How is carbon metabolized and processed in different ecosystems? In a recent study published in the journal Communications Earth & Environment, researchers led by Joely Maak, the study’s first author and researcher in the Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface”, examined the carbon cycle in a unique marine ecosystem.
Hydrothermal vents on the ocean floor release carbon dioxide that is many million years old. This old carbon originates from the Earth's interior, and it escapes either directly from the Earth's mantle or is produced when rocks that contain limestone or other carbonate minerals are heated or transformed in geologically active zones. These processes primarily occur where tectonic plates converge or diverge, and hot, rising material heats the sea floor. However, the fate of this carbon after it enters the sea has so far been largely unclear.
The Path of Hydrothermal Carbon
In a new study, researchers from MARUM – Center for Marine Environmental Sciences at the University of Bremen, and the National Sun Yat-Sen University, and the Exploration and Development Research institutes in Taiwan investigated a hydrothermal vent system at a depth of about ten meters off the coast of Kueishantao island in Taiwan. They tracked the path of this carbon in the surrounding sea and its uptake by microorganisms and other living things.
“We were able to show that millennia-old carbon from hydrothermal vents can power life in these extreme systems,” says Joely Maak, the study's lead author and researcher at MARUM.
The team used a special isotope in this study: radiocarbon (14C). This radioactive isotope is created in the Earth's upper atmosphere by cosmic radiation. The resulting 14C then enters the natural carbon cycle as carbon dioxide and is absorbed by plants, microorganisms, and, ultimately, animals. As long as an organism is alive, the proportion of 14C remains almost constant. However, once an organism dies and is no longer exchanging carbon with the atmosphere, the 14C gradually decays, and after several tens of thousands of years, it becomes virtually undetectable. Carbon from the Earth's interior is extremely old and has been separated from the atmosphere for a very long time, and therefore no longer contains 14C.
When this carbon enters the ocean through hydrothermal emissions, its signature differs significantly from that of modern atmospheric carbon. In fact, it is “radiocarbon-dead.” For the current study, the researchers are using this exact difference to trace the path of hydrothermal carbon through the marine ecosystem.
“Our approach was to use the old, 14C-free carbon from hydrothermal sources as a natural marker. We were surprised by how clearly the fingerprint could be traced through the entire food web, even into higher organisms,” says Dr. Hendrik Grotheer, geochemist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research.
Efficient Metabolic Pathway Shapes Entire Food Webs
The team’s previous work has shown that specialized bacteria at these vents have a special “secret weapon,” namely, the reductive tricarboxylic acid (rTCA) cycle. This energy-efficient metabolic pathway enables microorganisms to incorporate carbon dioxide into their biomass even under extreme conditions. Building on these results, the new study now shows that the carbon from the hydrothermal vents actually accounts for up to 30 percent of the bacteria’s biomass in the hydrothermal system and is passed on in the local food web. Crabs living directly at the hydrothermal vents also contain this ancient carbon because they feed on the microbes living in the hydrothermal system. Consequently, their body tissue appears measurably older than it actually is.
Project manager Dr. Enno Schefuß of MARUM explains, “Only by combining the study of specific bacterial markers (so-called ‘fatty acids’) and radiocarbon analyses of these, we could obtain these new findings – a combination of state-of-the-art technology and meticulous laboratory work.”
Photosynthesis Also Uses Hydrothermal Carbon
Using additional hydrogen isotopes, the researchers were also able to determine whether the carbon was assimilated via chemosynthesis or by photosynthesis. Unlike photosynthesis, in which plants use sunlight to generate energy, chemosynthesis works completely without sunlight. In this process, microorganisms use reduced chemicals from the Earth's interior to generate energy. Until now, it had never specifically been demonstrated that photosynthesis plays a role in the uptake of old carbon from hydrothermal systems. The current study was able to show, by using several isotope systems, that hydrothermal carbon is assimilated by photosynthesizing organisms further away from the vent.
“At the same time, the results show that despite the various assimilation mechanisms, only a small proportion of the total carbon released actually remains in the local ecosystem. The majority of the CO2 escapes direct biological use and is distributed into the ocean with the surrounding water masses or escapes into the atmosphere,” adds first author Joely Maak. “On the other hand, the release of components not covered in this study, such as dissolved organic carbon and micronutrients from marine hydrothermal vents, may influence the biogeochemistry of the oceans. This will be investigated in more detail in several projects in the second phase of the Cluster of Excellence, which has just been launched," notes co-project leader Dr. Marcus Elvert from MARUM.
International Collaboration for Successful Exploration of Hidden Ocean Processes
The study emphasizes the importance of long-term international cooperation between Taiwan and Bremen, demonstrating how modern isotope methods can help to reveal previously hidden biogeochemical processes in the sea. “This study shows the importance of long-term international cooperation for understanding complex oceanic processes,” says Dr. Solveig Bühring. “My Taiwanese project partner, Prof. Yu-Shih Lin, and I led the fieldwork. This collaboration began with a jointly funded DAAD scholarship and has evolved into an extremely successful research partnership. I very much hope that we can continue this close exchange in the future and jointly gain further insights into the role of hydrothermal systems in the global carbon cycle.”
The study receives additional funding from and is an integral part of research in the Cluster of Excellence “The Ocean Floor – Earth's Uncharted Interface.” The cluster aims to better understand ocean floor ecosystems under changing environmental conditions, as well as central material cycles, such as the carbon cycle.
Original publication
Joely Marie Maak, Marcus Elvert, Hendrik Grotheer, Yu-Shih Lin, Gesine Mollenhauer, In-Tian Lin, Solveig I. Bühring & Enno Schefuß: Physicochemical controls on ancient carbon assimilation into ecosystem biomass in shallow-water hydrothermal Systems. Commun Earth Environ (2026). DOI: 10.1038/s43247-026-03254-z
