How the ocean stores carbon In an effort to higher understand how the ocean stores carbon, UC Santa Barbara researchers and their colleagues challenge long-held ideas about how carbon dioxide is “fixed” in the dead of night, deep ocean. Led by UCSB microbial oceanographer Alison Santoro, the team reports that their work helps close a longstanding gap between estimates of nitrogen availability and measurements of dissolved inorganic carbon (DIC) fixation in deep waters.
“What we’re trying to get a better handle on is how much carbon is being fixed in the ocean,” Santoro said. “The numbers work now, which is great.”
This project was supported partly by the National Science Foundation.
The ocean as a planetary carbon sink
Who is doing the fixing? The ocean is Earth’s largest carbon sink, absorbing a few third of human carbon dioxide emissions and helping to keep up global temperatures. Because we rely so heavily on this natural buffering capability, scientists are keen to unravel the complex processes that control how carbon enters, moves through, and is stored within the ocean.
“We want to know how carbon moves around the deep ocean, because for the ocean to influence climate, carbon has to make it from the atmosphere to the deep ocean,” Santoro said.
Most of this inorganic carbon fixing is finished by microscopic life. At the surface, phytoplankton, that are single-celled, photosynthetic organisms, take up inorganic carbon dioxide (including dissolved carbon dioxide gas). As autotrophs, they make their very own food like land plants, using carbon dioxide and water to construct organic matter (sugars) and release oxygen.
Old hypotheses about deep-sea microbes
Scientists have generally assumed that almost all DIC fixation occurs within the surface of the Sun due to photosynthetic phytoplankton, but that a major amount of non-photosynthetic DIC fixation also occurs in deep, deep ocean regions. In these sunny waters, the method was regarded as dominated by autotrophic archaea that oxidize ammonia (a nitrogen-containing compound) for energy as a substitute of using sunlight.
However, when the researchers examined the nitrogen-based energy budget of those carbon-fixing microbes by sampling the water column, they soon realized that the maths didn’t work.
“When people set out to measure carbon fixation, there was a discrepancy between what people were measuring and what they thought were energy sources for microbes,” Santoro said. “We basically couldn’t get the budget to work for the organisms that are fixing the carbon.” Microbes need energy to repair carbon, however the deep ocean doesn’t appear to get enough energy from nitrogen to be reported within the water column, he explained.
A decade-long carbon cycle mystery
That similarity has preoccupied Santoro and the paper’s lead creator Barbara Bayer for nearly ten years as they sought to shut a critical gap in our understanding of the ocean’s carbon cycle. Earlier studies tested the concept carbon-fixing archaea might need been more efficient than scientists thought, requiring less nitrogen to repair the identical amount of carbon. However, their work showed that this explanation didn’t hold.
For the brand new study, the researchers shifted their focus and asked a distinct query: How necessary do these ammonia oxidizers really contribute to total dissolved inorganic carbon fixation within the Dark Ocean? To answer this, Bayer developed a goal experiment.
“She came up with a way to specifically stop her activity in the deep sea,” Santoro explained. By limiting the activity of those oxidizers with a special chemical, the team expected carbon fixation to diminish rapidly. The inhibitor, phenylacetylene, was confirmed to don’t have any other measurable effects on other social processes.
Their results indicated that despite blocking these ammonia oxidizers—mostly archaea which can be abundant in the dead of night ocean—carbon-fixing rates within the study areas didn’t drop as expected.
New suspects in deep-sea carbon fixation
If ammonia-oxidizing bacteria usually are not accountable for as much carbon fixation as once believed, other microbes must step in. The pool of potential contributors now includes additional sorts of microbes in the encompassing community, particularly bacteria and a few archaea.
“We think this means that heterotrophs—microorganisms that feed on organic carbon from decomposing microbes and other marine life—are taking in a lot more inorganic carbon in addition to the organic carbon they normally use,” meaning they’re also accountable for fixing among the carbon dioxide.
“And it’s really interesting because even though we know it’s a theoretical possibility, we didn’t really have a quantitative number of what fraction of carbon in the deep ocean was being fixed by these heterotrophs versus autotrophs. And now we do.”
Rethinking the deep marine food web
The latest findings do greater than make clear who’s fixing carbon at depth. They also provide fresh insights into how deep ocean food webs are formed and maintained.
“There are fundamental aspects of how the food web works in the deep sea that we don’t understand,” Santoro said, “and I think about figuring out how the base of the food web works in the deep sea.” “
More Mysteries of the Deep
Further work on this area for Santoro and his colleagues will delve into finer facets of carbon fixation within the ocean, equivalent to how the nitrogen cycle and the carbon cycle interact with other elemental cycles within the ocean, including those for iron and copper.
“The other thing we’re trying to figure out is once these organisms fix carbon in their cells, how does it become available to the rest of the food web?” He noted. “What kinds of organic compounds are they releasing from their cells that can feed the rest of the food web?”
The paper was also conducted by Nicola L. Paul, Justin B. Albers, and Craig A. Carlson at UCSB. Katharina Katzinger and Michael Wagner on the University of Vienna, in addition to Mac A. Seto on the Woods Hole Oceanographic Institution.