete the indigestible parts of their prey. Most ichthyoliths in marine sediments come from small pelagic species, including juvenile fishes and shark taxa, which sit at mid-trophic levels and whose abundance is sensitive to both productivity and the efficiency with which energy moves through the food web. By measuring ichthyolith accumulation rates, the researchers could reconstruct fish productivity directly. They analyzed 105 samples from five sediment cores spanning the North Pacific, Eastern Equatorial Pacific, North Atlantic, South Atlantic, and Southeast Indian Ocean, and combined these with previously published data from a sixth equatorial Pacific site.

The results were striking. At the Pacific and North Atlantic sites, ichthyolith accumulation rates rose dramatically during the PETM, with peak values five to eleven times higher than background levels. The highest rates appeared in the Eastern Equatorial Pacific, reaching 121 ichthyoliths per square centimeter per thousand years at Site 1221, while the North Pacific and North Atlantic sites peaked at 51 and 64 respectively. Crucially, the rise was not an artifact of sedimentation rates, since ichthyolith abundance showed negligible correlation with the total mass of sediment accumulating. Nor was it a sign of a community shift: most ichthyoliths were smaller than 63 micrometers, tooth-type diversity remained stable across the event, and rare tooth types were evenly distributed throughout the sequence, all pointing to persistent fish communities dominated by the same conical and triangular tooth morphotypes before, during, and after the warming.

With community composition ruled out as the driver, the evidence converged on a genuine increase in fish productivity. Warmer waters can accelerate fish growth and shorten generation times, which might boost tooth flux on its own, but the researchers found a nonlinear relationship between ichthyolith rates and temperature or latitude, suggesting trophic effects alone could not explain the geographic pattern. The missing piece came from the paired barite records. At the equatorial Pacific sites, barite-derived carbon export climbed to 187 grams of carbon per square meter per year within the main carbon isotope excursion, and its two prominent peaks, one at the onset of the event and another roughly 45,000 years later, coincided precisely with the two peaks in the fish record. In the North Atlantic, export reached 166 grams of carbon per square meter per year, far above what the region supports today.

Globally, the correlation between the two proxies was statistically robust: pooled carbon export data from all sites showed a significant positive relationship with ichthyolith accumulation, with a Pearson correlation coefficient of 0.55 and a Spearman coefficient of 0.61, both significant at the 0.001 level. The spatial pattern matched too, with the strongest responses in the Eastern Equatorial Pacific and North Atlantic, regions close to upwelling zones where nutrient-rich deep waters reach the surface, and the weakest in the mid- to high-latitude Southern Hemisphere, where both export and fish productivity remained low. The implication is that enhanced carbon export from the surface ocean fueled the boom in fish productivity, delivering more food to the mid-trophic-level consumers whose teeth preserve the signal.

To understand why export productivity rose despite the stratifying effects of warming, the team ran a series of simulations with the cGENIE Earth system model, using early Eocene paleogeography and progressively switching on individual mechanisms. Doubling atmospheric carbon dioxide from 1,112 to 2,224 parts per million on its own produced only a negligible long-term increase in organic carbon export, far smaller than the proxy records demand. The decisive factor was enhanced weathering on land. Lithium isotope evidence indicates terrestrial weathering intensified globally by 50 to 60 percent during the PETM, and when the model incorporated temperature-dependent phosphorus weathering, the ocean-wide average export flux rose from 0.7 to 1.2 moles per square meter per year, with a 1.9-fold weathering enhancement pushing the flux to about 1.6 moles, consistent with the two- to threefold rise inferred from the barite records. Regional iron fertilization from volcanic ash of the North Atlantic Igneous Province and enhanced dust production in North America likely amplified productivity near those sources, which the model, lacking regional iron input, underestimates. Notably, adding redox-dependent phosphorus recycling under low-oxygen conditions contributed almost nothing, because a deeper thermocline in the warmer, more stratified ocean limited the upward transport of recycled nutrients into the sunlit zone.

The findings carry a provocative message for the future. The fertilizing effect of terrestrial nutrient input operates on longer timescales than the decadal trends captured by satellites, and the PETM record shows it can outweigh the suppressive effects of stratification, sustaining higher ocean productivity under sustained greenhouse warming. Rising export productivity may even have helped the Earth system recover from the hyperthermal by burying organic carbon and drawing down carbon dioxide, a negative feedback visible in the elevated organic carbon contents of equatorial Pacific and North Atlantic sediments from the event. The researchers caution that higher export in the water column does not necessarily mean more organic matter reaches the seafloor, since warmer, acidified waters accelerate microbial degradation of sinking particles, and that PETM dynamics cannot be applied directly to decadal forecasts. Still, the study raises the possibility that today’s declining productivity trends could be moderated, or even reversed, over the long arc of a warming century and beyond, a nuance that current climate models, with their divergent projections, have yet to fully capture.

Subject of Research: Marine productivity and fish abundance during the Paleocene-Eocene Thermal Maximum

Article Title: Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum

Article References: Zhou, X., Zhang, R., Tsang, M.-Y., Li, M., & Yao, W. (2026). Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum. Nature Communications, 17(1), Article 10030. https://doi.org/10.1038/s41467-026-77863-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77863-9

Keywords: Paleocene-Eocene Thermal Maximum, marine productivity, ichthyoliths, marine barite, carbon export, paleoceanography, ocean warming, nutrient supply, cGENIE model, fish fossils, ocean stratification, climate change

The study, led by Riley Barton of the Marine and Coastal Research Laboratory at Pacific Northwest National Laboratory and Rensselaer Polytechnic Institute, together with Sasha Wagner, Christina M. Richardson, Alex Collins, Margaret A. Zimmer, Maya Montalvo, and Allison N. Myers-Pigg, focused on five small coastal mountain watersheds in California that were burned by the CZU Lightning Complex Fires of 2020. That fire event, ignited by an extraordinary lightning storm, tore through the Santa Cruz Mountains and left steep coastal drainages draped in ash and charred vegetation. Because these watersheds drain directly toward the Pacific, they offered an ideal natural laboratory for tracking how fire-altered carbon moves from land to sea.

The researchers zeroed in on a molecule called levoglucosan, an anhydrosugar that forms when cellulose and hemicellulose, the structural carbohydrates of plant tissue, are burned at moderate temperatures. Levoglucosan has long served as a molecular fingerprint for biomass burning in atmospheric science, because it is produced in abundance during combustion and can travel long distances in smoke. But in aquatic systems, levoglucosan has a very different personality: it is highly soluble, which means it dissolves readily in the first rains after a fire, and it is biolabile, meaning microbes can consume it quickly as a carbon and energy source. In other words, levoglucosan is the ephemeral, fast-cycling counterpart to the stubborn, centuries-persistent black carbon that has dominated discussions of fire-derived carbon in rivers.

Black carbon, by contrast, is the charred, condensed aromatic material that gives burned soils their dark color. It is famously recalcitrant, resisting microbial decay and persisting in soils, sediments, and the open ocean for hundreds to thousands of years. Because of this stability, black carbon has been treated as the primary long-term sink in the fire-carbon equation, and much of the aquatic research on pyrogenic carbon, often abbreviated PyC, has centered on quantifying these durable fractions. The new study asked a deceptively simple question: if you measure both the labile and the recalcitrant fire-derived carbon in the same streams at the same time, do they behave the same way?

The answer, emphatically, is no. Levoglucosan concentrations in the streams peaked during the earliest post-fire flows, the very first storm-driven runoff events after the flames were out, and then declined steadily over time. Remarkably, this decline was independent of stream discharge. It did not matter how much water was coursing through the channels; the levoglucosan signal faded because the supply was being exhausted, flushed out of the burned landscape during those initial pulses and consumed by microbes along the way. Black carbon told the opposite story. Its concentrations were discharge-driven, rising when flows rose, and, counterintuitively, they increased with time since the fire. The researchers attribute this delayed pattern to gradual solubilization: the charred material left on the landscape does not dissolve immediately but slowly becomes available to water over months and years as physical and chemical weathering break it down.

This divergence matters because it means that the two major categories of pyrogenic carbon operate on fundamentally different clocks. Labile anhydrosugars are a flash flood of bioavailable carbon, delivered within the first post-fire storm seasons and efficiently transported all the way to coastal lagoons and nearshore zones. Black carbon is a slow-release reservoir, accumulating in export pathways over longer timescales. A monitoring program that samples a burned watershed a year or two after a fire, or one that focuses only on the recalcitrant fraction, would completely miss the early, intense pulse of labile carbon, and with it, a significant chapter in the post-fire carbon budget.

The downstream consequences could be substantial. When levoglucosan and other soluble, easily degraded combustion products arrive in coastal lagoons and nearshore waters, they provide a fresh fuel source for microbial communities. Enhanced microbial processing means enhanced remineralization, the conversion of organic carbon back into carbon dioxide, potentially altering the carbon balance of these transitional ecosystems and influencing oxygen demand, nutrient cycling, and food web dynamics. Coastal lagoons along the California coast are already sensitive environments, subject to seasonal sandbar closure, variable salinity, and increasing fire pressure in their upstream catchments. A sudden influx of labile pyrogenic carbon could ripple through these systems in ways that measurements of black carbon alone would never reveal.

The broader implication is that wildfire’s impact on the global carbon cycle may be underestimated. Wildfire frequency and severity are increasing across the western United States and many other regions of the world, and each fire leaves behind a spectrum of organic matter ranging from the environmentally persistent to the rapidly degradable. If carbon accounting and biogeochemical models focus on the recalcitrant end of that spectrum, they will capture the long-term storage story but miss the fast-cycling one. The authors argue that expanding the focus beyond recalcitrant pyrogenic carbon to include labile compounds is essential to capture the full impact of terrestrial fires on carbon dynamics across the land-ocean aquatic continuum, the connected chain of rivers, lakes, estuaries, and coastal waters through which carbon travels from continents to the sea.

Methodologically, the study demonstrates the value of pairing molecular tracers with hydrologic context. By measuring levoglucosan and black carbon side by side across multiple burned watersheds and multiple flow conditions, the team could disentangle the effects of supply exhaustion from the effects of dilution and transport. The independence of the levoglucosan decline from discharge is a particularly telling signature: it points to a finite, depletable pool of soluble combustion products rather than a continuous source regulated by water flux. Such distinctions are exactly what models of post-fire carbon export need in order to predict how landscapes will behave as fire regimes intensify.

The research, supported by the National Science Foundation and the U.S. Department of Energy’s Office of Science, and conducted as part of Barton’s doctoral dissertation, arrives at a moment when fire science is racing to keep pace with a changing fire regime. As more watersheds burn and more rain falls on fresh ash, the first flush of labile carbon into rivers and coasts may prove to be one of the most dynamic and underappreciated consequences of wildfire. Recognizing that not all fire-derived carbon behaves alike, and that the most fragile molecules may carry the biggest early signal, is a crucial step toward understanding what burning landscapes mean for the carbon cycle from mountaintop to ocean.

Subject of Research: Post-fire mobilization of levoglucosan and black carbon in coastal California watersheds

Article Title: Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds

Article References: Barton, R., Wagner, S., Richardson, C. M., Collins, A., Zimmer, M. A., Montalvo, M., & Myers-Pigg, A. N. (2026). Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds. Biogeochemistry. https://doi.org/10.1007/s10533-026-01367-3

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01367-3

Keywords: levoglucosan, black carbon, pyrogenic carbon, wildfire, watersheds, biogeochemistry, carbon cycle, anhydrosugar, coastal lagoons, organic carbon, CZU Lightning Complex Fire, carbon export

The study, published in PLOS Ecosystems by Emmanuel Boss, Charlotte Begouen Demeaux, and Michael J. Behrenfeld, revisits how the euphotic depth should be computed. The authors revive an argument made two decades ago by the influential biological oceanographer Karl Banse, who died in 2025: phytoplankton, the microscopic algae that anchor the marine food web, do not sense light as a percentage of surface illumination. They experience only the absolute flux of photons arriving at whatever depth they happen to occupy. A definition based on relative light levels, the authors contend, is therefore physiologically meaningless.

The one-percent rule has undeniable practical appeal. It can be estimated with an uncalibrated radiometer, a simple Secchi disk, or satellite ocean-color products, and it appears in studies of phytoplankton seasonal cycles, carbon flux calculations, and remote-sensing data distributed worldwide. But the new analysis, built on nearly 21,000 profiles from autonomous BGC-Argo floats spanning October 2012 to December 2023, shows that the absolute amount of light found at the one-percent horizon varies enormously with latitude, season, and cloud cover. At any given moment, roughly seventy percent of the ocean lies beneath clouds, and seasonal swings in solar angle halve surface irradiance by thirty degrees latitude and extinguish it entirely near the poles in winter.

Instead of a percentage, the authors recommend anchoring the euphotic depth to a fixed absolute light level called the compensation irradiance: the minimum daily light at which photosynthesis can outpace algal respiration, allowing net growth. Drawing on recent Arctic field campaigns and laboratory work, they propose a value of 0.0035 plus or minus 0.002 mol photons per square meter per day, a figure orders of magnitude lower than many earlier estimates and close to the theoretical minimum of about 0.0009 mol photons per square meter per day derived from the bioenergetics of photosynthesis.

The evidence for such extraordinarily low-light growth comes from converging sources. In Baffin Bay, researchers using chlorophyll fluorescence and particle backscattering sensors below sea ice detected the onset of phytoplankton accumulation at an average light level of 0.0043 mol photons per square meter per day. A separate Arctic study coupling biomass measurements with radiocarbon-based production assays found a compensation irradiance of 0.0035 mol photons per square meter per day, essentially identical to the value now proposed. In the Southern Ocean near the Kerguelen Islands, incubation experiments previously documented positive net primary production at the 0.01 percent light level, corresponding to roughly 0.006 mol photons per square meter per day.

Critically, the new work extends the argument beyond polar waters. Analyzing cytometric counts of Prochlorococcus, the abundant photosynthetic bacterium that dominates nutrient-poor oceans, from the Atlantic Meridional Transect cruise 13, the authors found significant cell concentrations down to depths where daily light falls to the proposed 0.0035 threshold, even in subtropical latitudes. Oxygen-based measurements of gross primary production from other Atlantic transects likewise show detectable production at the 0.1 percent light level. Because the ocean is well connected and microbial strains disperse globally, the authors reason that an algal lineage capable of near-theoretical-minimum growth in the Arctic almost certainly has counterparts elsewhere.

The quantitative consequences are striking. Across the BGC-Argo dataset, the one-percent light horizon sits, on average, about 80 meters, or 48 percent, shallower than the depth of the proposed compensation isolume. When the authors integrated chlorophyll and phytoplankton carbon from the surface down to each horizon, the conventional one-percent cutoff missed more than sixty percent of the biomass inventory captured by the deeper boundary: average integrated chlorophyll was 40.9 versus 65.3 milligrams per square meter, and phytoplankton carbon 1.1 versus 1.8 grams per square meter. A substantial reservoir of phytoplankton, in other words, lives below the depth where most studies assume production stops.

That oversight matters for the biological pump, the suite of processes that transports carbon from the surface ocean into the deep sea, where it can remain sequestered for decades to centuries. Many pump estimates treat everything below the one-percent horizon as a realm of pure respiration, assuming any phytoplankton found there will simply sink. If production actually continues deeper, those assumptions inflate the apparent strength of the pump. Similar biases arise in estimates of the mixed-layer pump, the seasonal export of carbon when springtime shallowing of the mixed layer strands biomass in darkening water; using the shallower horizon overstates how much carbon that mechanism delivers to depth.

Numerical ecosystem and biogeochemical models, notably, already sidestep the percentage convention. They instead assign phytoplankton a constant basal respiration rate, typically 0.02 to 0.03 per day, and growth occurs only where photosynthesis exceeds that cost. The authors show, through a standard photosynthesis formulation, that such respiration rates imply a compensation irradiance of roughly 0.036 mol photons per square meter per day, still an order of magnitude above the lowest field estimates. Even models, it seems, do not yet allow production at the dimmest levels where real algae demonstrably persist.

The authors acknowledge open questions. Temperature might, in principle, raise respiration and thus the required light at lower latitudes, but meta-analyses suggest the temperature dependence of algal net growth weakens and effectively vanishes as light becomes limiting, lending support to a single global threshold. Deep mixed layers and internal waves complicate the light environment individual cells experience, and the appropriate isolume for a given question may differ: migrating zooplankton follow far dimmer isolumes, while nutrient and particle-attenuation horizons align with much brighter ones. Still, the recommendation is unambiguous: after nearly a century, oceanographers should retire the one-percent rule for defining the euphotic depth and adopt a physiologically grounded, absolute light threshold, a shift the authors suggest could be implemented with existing satellite products and may ultimately revise how the living ocean is measured from top to bottom.

Subject of Research: Redefining the ocean's euphotic depth using a physiologically based compensation irradiance instead of the one-percent surface light convention

Article Title: Revisiting how the euphotic depth is computed

Article References: Revisiting how the euphotic depth is computed. (n.d.). https://doi.org/10.1371/journal.pesy.0000032

Image Credits: AI Generated

DOI: 10.1371/journal.pesy.0000032

Keywords: euphotic depth, phytoplankton, compensation irradiance, oceanography, primary production, biological pump, BGC-Argo, Prochlorococcus, light attenuation, carbon export, Arctic Ocean, marine ecosystems

The twilight zone, formally known as the mesopelagic zone, occupies the depth range between the base of the euphotic layer and roughly one thousand meters. It is a critical waypoint in the biological carbon pump, the suite of processes by which carbon fixed by photosynthesis in the surface ocean is transported to depth, where a portion of it can be sequestered away from the atmosphere for decades to centuries. Most of that vertical transport happens on aggregates of particulate organic matter, flakes and pellets and colloidal clusters that vary enormously in size, density and composition. Microbes colonize these particles and remineralize their carbon, and viruses, by infecting and lysing those microbes, can accelerate the breakdown and reshape which carbon makes it to the deep ocean. Yet the viral ecology of sinking particles has remained one of the least charted corners of marine microbiology, because particles are hard to sample intact and viruses are hard to assign to hosts.

To overcome these obstacles, the team collected seawater from three oceanic regions and separated the particulate matter into multiple size fractions, then sequenced the metagenomes of samples taken at different depths from the surface down through the twilight zone and into deeper waters. From this material they recovered a total of 5,016 viral operational taxonomic units, or vOTUs, a level of diversity that underscores how rich and poorly described the particle-associated virosphere really is. By analyzing how these viral populations were distributed across particle sizes and water layers, the researchers could ask a question that has rarely been addressed at this scale: do viruses simply float passively in the water, or do they travel with the particles that sink out of the sunlit zone?

The answer turned out to depend strongly on both depth and particle size. Small particles near the surface harbored proportionally more lytic viruses, the kind that replicate aggressively inside a host cell and then burst it open, releasing new virions and dissolved organic matter in the process. Large particles at greater depths, by contrast, were enriched in lysogenic viruses, which integrate their genomes into those of their hosts and ride along with the dividing cells, replicating only when triggered. This pattern fits ecological theory: in productive surface waters, where microbial hosts are abundant and fast-growing, the kill-the-winner dynamics of lytic infection pay off, while in the resource-poor twilight zone, a dormant, integrated lifestyle is a safer bet for a virus whose potential hosts are scarce and scattered.

Host assignment revealed the players behind these interactions. More than 1,000 of the recovered vOTUs could be linked to prokaryotic hosts, and the dominant hosts belonged to the phylum Pseudomonadota, a group that includes many of the most abundant heterotrophic bacteria in the ocean. Using co-occurrence patterns between viral and host genomes, together with metagenome-assembled genomes recovered from the same samples, the researchers built a network of putative virus-host relationships spanning the particle size spectrum. The virus-to-microbe ratios and correlation analyses further suggested that the abundance of hosts shapes not only how many viruses are present but also the balance between lytic and lysogenic strategies in the community.

One of the most striking findings concerns what these viruses can actually do. Metabolic annotation of the viral genomes identified 178 auxiliary metabolic genes, or AMGs, which are host-like genes carried by viruses that can augment the metabolism of the cells they infect. The study also detected 12 families of carbohydrate-active enzymes, known as CAZymes, including glycoside hydrolases, glycosyltransferases and polysaccharide lyases. These enzymes are capable of degrading or remodeling the complex polysaccharides that make up the structural scaffolding of marine particles. Notably, the AMGs and CAZymes were found primarily in lysogenic members of the class Caudoviricetes, the tailed viruses that dominate marine viral communities. In practical terms, this means that viruses integrated into particle-dwelling bacteria may be supplying their hosts with extra tools to break down particulate organic carbon, thereby influencing how fast sinking particles are degraded and how much carbon escapes remineralization on the way down.

The functional complementation implied by these genes has direct consequences for carbon cycling. If a lysogenic virus equips its host with a glycoside hydrolase that can cleave a polysaccharide the host could not otherwise digest, the host gains access to a new energy source, and the particle loses structural material more quickly. Multiply that interaction across thousands of viral types and the countless particles sinking through the water column, and viral genomes emerge as a significant, previously underappreciated lever on the efficiency of the biological carbon pump. The study’s authors frame their results as new insight into the roles of viruses in carbon cycling and vertical connectivity in the ocean, and the metabolic evidence gives that framing concrete biochemical substance.

Equally important is the study’s evidence for connectivity between water layers. Co-occurrence analysis revealed extensive overlap of vOTUs across depths, and marker viruses identified with statistical comparisons of community composition showed that viruses originating in the surface ocean are transported via particles into the twilight zone and beyond, even reaching bathypelagic depths. In other words, the sinking particle highway does not carry carbon alone; it carries an entire mobile virome. Some viral types were significantly enriched at the upper or lower boundaries of the twilight zone in the different study regions, suggesting that particle-associated viruses can serve as biological tracers of vertical export, linking the ecology of the euphotic zone to the microbial processing of carbon far below.

The three-region comparative design strengthens these conclusions. By sampling across distinct oceanographic settings, the researchers could distinguish region-specific patterns from general ones, and the consistency of the size-dependent shift from lytic to lysogenic dominance suggests a robust ecological rule rather than a local curiosity. The use of tools such as vConTACT2 for viral clustering, non-metric multidimensional scaling for community ordination, and STAMP for statistical profiling of marker viruses reflects the maturing analytical toolkit of viral metagenomics, which now allows oceanographers to treat viruses as quantifiable components of biogeochemical models rather than as an unstructured background of biological noise.

The broader implications reach into climate science. Models of the biological carbon pump currently struggle to predict how efficiently carbon is transferred from the surface to the deep ocean, and microbial degradation on sinking particles is one of the largest sources of uncertainty. If particle-associated viruses systematically accelerate the solubilization and remineralization of particulate organic carbon, and if their lytic versus lysogenic balance shifts predictably with depth and particle size, then viral activity belongs explicitly in carbon export models. This study provides the first large-scale, size-resolved baseline for doing exactly that, cataloguing thousands of viral populations, their hosts and their metabolic genes along the particle size spectrum. As sequencing of twilight-zone particles continues, the viral ferry system described here is likely to prove a fundamental feature of ocean biogeochemistry, one that quietly shapes how much carbon the sea sends to the deep and how long it stays there.

Subject of Research: Viral communities associated with particles of different sizes in the ocean twilight zone and their roles in carbon cycling and vertical connectivity

Article Title: Connectivity and potential functions of viral communities along the particle size spectrum in the twilight zone

Article References: Zhang, Y., Wang, P., Liu, H., & Jing, H. (2026). Connectivity and potential functions of viral communities along the particle size spectrum in the twilight zone. BMC Biology. https://doi.org/10.1186/s12915-026-02750-0

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02750-0

Keywords: ocean twilight zone, marine viruses, viral metagenomics, particle-associated viruses, biological carbon pump, auxiliary metabolic genes, carbohydrate-active enzymes, lysogeny, Pseudomonadota, carbon export, vOTUs, vertical connectivity