Tiny poops in the ocean may help solve the carbon problem

A Dartmouth-led study proposes a new method for recruiting trillions of microscopic sea creatures called zooplankton in the fight against climate change by converting carbon into food the animals would eat, digest, and send deep into the ocean as carbon-filled feces.

The technique harnesses the animals' ravenous appetites to essentially accelerate the ocean's natural cycle for removing carbon from the atmosphere, which is known as the biological pump, according to the paper in Nature Scientific Reports.

It begins with spraying clay dust on the surface of the ocean at the end of algae blooms. These blooms can grow to cover hundreds of square miles and remove about 150 billion tons of carbon dioxide from the atmosphere each year, converting it into organic carbon particulates. But once the bloom dies, marine bacteria devour the particulates, releasing most of the captured carbon back into the atmosphere.

The researchers found that the clay dust attaches to carbon particulates before they re-enter the atmosphere, redirecting them into the marine food chain as tiny sticky pellets the ravenous zooplankton consume and later excrete at lower depths.

"Normally, only a small fraction of the carbon captured at the surface makes it into the deep ocean for long-term storage," says Mukul Sharma, the study's corresponding author and a professor of earth sciences. Sharma also presented the findings Dec. 10 at the American Geophysical Union annual conference in Washington, D.C.

"The novelty of our method is using clay to make the biological pump more efficient—the zooplankton generate clay-laden poops that sink faster," says Sharma, who received a Guggenheim Award in 2020 to pursue the project.

"This particulate material is what these little guys are designed to eat. Our experiments showed they cannot tell if it's clay and phytoplankton or only phytoplankton—they just eat it," he says. "And when they poop it out, they are hundreds of meters below the surface and the carbon is, too."

The team conducted laboratory experiments on water collected from the Gulf of Maine during a 2023 algae bloom. They found that when clay attaches to the organic carbon released when a bloom dies, it prompts marine bacteria to produce a kind of glue that causes the clay and organic carbon to form little balls called flocs.

The flocs become part of the daily smorgasbord of particulates that zooplankton gorge on, the researchers report. Once digested, the flocs embedded in the animals' feces sinks, potentially burying the carbon at depths where it can be stored for millennia. The uneaten clay-carbon balls also sink, increasing in size as more organic carbon, as well as dead and dying phytoplankton, stick to them on the way down, the study found.

In the team's experiments, clay dust captured as much as 50% of the carbon released by dead phytoplankton before it could become airborne. They also found that adding clay increased the concentration of sticky organic particles—which would collect more carbon as they sink—by 10 times. At the same time, the populations of bacteria that instigate the release of carbon back into the atmosphere fell sharply in seawater treated with clay, the researchers report.

In the ocean, the flocs become an essential part of the biological pump called marine snow, Sharma says. Marine snow is the constant shower of corpses, minerals, and other organic matter that fall from the surface, bringing food and nutrients to the deeper ocean.

"We're creating marine snow that can bury carbon at a much greater speed by specifically attaching to a mixture of clay minerals," Sharma says.

Zooplankton accelerate that process with their voracious appetites and incredible daily sojourn known as the diel vertical migration. Under cover of darkness, the animals—each measuring about three-hundredths of an inch—rise hundreds, and even thousands, of feet from the deep in one immense motion to feed in the nutrient-rich water near the surface. The scale is akin to an entire town walking hundreds of miles every night to their favorite restaurant.

When day breaks, the animals return to deeper water with the flocs inside them where they are deposited as feces. This expedited process, known as active transport, is another key aspect of the ocean's biological pump that shaves days off the time it takes carbon to reach lower depths by sinking.

Earlier this year, study co-author Manasi Desai presented a project conducted with Sharma and fellow co-author David Fields, a senior research scientist and zooplankton ecologist at the Bigelow Laboratory for Ocean Sciences in Maine, showing that the clay flocs zooplankton eat and expel do indeed sink faster. Desai, a former technician in Sharma's lab, is now a technician in the Fields lab.

Sharma plans to field-test the method by spraying clay on phytoplankton blooms off the coast of Southern California using a crop-dusting airplane. He hopes that sensors placed at various depths offshore will capture how different species of zooplankton consume the clay-carbon flocs so that the research team can better gauge the optimal timing and locations to deploy this method—and exactly how much carbon it's confining to the deep.

"It is very important to find the right oceanographic setting to do this work. You cannot go around willy-nilly dumping clay everywhere," Sharma says. "We need to understand the efficiency first at different depths so we can understand the best places to initiate this process before we put it to work. We are not there yet—we are at the beginning."

In addition to Desai and Fields, Sharma worked with the study's first authors Diksha Sharma, a postdoctoral researcher in his lab who is now a Marie Curie Fellow at Sorbonne University in Paris, and Vignesh Menon, who received his master’s degree from Dartmouth this year and is now a PhD student at Gothenburg University in Sweden.

Additional study authors include George O'Toole, professor of microbiology and immunology in Dartmouth's Geisel School of Medicine, who oversaw the culturing and genetic analysis of bacteria in the seawater samples; Danielle Niu, who received her doctorate in earth sciences from Dartmouth and is now an assistant clinical professor at the University of Maryland; Eleanor Bates '20, now a PhD student at the University of Hawaii at Manoa; Annie Kandel, a former technician in Sharma's lab; and Erik Zinser, an associate professor of microbiology at the University of Tennessee focusing on marine bacteria.

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The study's first authors are Diksha Sharma, a postdoctoral researcher in the Sharma lab who is now a Marie Curie Fellow at Sorbonne University in Paris, and Vignesh Menon, who received his master’s degree from Dartmouth this year and is now a PhD student at Gothenburg University in Sweden.

Additional authors include George O'Toole, professor of microbiology and immunology in the Geisel School of Medicine, who oversaw the culturing and genetic analysis of bacteria in the seawater samples; Danielle Niu, who received her doctorate in earth sciences from Dartmouth and is now an assistant clinical professor at the University of Maryland; Eleanor Bates '20, now a PhD student at the University of Hawaii at Manoa; Annie Kandel, a former technician in Sharma's lab; and Erik Zinser, an associate professor of microbiology at the University of Tennessee focusing on marine bacteria.