The deep ocean is fixing carbon in ways no one expected

"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 said. "The numbers work out now, which is great."

This project was supported in part by the National Science Foundation.

The ocean as a planetary carbon sink

Who's doing the fixing? The ocean is the Earth's largest carbon sink, soaking up roughly a third of human carbon dioxide emissions and helping to keep global temperatures in check. Because we rely so heavily on this natural buffering capacity, scientists are keen to untangle the complex processes that control how carbon enters, moves through, and is stored 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 said.

Much of this inorganic carbon fixation is carried out by microscopic life. At the surface, phytoplankton, which are single-celled, photosynthetic organisms, take up inorganic carbon dioxide (including dissolved carbon dioxide gas). As autotrophs, they manufacture their own food in a way similar to land plants, using carbon dioxide and water to build organic matter (sugars) and release oxygen.

Old assumptions about deep-ocean microbes

Scientists have generally believed that most DIC fixation occurs in the sunlit surface layer thanks to photosynthetic phytoplankton, but that a meaningful amount of non-photosynthetic DIC fixation also takes place in the deeper, darker regions of the ocean. In these sunless waters, the process was thought to be dominated by autotrophic archaea that oxidize ammonia (a nitrogen-containing compound) for energy instead of using sunlight.

However, when researchers examined the nitrogen-based energy budget of these carbon-fixing microbes by sampling the water column, they soon realized that the math did not work out.

"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 said. "We basically couldn't get the budget to work out for the organisms that are fixing carbon." The microbes require energy to fix carbon, she explained, but there did not appear to be enough nitrogen-derived energy in the deep ocean to support the high carbon fixation rates that were being reported throughout the water column.

A decade-long carbon cycle mystery

This mismatch has occupied the attention of Santoro and the paper's lead author Barbara Bayer for nearly ten years as they have sought to close a key gap in our understanding of the ocean's carbon cycle. Earlier studies tested the idea that perhaps the carbon-fixing archaea were far more efficient than scientists assumed, needing less nitrogen to fix the same amount of carbon. Their work, however, showed that this explanation did not hold up.

For the new study, the researchers shifted their focus and asked a different question: How much do these ammonia oxidizers actually contribute to the overall dissolved inorganic carbon fixation in the dark ocean? To answer that, Bayer designed a targeted experiment.

"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. The inhibitor, phenylacetylene, was confirmed to have no other measurable effects on other community processes.

Their results indicated that despite inhibiting these ammonia oxidizers -- mostly archaea that are abundant in the dark ocean -- the rate of carbon fixation in the study areas didn't drop as much as expected.

New suspects in deep-sea carbon fixation

If ammonia-oxidizing archaea are not responsible for as much carbon fixation as once believed, other microbes must be stepping in. The pool of likely contributors now includes additional types of 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, "meaning that they're also responsible for fixing some carbon dioxide.

"And that's really interesting because even though we know this to be a theoretical possibility, we didn't really have a quantitative number on what fraction of the carbon in the deep ocean was getting fixed by these heterotrophs versus autotrophs. And now we do."

Rethinking the deep-ocean food web

The new findings do more than clarify who is fixing carbon at depth. They also provide fresh insight into how the deep ocean's food web is structured and sustained.

"There are basic aspects of how the food web works in the deep ocean that we don't understand," Santoro said, "and 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

Further work in this realm for Santoro and her collaborators will dive into the finer aspects of carbon fixation in the ocean, such as how the nitrogen cycle and carbon cycle interact with other elemental cycles in the ocean, including for 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?" she 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?"

Research in this paper was also conducted by Nicola L. Paul, Justine B. Albers and Craig A. Carlson at UCSB; Katharina Kitzinger and Michael Wagner at the University of Vienna as well as Mak A. Saito at Woods Hole Oceanographic Institution.