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When “marine snow” made of dead plankton’s shells, fish poop, dust particles, and other debris descends to the ocean floor, it carries atmospheric carbon the plankton used to make their calcite shells. It’s one of the ways the ocean stores carbon, helping to keep greenhouse gases from turning the planet into an oversize toaster oven. Yet scientists realized that something has been dissolving those calcite shells and releasing carbon dioxide, reducing the ocean’s carbon-trapping capacity. A study published in the Proceedings of the National Academy of Sciences USA identified the culprit: dense microbe “cities” living inside the marine snow.

The individual cities are microscopic, but collectively they have powerful effects on Earth’s climate because the ocean is home to an inconceivable number of microbes. A shot glass full of seawater can contain millions of bacterial cells. “If you were to take every bacterial cell in the ocean and string them end to end like a chain of pearls, it would stretch 50 times around the Milky Way,” says study co-author Andrew Babbin, an oceanographer at the Massachusetts Institute of Technology.

To study the microbial cities, “we brought the ocean into the laboratory,” says Benedict Borer, lead study author and a biogeochemist at Rutgers University. The scientists introduced microbes to a microfluidic chip designed to mimic marine-snow particles and added fluorescent molecules whose glow changed with oxygen levels and acidity. (The system was so sensitive that at first, people breathing in the lab were affecting measurements.)

The researchers found that the cities’ chemical microenvironments increase calcite dissolution. Many oxygen-breathing microbes feed on carbon, then release carbon dioxide, which turns into carbonic acid in seawater. The sheer number of microbes breathing in such tight quarters creates concentrated pockets of carbonic acid in and around the marine-snow particles, which dissolve the snow’s calcite.

As marine-snow particles dissolve and get lighter, they also sink more slowly, the researchers say, giving carbon extra time to escape before it can reach long-term storage in the deep ocean and potentially increasing its release back into the environment. More research is needed to calculate microbial cities’ full influence on ocean acidity because dissolved calcite can counteract the carbonic acid to an extent.

“Large-scale biogeochemical processes often depend on very small-scale interactions,” says Hongjie Wang, an oceanographer at the University of Rhode Island, who was not involved in the study. Babbin agrees: “Ultimately everything that’s happening at these microscales—that’s really what’s terraforming our planet.”

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Thomas Fuchs

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