Hydrothermal vents, known for their extreme conditions on the ocean floor, release carbon dioxide that is remarkably ancient—millions of years old. This carbon originates deep within the Earth’s mantle, seeping out through geologically active regions where tectonic plates converge or diverge. When rocks containing calcium carbonate minerals like limestone are subjected to intense heat, they undergo transformations that liberate this primordial carbon. Yet, despite its significant presence, the interaction of this ancient carbon with marine ecosystems has remained largely enigmatic until recently.
Scientists from the MARUM Center for Marine Environmental Sciences at the University of Bremen, alongside collaborators from Taiwan’s National Sun Yat-Sen University and regional research institutions, embarked on an ambitious study to track the journey of hydrothermally sourced carbon. Their fieldwork centered on shallow hydrothermal vents situated just ten meters below the surface near Kueishantao Island, Taiwan. By combining cutting-edge isotope tracing techniques and detailed biochemical analyses, the team revealed that this ancient carbon reservoir directly fuels life in these extreme subsea environments, overturning previous assumptions about oceanic carbon cycling.
Central to their approach was the use of radiocarbon (¹⁴C), a radioactive isotope generated by cosmic rays in the Earth’s upper atmosphere. Freshly formed ¹⁴C integrates into the biosphere via photosynthesis and microbial uptake, maintaining a measurable presence in living organisms. However, once an organism dies, this isotope decays with a half-life of approximately 5,730 years, rendering carbon older than tens of thousands of years effectively devoid of ¹⁴C. The carbon emitted by hydrothermal vents, sourced from the Earth’s interior and isolated from atmospheric exchange over geological timescales, is entirely radiocarbon-dead. This contrast provides a natural isotopic fingerprint, enabling researchers to differentiate ancient carbon from modern organic material.
By tracing the absence of ¹⁴C, the researchers could map how hydrothermal carbon permeates the ecosystem around the vents. They demonstrated that the microbes inhabiting these hydrothermal environments incorporate up to 30% of their biomass from this ancient carbon source. These microorganisms leverage an unusual and highly efficient metabolic mechanism known as the reductive tricarboxylic acid (rTCA) cycle. Unlike conventional photosynthesis, the rTCA cycle allows bacteria to fix carbon dioxide without sunlight, capitalizing on the chemical energy yielded by reduced compounds from Earth’s interior. This metabolic innovation grants these bacteria a competitive advantage under the chemically harsh and light-deprived conditions near hydrothermal systems.
Remarkably, the uptake of hydrothermal carbon extends beyond microbial life and impacts higher trophic levels. Crabs residing directly atop the venting structures have been found to harbor this ancient carbon within their tissues, a consequence of feeding on carbon-fixing microbes. This trophic transfer means that the body carbon of these vent fauna appears anomalously old when dated radiometrically, providing compelling evidence for the deep integration of hydrothermal carbon into local food webs. The findings highlight the profound ecological importance of geologically sourced carbon that has long been overlooked.
The investigation further differentiated between carbon assimilation via chemosynthesis and photosynthesis within the vicinity of these hydrothermal vents. Chemosynthesis enables organisms to produce biomass using chemical energy derived from the oxidation of inorganic molecules, independent of sunlight. By employing hydrogen isotope analysis in concert with radiocarbon measurements, the team discovered that photosynthetic organisms situated farther from the vent also assimilate hydrothermal carbon. This was an unexpected revelation, indicating that the influence of vent-derived carbon extends into the photic zone and becomes intertwined with sunlight-driven biological processes.
Despite these assimilation pathways, the study underscored that only a fraction of the carbon dioxide released by hydrothermal venting is retained and biologically utilized within the local ecosystem. The bulk of this ancient carbon escapes biological consumption, dispersing into surrounding oceanic water masses and even bubbling into the atmosphere. However, the vent emissions comprise not only carbon dioxide but also dissolved organic carbon and a suite of micronutrients and trace elements, which might subtly modulate oceanic biogeochemical cycles at broader scales. Ongoing research efforts aim to elucidate how these additional components influence marine productivity and elemental cycling.
This research project exemplifies the benefits of sustained international scientific collaboration. The partnership between German and Taiwanese institutions facilitated a rigorous combined approach, leveraging expertise in isotope geochemistry, microbial ecology, and oceanography. Such collaborations are critical for unraveling complex environmental phenomena and expanding scientific frontiers in ocean systems. The team credits the cooperative ethos, shared resources, and cross-disciplinary dialogue for their success in illuminating hidden processes governing carbon flow in shallow hydrothermal realms.
Furthermore, this study emphasizes advanced isotope analytical techniques as indispensable tools to explore biogeochemical interactions that remain otherwise cryptic. The fusion of radiocarbon fingerprinting with lipid biomarker analyses allowed the researchers to detect carbon assimilation at molecular and ecosystem scales with unprecedented clarity. This reinforces the transformative potential of isotope geochemistry in studying Earth’s carbon reservoirs, particularly within extreme and transitional environments such as hydrothermal fields where conventional observations are challenging.
Positioned within the broader objectives of the Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface,” this investigation contributes vital insights into ocean floor ecosystems’ dynamics under environmental variability. It sheds light on how geological carbon fluxes interact with biological communities, influencing ocean chemistry and potentially climate regulation. MARUM’s commitment to interdisciplinary and transparent research ensures that such knowledge not only advances academia but also informs societal understanding and marine conservation strategies aligned with global sustainability agendas.
In conclusion, the study fundamentally alters how scientists view the role of ancient carbon released by marine hydrothermal vents. It reveals that millennia-old carbon is not inert but actively integrated into local food webs via specialized microbial metabolisms and further propagated into higher organisms. This discovery broadens our understanding of marine carbon cycling, showing that Earth’s interior processes contribute directly to sustaining life in otherwise inhospitable environments. As isotope technologies continue to evolve, we anticipate even deeper explorations into how hidden planetary mechanisms shape the biosphere’s composition and functioning.
Subject of Research: Assimilation of ancient hydrothermal carbon into marine ecosystem biomass and its biogeochemical cycling.
Article Title: Physicochemical controls on ancient carbon assimilation into ecosystem biomass in shallow-water hydrothermal systems
News Publication Date: 2-Feb-2026
Web References: 10.1038/s43247-026-03254-Z
Image Credits: White water of Kueishantao: Sulfur-containing hydrothermal fluids make the sea appear milky. Photo: MARUM – Center for Marine Environmental Sciences, University of Bremen; S. Bühring
Keywords: hydrothermal vents, ancient carbon, radiocarbon dating, marine ecosystems, microbial metabolism, reductive tricarboxylic acid cycle, chemosynthesis, photosynthesis, isotope geochemistry, biogeochemical cycles, Kueishantao, carbon cycling, oceanography
