In a groundbreaking study, researchers from UC Santa Barbara and their collaborators have unveiled findings that challenge long-standing assumptions about carbon fixation in the deep ocean. Led by microbial oceanographer Alyson Santoro, the team published their results in Nature Geoscience, revealing insights that help bridge a significant gap between nitrogen availability estimates and measurements of dissolved inorganic carbon (DIC) fixation in deep waters.
“Something that we’ve been trying to get a better handle on is how much of the carbon in the ocean is getting fixed,” Santoro stated. “The numbers work out now, which is great.” This research was partially funded by the National Science Foundation.
The Ocean as a Planetary Carbon Sink
The ocean is Earth’s largest carbon sink, absorbing approximately one-third of human carbon dioxide emissions and playing a crucial role in regulating global temperatures. Given our reliance on this natural buffer, scientists are eager to understand the complex processes governing carbon’s entry, movement, and storage in the sea.
“We want to know how carbon moves around the deep ocean because, in order for the ocean to impact the climate, carbon has to make it from the atmosphere to the deep ocean,” Santoro explained. Microscopic life plays a significant role in this process. At the ocean’s surface, phytoplankton—a type of single-celled, photosynthetic organism—takes up inorganic carbon dioxide, using it to create organic matter and release oxygen.
Old Assumptions about Deep-Ocean Microbes
Traditionally, scientists believed that most DIC fixation occurred in the sunlit surface layer due to photosynthetic phytoplankton. However, a substantial amount of non-photosynthetic DIC fixation was also thought to happen in the ocean’s deeper, darker regions, primarily driven by autotrophic archaea that oxidize ammonia for energy.
Upon examining the nitrogen-based energy budget of these carbon-fixing microbes, researchers discovered a discrepancy. “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 noted. The microbes require energy to fix carbon, but there seemed to be insufficient nitrogen-derived energy in the deep ocean to support the high carbon fixation rates reported.
A Decade-Long Carbon Cycle Mystery
This mismatch has puzzled Santoro and lead author Barbara Bayer for nearly a decade as they sought to close a crucial gap in understanding the ocean’s carbon cycle. Previous studies tested whether carbon-fixing archaea might be more efficient than assumed, requiring less nitrogen to fix the same amount of carbon. However, this hypothesis did not hold up.
In their new study, the researchers shifted their focus, asking how much ammonia oxidizers actually contribute to overall DIC fixation in the dark ocean. Bayer designed a targeted experiment to address this question.
“She came up with a way to specifically inhibit their activity in the deep ocean,” Santoro explained. By limiting the activity of these oxidizers with a specialized chemical, the team expected to see a sharp drop in carbon fixation.
Despite inhibiting these ammonia oxidizers, the rate of carbon fixation in the study areas did not decrease as much as anticipated.
New Suspects in Deep-Sea Carbon Fixation
If ammonia-oxidizing archaea are not as responsible for carbon fixation as previously thought, other microbes must be involved. The potential contributors include various microbes in the surrounding community, particularly bacteria and some archaea.
“We think that what this means is that the heterotrophs—microorganisms that feed on organic carbon from decomposing microbes and other marine life—are taking up a lot of inorganic carbon in addition to the organic carbon that they usually consume,” Santoro said. “This means they’re also responsible for fixing some carbon dioxide.”
This finding is significant because it provides a quantitative understanding of the fraction of carbon in the deep ocean fixed by heterotrophs versus autotrophs, which was previously unknown.
Rethinking the Deep-Ocean Food Web
These new insights not only clarify who is fixing carbon at depth but also offer a fresh perspective on the deep ocean’s food web structure and sustainability.
“There are basic aspects of how the food web works in the deep ocean that we don’t understand,” Santoro said. “I think of this as figuring out how the very base of the food web in the deep ocean works.”
More Mysteries of the Deep
Future research by Santoro and her collaborators will delve deeper into the intricacies of carbon fixation in the ocean, including interactions between the nitrogen cycle, carbon cycle, and other elemental cycles like iron and copper.
“The other thing we’re trying to figure out is once these organisms fix the carbon into their cells, how does it become available to the rest of the food web?” Santoro noted. “What kinds of organic compounds might they be leaking out of their cells that could be feeding the rest of the food web with?”
This research also involved contributions from Nicola L. Paul, Justine B. Albers, and Craig A. Carlson at UCSB; Katharina Kitzinger and Michael Wagner at the University of Vienna; and Mak A. Saito at Woods Hole Oceanographic Institution.
