In the deep sea, it never stops snowing. The type of snow that falls in the ocean, known as marine snow, is very different from the flaky geometric wonders that fall on land. Marine snow describes tiny clumps of organic and inorganic detritus – think mucus, poop, silt, dead plankton, etc. – that rain from the shallows into the deep.
As they sink from the surface to the seafloor, they take tons of carbon with them. Scientists estimate that marine snow transports more than 10 gigatons of carbon away from the surface every year. However, only two gigatons are believed to sink deep enough to be stored for hundreds to thousands of years.
Scientists have long wondered what factors limit this essential part of the carbon cycle. Now scientists at UC San Diego’s Scripps Institution of Oceanography and the University of Lincoln recently discovered one cause of this discrepancy.
Their new study, published Oct. 8 in the journal Science , revealed that the viscosity—or gooeyness—of marine snow can have a major impact on how quickly it’s broken down and how far it sinks, factors that influence how much carbon it transports to the deep ocean. They found that gooey marine snow can shuttle more than six times the carbon to the deep ocean than less viscous particles.
This is the first time scientists have studied live bacteria interacting with the physical structure of marine snow, or any mucus for that matter. These types of interactions are critical, however, for maintaining the health of life on our planet, as coral reefs, kelp forests, land plant root systems, and animal and human digestive tracts all rely on interactions between mucus and bacteria. The researchers behind the new study say their findings and new mucus imaging method will help scientists studying how bacteria behave in mucus.
This breakthrough in slimey science started in 2021 when Bryce Inman, a postdoctoral scholar at Scripps Oceanography at the time, began searching for a way to visualize what he calls “the muscoscape.”
“Observing individual bacteria interacting with the physical structure of mucus had never been possible,” said Inman.
So for four years, Inman and his colleagues, including Farooq Azam, a microbiologist and professor emeritus at Scripps, tried and failed to make it possible. It wasn’t until 2025 that they finally found a way to make it work.
Using a combination of 3D confocal microscopy and specialized fluorescent molecules, the researchers were able to map the “physical terrain” of individual clumps of marine snow and see how bacteria interacted with it.
Being able to see the individual bacteria and all the nooks and crannies of the marine snow allowed the researchers to gain a better understanding of how the bacteria impacted the structure of the snow, specifically its viscosity.
Like snow on land, no two marine snow particles are the same. They are all made up of different combinations of organic and inorganic molecules that differ in their gooeyness.
Using their new method, detailed in a companion paper in Nature Communications , the researchers were able to determine that bacteria can colonize less-viscous areas near the marine snow’s surface far more easily than its more-viscous regions deeper inside.
“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 a series of laboratory experiments, the researchers found that more viscous marine snow takes longer for bacteria to degrade and fragment. Computer simulations also suggest that the more viscous marine snow is, the deeper it will sink. Inman and his colleagues estimate that
high-viscosity marine snow can carry more than six times the carbon to the deep ocean than their low-viscosity counterparts.
“How far marine snow can sink is a surprisingly complicated problem, and we have added another – very gooey – piece to the puzzle,” said Inman.
What Inman and his colleagues found will not only help scientists studying Earth’s carbon cycle, but also those studying the role mucus plays in everything from human health to coral reef recovery.
Mucus, slime, whatever you want to call it, “has a surprising amount of structure,” Inman said. “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.”
Additional funding for this project was provided by the Simons Foundation and the Engineering and Physical Sciences Research Council.
Science
Marine snow viscosity regulates microbial degradation and the ocean carbon sink
8-Oct-2026