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May 18, 2023

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When the research submarine Alvin sank off the coast of Massachusetts in 1968, it took the crew's lunch with it. Sandwiches wrapped in wax paper, a few thermoses of broth, and an apple or two came to rest with the legendary exploration vessel. And to the shock of the scientists who later returned to recover the wreck, there they remained—practically unspoiled despite sitting more than a kilometer below the surface for nearly a year.

A sandwich left out on your countertop or casually thrown into the sea would be lucky to last more than a day or two before going bad or getting gobbled up. So why didn't something eat the Alvin crew's lunch?

New evidence suggests that the extreme pressures of the deep sea slow down microbial carbon degradation, the process responsible for spoiling sandwiches and recycling organic carbon into carbon dioxide, a critical step in the carbon cycle. The research team behind the new study says that their findings could have important implications for carbon budgets, which are used in climate models, and future geoengineering strategies that propose storing excess carbon on the seafloor. The results were published in Nature Geoscience.

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For decades, scientists have wondered whether microbial carbon degradation is suppressed in the deep sea. But answering this seemingly simple question has proven challenging.

Shallow-water microbes continually fall into the deep ocean from the sunlit surface. These unwilling interlopers would presumably break down carbon more slowly at depth because they have not adapted to the pressure.

"These microbes survive, barely, in the deep sea. But they are not feeling really comfortable there," said marine microbiologist Gerhard Herndl of the University of Vienna.

But other microbes don't mind pressure much at all. Some will even die if they’re decompressed. Some of these pressure-loving piezophiles seem to have hearty appetites for organic carbon, leading some scientists to think that microbial activity in the deep sea could actually be rather high—though it's possible that when scientists sample these communities, "we’re just isolating the ‘weeds’ that grow quickly," said marine microbiologist Douglas Bartlett of the Scripps Institution of Oceanography, who was not involved in the new study.

Complicating everything further is the enormous technical challenge of working in the deep. Keeping a deep-sea sample under pressure after bringing it to the surface requires a tough titanium chamber that can tolerate pressure differences hundreds of times greater than that between the inside and outside of the International Space Station.

"That's really hard engineering to do," Bartlett said. So scientists have mostly measured deep-sea carbon degradation rates in depressurized samples brought up to the surface.

But without a way to make measurements under natural deep-sea conditions—pressure and all—it's impossible to know whether the observations researchers have made in decompressed samples reflect what's going on in the depths.

Instead of bringing deep-sea samples to the surface for experiments, they brought their experiments to the deep sea.

After years of trying to get pressure chambers to work, Herndl and his colleagues took a different approach; instead of bringing deep-sea samples to the surface for experiments, they’d bring their experiments to the deep sea.

Previously, researchers in Japan worked with Herndl's group to develop a device that can be lowered from a ship to make measurements under water. The device takes a water sample, performs an experiment, and then adds a special fluid into the sample to "fix" it, preserving microbes exactly as they were in the deep sea. Then the sample is brought to the surface for measurements.

In the Pacific, Atlantic, and Southern Oceans, experiments with this device revealed that as a whole, microbial communities consumed carbon about one third as quickly at 4,000 meters deep as at the surface.

Roughly 85% of microbes consumed carbon at about the same rate regardless of depth, and only about 5% of the microbes in seawater samples were pressure-loving piezophiles. The remaining 10% of microbes were pressure hating. These communities "respond tremendously when you release them from pressure," gobbling up carbon much faster than they do in the deep sea, Herndl said. Because these organisms are much more active at sea surface pressure, previous estimates of the carbon degradation rates of deep-sea microbial communities were "really grossly overestimated," he added.

The discovery could have important implications for geoengineering and for the carbon budgets that scientists use to build climate models.

"One of the issues of our time now is what to do about climate impacts."

"One of the issues of our time now is what to do about climate impacts," Bartlett said. Pumping carbon dioxide into the atmosphere drives climate change, prompting some to devise creative carbon storage solutions. "People consider ways to bring more particulate organic carbon into the deep ocean to bury it and to sequester that carbon," so knowing the rate that microbes break down organic carbon in the deep sea "is really important," he said.

With respect to carbon budgeting, Herndl added, the discovery resolves a long-standing problem. Previous estimates of deep-ocean carbon degradation rates found a troubling mismatch: The supply of organic material sinking down from the surface seemed far smaller than deep-sea microbes’ appetite for that carbon. If the budgets really are misbalanced, "then apparently we don't understand how the deep ocean works," Herndl said.

But the new, lower carbon demand measured in this study lines up neatly with supply. The mismatch looks like it was simply a matter of overestimating carbon degradation rates in depressurized samples, Herndl and Bartlett said.

"It seems like that was the magic bullet—the solution that had eluded microbial oceanographers all these years," Bartlett said, "not [measuring] microbial activity under the actual deep-sea conditions."

"Microbes are by far the main contributors to carbon processing in the deep ocean," Herndl said. "So it makes a difference when you [calculate] a global carbon budget…it makes a difference whether you estimate microbial activity in the deep correctly or not."

—Elise Cutts (@elisecutts), Science Writer

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