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Here’s what you’ll learn when you read this story:
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Although most dissolved inorganic carbon fixation occurs at the ocean’s surface, some of that fixation budget originates from the deep ocean.
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In a new study, scientists suggest that microbial heterotrophs are aiding ammonia-oxidizing autotrophs in fixing inorganic carbon in the deep ocean.
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This helps us better understand both the deep ocean food web and our ocean’s carbon-storing abilities.
When it comes to cooling our steadily warming planet, the name of the game is pulling as much carbon from the atmosphere as possible—and when it comes to playing this carbon-fixing game, the ocean is the undisputed GOAT. According to the United Nations, the ocean soaks up more than 30 percent of all carbon dioxide emissions and a remarkable 90 percent of the excess heat generated by those emissions. When it comes to the fight against climate change, humanity has no greater ally.
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However, scientists are still pondering the various mechanisms that the deep ocean uses to fix that carbon. Most inorganic carbon fixing takes place at sea level, thanks to surface-dwelling phytoplankton who—like other land-based autotrophs (i.e. an organism that makes its own food)—produce organic sugars and oxygen in exchange for carbon dioxide. However, some non-photosynthetic fixation also occurs in the deeper layers of the ocean. The prevailing theory has long been that archaea capable of oxidizing ammonia for energy continued this carbon-fixing work without the need for sunlight. Yet, when you crunch the numbers, something didn’t add up.
“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,” Alyson Santoro, a microbial oceanographer from the University of California Santa Barbara (UCSB) and senior author of a new paper in Nature Geoscience investigating this discrepancy, said in a press statement. “We basically couldn’t get the budget to work out for the organisms that are fixing carbon.”
In other words, these deep ocean autotrophs needed nitrogen-based energy to fix carbon, but the data showed that there simply wasn’t enough energy to go around for ammonia-oxidizing autotrophs to be the sole source of fixation. Instead of investigating whether these autotrophs were more efficient at this process than we realized (spoiler, they’re not), the researchers asked a different question: Were ammonia oxidizers the only organisms capable of fixing carbon?
To answer that question, Santoro and her team—including lead author Barbara Bayer from the University of Vienna—inhibited these oxidizers with the chemical phenylacetylene. If these autotrophs were a major source of fixation, then those rates should drop drastically. However, the rates didn’t drop nearly as much as would be expected, leading the researchers to consider that microbial heterotrophs in the deep ocean must be playing a role.
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“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,” Santaro said in a press statement. “I think of this as figuring out how the very base of the food web in the deep ocean works.”
While the ocean is soaking up as much carbon as possible, there’s no such thing as a free lunch. A study earlier this year reported that even under ideal climate scenarios (where humanity reaches a net-negative carbon future), the Southern Ocean will likely “burp up” excess heat at levels that mimic anthropomorphic climate change for decades—or even centuries. Although this oceanic indigestion could be in our future, it’s as vital as ever to strive toward a carbon-neutral reality to limit the very worst of these climatic outcomes.
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