The deep ocean fixes more inorganic carbon than previously recognized because heterotrophic microbes—alongside ammonia‑oxidizing autotrophs—contribute to carbon fixation. Researchers inhibited ammonia oxidizers with phenylacetylene and found that fixation rates fell much less than expected, revealing a significant heterotrophic role. This result clarifies the deep ocean food web and improves estimates of the ocean’s carbon‑storage capacity, though climate risks such as Southern Ocean heat release remain.
Hidden Deep‑Sea Microbes Are Pulling Carbon From the Atmosphere, Scientists Find
A Creature Is Pulling Carbon From the Atmosphereifish - Getty Images
New research shows that the deep ocean’s capacity to remove carbon from the atmosphere is driven not only by well-known ammonia‑oxidizing autotrophs but also by previously underappreciated heterotrophic microbes. The discovery refines our picture of the deep‑sea food web and improves estimates of how much carbon the ocean can sequester.
Why It Matters
The ocean is humanity’s foremost ally in moderating climate: according to the United Nations, it absorbs more than 30% of global carbon dioxide emissions and takes up roughly 90% of the excess heat generated by those emissions. Most inorganic carbon fixation happens in the sunlit surface layer, where phytoplankton use photosynthesis. But some inorganic carbon is fixed far below the photic zone through non‑photosynthetic processes—mechanisms scientists are still working to fully understand.
What Researchers Knew — And What Didn’t Add Up
For years, researchers assumed archaeal microbes that oxidize ammonia supplied the energy for deep‑sea autotrophic carbon fixation. Yet when teams tried to balance measured carbon fixation against the available chemical energy from ammonia oxidation, the numbers didn’t add up. “We basically couldn’t get the budget to work out for the organisms that are fixing carbon,” said Alyson Santoro, a microbial oceanographer at the University of California Santa Barbara and senior author of the study published in Nature Geoscience.
How the Study Tested the Puzzle
To test whether ammonia oxidizers were the sole contributors to deep‑ocean inorganic carbon fixation, Santoro and colleagues—led by Barbara Bayer at the University of Vienna—used the inhibitor phenylacetylene to suppress ammonia‑oxidizing organisms in seawater samples. If those autotrophs accounted for most fixation, inhibition should produce a dramatic drop in fixation rates.
Surprising Result: Fixation rates declined far less than expected when ammonia oxidizers were inhibited, indicating other microbes are responsible for a sizeable share of deep‑sea inorganic carbon fixation.
Implications
The results suggest that heterotrophic microbes—organisms typically thought to consume organic carbon—are also directly contributing to inorganic carbon fixation in the deep ocean. The team produced the first quantitative estimate of how much deep‑sea carbon fixation can be attributed to heterotrophs versus autotrophs, clarifying the base of the deep ocean food web and improving models of ocean carbon sequestration.
These findings do not remove broader climate risks. A separate study earlier this year warned that even under optimistic, net‑negative emission scenarios, the Southern Ocean could release stored heat at rates that mimic anthropogenic warming for decades or centuries. Such possibilities underscore the urgency of reducing emissions and working toward a carbon‑neutral future.
Study: Santoro, B. Bayer and colleagues. Published in Nature Geoscience.
