Sticky ocean “snow” carries six times more carbon to the deep, study finds

Marine snow — tiny clumps of waste, dead organisms and other debris that drift through the dark ocean — removes more than 10 gigatons of carbon from surface waters each year, researchers estimate. Only about two gigatons reach depths where that carbon can stay stored for hundreds or thousands of years. Why so much is lost has long puzzled scientists.

Researchers from UC San Diego’s Scripps Institution of Oceanography and the University of Lincoln have identified a surprisingly important factor: how sticky the marine snow is. Their findings, published October 8 in the journal Science, show that the viscosity of marine snow influences how quickly bacteria break it apart and how far it can sink. According to the researchers, the stickiest particles can transport more than six times as much carbon to the deep ocean as particles with a thinner, less viscous consistency.

How ocean slime helps store carbon

Marine snow plays an important role in Earth’s carbon cycle. By transporting carbon into deep water, sinking particles can keep some of it isolated from the atmosphere for centuries or longer. But not every particle survives the journey: bacteria feed on marine snow as it descends, gradually breaking down the material and affecting how much carbon reaches the depths.

The new research provides the first detailed look at how living bacteria interact with the physical structure of marine snow and, more broadly, mucus itself. Similar relationships between bacteria and mucus help sustain coral reefs, kelp forests, plant root systems, and the digestive systems of humans and other animals. Until recently, scientists lacked the technology to observe these interactions at the microscopic scale needed to understand them.

A four-year quest to see inside marine snow

The project began in 2021, when Bryce Inman, then a postdoctoral researcher at Scripps Oceanography, set out to explore what he calls “the muscoscape.”

“Observing individual bacteria interacting with the physical structure of mucus had never been possible,” said Inman.

Together with colleagues including Farooq Azam, a microbiologist and professor emeritus at Scripps, Inman spent four years trying to overcome that limitation. Multiple attempts were unsuccessful before the team finally developed a working technique in 2025. Their solution combined 3D confocal microscopy with specially designed fluorescent molecules. For the first time, the scientists could examine the “physical terrain” of individual marine snow particles while watching how living bacteria moved through and interacted with them.

The images revealed an intricate microscopic environment, with structural differences that affect where bacteria can settle and how easily they can reach the material inside.

Why stickier marine snow sinks deeper

Marine snow particles vary considerably, each with its own mixture of organic and inorganic materials, creating differences in composition, structure and viscosity. The researchers’ new imaging approach, described in a companion study published in Nature Communications, revealed that bacteria preferentially colonize areas near a particle’s surface where the material is less viscous. The denser, stickier regions deeper inside are much harder for bacteria to penetrate.

“Bacteria can latch on to less viscous portions of each particle, but cannot reach more viscous regions until they break it down,” said Inman. “Think of it like a worm in an apple: the flesh is easy to penetrate, but the core is tough to chew through”

In laboratory experiments, the scientists observed that particles with greater viscosity resisted bacterial degradation and fragmentation for longer periods. Computer simulations further suggested that these stickier particles can descend to greater depths before breaking apart. Based on these results, the team estimates that highly viscous marine snow can deliver more than six times as much carbon to the deep ocean as less viscous particles.

“How far marine snow can sink is a surprisingly complicated problem, and we have added another – very gooey – piece to the puzzle,” said Inman.

A discovery with implications beyond the ocean

Although the research focuses on marine snow, its implications extend to many environments where bacteria interact with mucus. Mucus is often regarded as a relatively simple, slippery substance, but the new imaging technology reveals a complex internal landscape with microscopic structures that can influence how bacteria behave. Understanding these physical features could help scientists investigate biological processes ranging from human digestive health to the recovery of damaged coral reefs.

“There are bulges, crevasses, all this sort of structure to it that we really had no idea about at a scale smaller than bacteria. This is the first time we’re really examining this physical habitat,” Inman said.

Additional funding for the research was provided by the Simons Foundation and the Engineering and Physical Sciences Research Council.