Researchers from Germany and Taiwan have traced ancient carbon dioxide venting from the seafloor near Kueishantao Island into the bodies of microbes and crabs, offering one of the clearest pictures yet of how geologic carbon enters a living food web. The findings, published in Communications Earth and Environment, show that CO2 stripped of radiocarbon-14, a signature of carbon thousands of years old, is fixed by bacteria and then consumed by animals in the shallow waters off Taiwan’s northeast coast. The work raises pointed questions about how similar vent systems worldwide may quietly shape local ecosystems and complicate standard models of ocean carbon cycling.
Radiocarbon as a Deep-Carbon Fingerprint
The central method behind the study is deceptively simple: use the absence of radiocarbon-14 as a tracer. Carbon that has been locked underground for millennia loses virtually all of its 14C through radioactive decay. When that carbon re-enters the ocean as CO2 through hydrothermal vents, it carries a distinct isotopic signature that scientists can follow through sediments, water, and tissue samples. A team involving MARUM, National Sun Yat-sen University, and the Alfred Wegener Institute applied this logic at Kueishantao, a small volcanic island where shallow vents release gas dominated by CO2 into surrounding waters.
By measuring compound-specific carbon isotopes in sediments, filtered particulate organic carbon from the water column, and crab tissue, the researchers built a chain of evidence showing that 14C-depleted CO2 is assimilated into microbial biomass and then transferred into vent-endemic fauna. The isotope correlations reported in the paper connect physicochemical conditions at the vents, including pH and temperature, to rates of carbon uptake by chemosynthetic bacteria. That connection matters because it suggests the efficiency of ancient carbon fixation is not random but governed by measurable environmental variables.
Radiocarbon is especially powerful in this setting because other carbon isotopes alone cannot distinguish between recently fixed organic matter and material derived from geologic reservoirs. While ratios of 13C to 12C can hint at different metabolic pathways, only the near-total absence of 14C flags carbon that has been isolated from the atmosphere long enough for radioactive decay to run its course. At Kueishantao, that absence acts as a fingerprint linking the carbon in microbial fatty acids and crab tissues back to the vent fluids themselves.
From Microbes to Vent Crabs
The most striking aspect of the research is how far up the food chain the ancient carbon signal travels. Xenograpsus testudinatus, a crab species found almost exclusively at Kueishantao’s vents, showed clear isotopic evidence of feeding on biomass built from vent-derived carbon. Earlier isotopic work on the same species had already established that crab tissues reliably record diet sources and trophic relationships tied to the vent environment. The new study extends that finding by linking the crab’s carbon signature directly to the ancient, 14C-depleted pool rather than to photosynthetically produced organic matter from the sunlit ocean above.
This distinction is not trivial. Most marine food webs run on recently fixed carbon, produced by phytoplankton using atmospheric CO2. At Kueishantao, a parallel economy exists: bacteria fix carbon that left Earth’s interior long ago, and that carbon sustains an animal population dense enough to dominate the local seafloor. Previous surveys of the site documented abundant chemosynthetic bacteria and a distinct macrobiota community shaped by the extreme chemistry of the vents, including high sulfur concentrations and low pH. The new paper adds a quantitative carbon-flow dimension to that ecological portrait, showing that the same chemistry that makes the site inhospitable to many species provides raw material for microbial primary production.
In practical terms, the researchers traced specific fatty acids that are characteristic of bacterial membranes and measured their radiocarbon content. Because these compounds are synthesized directly by microbes, their 14C-depletion indicates that the microbes are fixing vent CO2 rather than relying on recycled surface-derived organic matter. When similar depleted signatures appear in crab tissues, the simplest explanation is that the crabs are feeding on bacterial mats or on other organisms that graze on those mats, effectively importing deep carbon into higher trophic levels.
Open Data and Independent Verification
One feature that sets this work apart from typical vent studies is the transparency of its underlying data. The team deposited fatty-acid concentrations and isotope values for sediments, water-column particulate organic carbon, and crab tissue in the PANGAEA data repository. A companion dataset provides individual fatty-acid stable isotope measurements and sampling metadata, including event labels, coordinates, and depths for each location. Together, these records allow other researchers to recheck the isotope correlations and microbial indicators that anchor the paper’s conclusions.
That level of openness matters in a field where vent ecosystems are difficult and expensive to sample. Independent teams studying carbon cycling at other shallow or deep-sea vent sites can now compare their own isotope profiles against the Kueishantao baseline without organizing a new expedition. The datasets also provide a reference point for monitoring how the system changes over time, whether because of natural variability in vent output or external disturbances such as earthquakes and typhoons.
Beyond vent research, the Kueishantao records contribute to a broader movement toward open geochemical and ecological data. Public repositories such as NCBI have long supported genomic and microbiological work; extending similar openness to isotope and fatty-acid measurements makes it easier to integrate molecular, geochemical, and ecological perspectives on how extreme environments function.
Why Shallow Vents Deserve More Attention
Most public attention to hydrothermal vents focuses on the deep ocean, where black smokers tower above mid-ocean ridges at depths of two kilometers or more. Kueishantao’s vents sit in shallow water, making them far more accessible to researchers but also more directly connected to coastal ecosystems. The gas pouring from these vents is dominated by CO2, and the surrounding waters are naturally acidified, creating conditions that resemble projections for broader ocean chemistry under continued fossil-fuel emissions.
That resemblance has led some scientists to treat shallow vent fields as natural laboratories for ocean acidification research. But the new study complicates that framing. If ancient, geologically sourced carbon is being actively fixed and cycled through local food webs, then the biology at these sites is not simply tolerating low pH. It is exploiting it. Chemosynthetic bacteria at Kueishantao use the high CO2 concentrations and associated chemical gradients as an energy and carbon source, turning what would otherwise be a stressor into a foundation for productivity.
This perspective has implications for how researchers interpret experiments and observations at naturally acidified sites. Organisms living near vents may be adapted not only to low pH but also to unusual food sources and redox conditions. As a result, their responses to acidification cannot be assumed to mirror those of species in more typical coastal settings. Recognizing the role of deep carbon inputs helps separate the effects of chemistry alone from the broader ecological context in which that chemistry operates.
Rethinking Carbon Budgets and Ecosystem Boundaries
The Kueishantao findings also feed into a larger discussion about Earth’s carbon budget. Standard models of the global carbon cycle distinguish between fast, biologically mediated exchanges among the atmosphere, surface ocean, and biosphere, and slow, geologic exchanges involving volcanic degassing and rock weathering. Shallow hydrothermal systems blur that boundary. When vent-derived CO2 is rapidly incorporated into microbial biomass and passed to animals, geologic carbon briefly joins the fast cycle before eventually returning to the ocean-atmosphere pool through respiration and decomposition.
On a global scale, the amount of carbon processed in this way at any single vent field is small compared with human emissions or large-scale ocean uptake. Yet the study shows that the local consequences can be substantial. At Kueishantao, deep carbon supports a dense, specialized community that would not exist without the vents. Similar systems along volcanic arcs, back-arc basins, and coastal fault zones may host their own pockets of geologically fueled productivity, each with distinct ecological and biogeochemical signatures.
For scientists working to predict how coastal ecosystems will respond to climate change, these findings are a reminder that not all carbon sources are created equal. Ancient CO2 released from the seafloor can alter local food webs, water chemistry, and even sediment properties in ways that differ from atmospheric CO2 dissolving into the ocean. Incorporating such nuances into regional carbon budgets and ecosystem models will require more detailed mapping of shallow vent fields and more studies that link geologic processes to biological outcomes.
Ultimately, the work at Kueishantao underscores how tightly intertwined Earth’s interior and surface environments can be. By following a radiocarbon fingerprint from bubbling vents through microbial membranes and into the muscle of a crab, the researchers have illuminated one pathway by which deep time intersects with present-day life, a reminder that even in a rapidly changing ocean, some of the carbon sustaining modern ecosystems has been on a much longer journey.
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*This article was researched with the help of AI, with human editors creating the final content.
