Tiny poops in the ocean may help solve the carbon problem

Humanity’s fight against climate change could be aided by some of the world’s smallest animals and their tiny poops, according to a new study in Nature Scientific Reports.

A team led by Dartmouth researchers floats a method of recruiting trillions of microscopic sea creatures known as zooplankton to capture carbon dioxide originating from Earth’s atmosphere and deposit it deep into the sea as feces. The technique consists of spraying clay dust on the surface of the ocean at the site of large blooms of microscopic plants called phytoplankton. These blooms can grow to cover hundreds of square miles and are grazed upon by zooplankton.

Mukul Sharma, the study’s corresponding author and a professor of earth sciences, says that phytoplankton blooms remove about 150 billion tons of carbon dioxide from the atmosphere each year through photosynthesis, converting it into organic carbon particulates. But once the phytoplankton die, marine bacteria devour the organisms’ rotting carcasses and the particulates, releasing most of the captured carbon back into the atmosphere.

The magic ingredient

The team conducted laboratory experiments on water collected from the Gulf of Maine during a 2023 phytoplankton bloom. They found that clay dust applied to the surface at the end of a bloom attaches to the organic carbon released by phytoplankton, prompting marine bacteria to produce a kind of glue that causes the clay and organic carbon to form tiny sticky balls called flocs.

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The flocs become part of the daily smorgasbord of particulates that zooplankton gorge on, the researchers report. Once digested, the clay embedded in the animals’ feces sinks, potentially burying the carbon at depths where it can be stored for millennia. The uneaten flocs 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.

Sticky particles

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.

Spreading clay on the water’s surface essentially accelerates the natural cycle known as the biological pump that removes carbon from the atmosphere and stores it in the ocean, says Sharma, who will present the findings Dec. 10 at the American Geophysical Union annual conference in Washington, D.C.

“Normally, only a small fraction of the carbon captured at the surface makes it into the deep ocean for long-term storage. The novelty of our method is using clay to make the biological pump more efficient,” says Sharma, who received a Guggenheim Award in 2020 to pursue the project.

“We want to take advantage of the ocean’s biology to trap the carbon dioxide removed by phytoplankton and, by sending these little pods through the marine food chain, confine it to the deep ocean,” he says.

Carbon-clay flocs

In the ocean, the carbon-clay flocs would 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 even more 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.

“Zooplankton are eating and pooping machines,” Sharma says. “When you slice apart their poop, you see the remains of all these phytoplankton that have not been digested.” The flocs of clay and carbon produced by the Dartmouth-led team’s method would mix with all the other matter zooplankton consume, Sharma says.

Active transport

When day breaks, the carbon flocs hitch a ride inside the animals to deeper water where it’s deposited as feces. This dynamic, known as active transport, is another key aspect of the ocean’s biological pump. It 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 zooplankton eat the clay flocs and that the pellets do indeed sink faster.

“The zooplankton generate clay-laden poops that sink faster,” Sharma says. “This particulate material is what these little guys are designed to eat. Our experiments showed that they cannot tell if it’s clay and phytoplankton or only phytoplankton—they just eat it. And when they poop it out, they are hundreds of meters below the surface and all that carbon is, too.”

Phytoplankton blooms

Sharma plans to next conduct field experiments 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.”

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.