By Abbas Nazil
Researchers have discovered that the deep ocean fixes carbon in ways far different from long-held assumptions, revealing the critical role of previously overlooked microbes in regulating Earth’s climate.
The study, conducted by the University of California, Santa Barbara, and collaborators, found that ammonia-oxidizing archaea, long believed to dominate carbon fixation in the dark ocean, contribute far less than expected.
Experiments showed that other microorganisms, particularly heterotrophic bacteria and some archaea, play a much larger role in capturing inorganic carbon, reshaping the understanding of carbon movement through the ocean’s depths.
Lead researcher Alyson Santoro explained that for nearly a decade, scientists struggled to reconcile measurements of dissolved inorganic carbon fixation with estimates of nitrogen availability.
Previous assumptions suggested that ammonia-oxidizing archaea could account for high carbon fixation rates using nitrogen-based energy, but experiments with phenylacetylene, a chemical that selectively inhibited these archaea, showed only minimal reductions in carbon fixation.
The finding indicated that other members of the microbial community are actively taking up carbon dioxide, demonstrating that heterotrophs, which usually consume organic carbon from decaying organisms, are also fixing inorganic carbon.
The results provide quantitative evidence of the fraction of carbon in the deep ocean processed by heterotrophs versus autotrophs, a detail that was previously unknown.
The study not only clarifies the microbial contributors to deep-ocean carbon fixation but also offers new insight into the structure and dynamics of the deep-sea food web, revealing how carbon flows at the base of this ecosystem.
Researchers plan to further investigate the interaction between carbon and nitrogen cycles and the role of other elemental cycles, including iron and copper, in supporting microbial life at depth.
The team also aims to determine how fixed carbon becomes available to other organisms and what types of organic compounds are released to sustain the wider food web.
This research, published in *Nature Geoscience*, highlights the importance of understanding microbial processes in the ocean, which serve as a major planetary carbon sink and play a critical role in stabilizing global climate.
Collaborators on the study included Barbara Bayer, Katharina Kitzinger, Nicola Paul, Justine Albers, Mak Saito, Michael Wagner, and Craig Carlson, who collectively advanced knowledge of deep-ocean biogeochemistry and its implications for climate regulation.
