Researchers have discovered that heterotrophic microbes play a larger role in fixing carbon in the deep ocean than previously thought, challenging long-held assumptions. Led by Alyson Santoro at UC Santa Barbara, the study reveals that ammonia-oxidizing archaea contribute less than expected to this process. The findings, published in Nature Geoscience, help explain discrepancies in carbon and nitrogen cycles in dark ocean waters.
The ocean serves as Earth's primary carbon sink, absorbing about a third of human-generated carbon dioxide emissions to mitigate global warming. Scientists have long puzzled over how inorganic carbon is fixed in the sunless depths, where photosynthesis cannot occur. Traditionally, experts believed that autotrophic archaea, which oxidize ammonia for energy, dominated this non-photosynthetic carbon fixation.
However, measurements of carbon fixation rates in deep waters exceeded what available nitrogen energy could support, creating a mismatch in the microbial energy budget. This decade-long mystery prompted Alyson Santoro and her team, including lead author Barbara Bayer, to investigate further. They conducted experiments in the deep ocean, using the inhibitor phenylacetylene to specifically block ammonia oxidizers without affecting other microbial processes.
Surprisingly, carbon fixation rates did not decline as anticipated after inhibiting these archaea. "There was a discrepancy between what people would measure when they went out on a ship to measure carbon fixation and what was understood to be the energy sources for microbes," Santoro explained. The results indicate that other microbes, particularly heterotrophs that consume organic matter from decaying organisms, are responsible for a significant portion of inorganic carbon uptake.
"We think that what this means is that the heterotrophs are taking up a lot of inorganic carbon in addition to the organic carbon that they usually consume," Santoro said. This shifts understanding of the deep ocean food web's base, where these microbes incorporate carbon dioxide into their cells, potentially leaking organic compounds to sustain broader ecosystems.
The study closes a gap between nitrogen availability and dissolved inorganic carbon fixation estimates. "The numbers work out now, which is great," Santoro noted. Future research will explore interactions with other elemental cycles, like iron and copper, and how fixed carbon enters the food web. Collaborators included researchers from the University of Vienna and Woods Hole Oceanographic Institution.