It’s that slime at the heart of the ocean carbon pump.

Marine snow, the constant rain of mucus, poop, dead plankton, silt and other detritus from the shallows, carries more than 10 gigatons of carbon away from the surface every year, but only about 2 gigatons make it deep enough to be stored for centuries to millennia.

A new study published Oct 8 in Science says why so much is lost: viscosity.

What they found

Scientists at UC San Diego’s Scripps Institution of Oceanography and the University of Lincoln, led by Bryce Inman, discovered that the gooier, more viscous, a particle is, the better it transports carbon.

• Bacteria get stuck on the outside: Less viscous areas near the surface of a particle are easy for bacteria to colonize. The more viscous interior is tough to penetrate. “Think of it like a worm in an apple: The flesh is easy to penetrate, but the core is tough to chew through,” Inman said.
• Takes longer to break down: Lab experiments showed viscous snow resists bacterial degradation and fragmentation much longer.
• Sinks deeper: Computer simulations showed that extra resistance time lets it sink further. The team estimates high- viscosity marine snow can shuttle more than six times as much carbon to the deep ocean as low-viscosity 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.

How they saw it

This is the first time anyone has watched live bacteria interacting with the physical structure of marine snow, or any mucus.

Starting in 2021, Inman spent 4 years trying to visualize what he calls “the mucoscape”.

In 2025 it worked: using 3D confocal microscopy plus specialized fluorescent molecules called molecular rotors, they mapped the “physical terrain”, bulges, crevasses, viscous pockets, of individual clumps at a scale smaller than bacteria.

The imaging method is detailed in a companion paper in Nature Communications. 

Why it matters beyond carbon

The findings help refine climate models for the ocean carbon sink, but Inman says the technique also matters for any system where bacteria live in mucus, coral reefs, kelp forests, plant roots, and animal and human digestive tracts.

Marine snow is exactly what it sounds like — it’s snowing in the ocean, 24/7.

What it is

Not frozen water. It’s a shower of tiny, fluffy clumps falling from the sunlit surface to the deep seafloor. The term was coined by divers seeing it like snowflakes in a headlamp beam.

Each flake is an aggregate, no two are the same:

• Biological glue: transparent exopolymer particles (TEP), basically phytoplankton mucus and slime
• Leftovers: dead phytoplankton and zooplankton, fecal pellets, shed exoskeletons
• Mineral ballast: silt, calcium carbonate shells, silica
• Living passengers: bacteria (orange in that image you saw), microalgae, protozoa hitching a ride

Size ranges from a few micrometers to centimeters. A big aggregate looks like a dirty snowball with mucus parachutes.

How it forms

1. Phytoplankton bloom in the surface and excrete sticky sugars
2. Stickiness makes everything collide and clump, think dust bunnies under the bed
3. Bacteria colonize it immediately and start excreting enzymes to dissolve it
4. If it survives long enough, it sinks, typically 10 to 100 and more meters per day, some much faster if ballasted

Why we care: the biological carbon pump

This is one of Earth’s main carbon sinks:

• Surface phytoplankton fix CO2 into organic carbon
• When they die or get eaten, that carbon is packaged into marine snow
• As it sinks, it takes tons of carbon with it. Scientists estimate >10 gigatons per year leave the surface this way, but only ∼2 gigatons sink deep enough to be stored for 100s- 1000s of years

Everything else gets eaten or respired on the way down by bacteria and zooplankton, turning back into CO2 in the upper ocean.

That’s why this new Science paper matters — it shows viscosity is a regulator. The gooier interior acts like a hard apple core that bacteria can’t chew through easily, so gooey flakes resist degradation, sink deeper, and shuttle >6 times more carbon to the deep.

There’s also a recent twist from 2024- 2025 work: sometimes marine snow makes parachute- like mucus structures that actually slow sinking and keep it in the top 100m longer, giving microbes more time to break it down, stalling carbon storage.

So, it’s a tug- of- war at the microscale: ballast and gooeyness help it sink and store carbon; fragmentation, grazing by zooplankton, and slime parachutes keep it up and recycle carbon.

Without marine snow, the Earth would be much hotter. It’s essentially dead stuff at the bottom of the ocean slowing climate change, and it’s also the base food delivery service for most deep- sea life that never sees sunlight.

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Marine snow viscosity regulates microbial degradation and the ocean carbon sink

Editor’s summary and Abstract in plain language

The paper asks: when marine snow sinks, will its carbon be released in the upper ocean or sequestered at depth? That depends on sinking speed vs degradation rate.

Using molecular rotors to measure the nanoscale viscosity field of natural aggregates, they show:

aggregates are highly structured microhabitats with less-viscous regions that are accessible for colonization and more- viscous regions that correlate to a barrier to bacterial infiltration

More- viscous aggregates take longer to degrade, sink farther, and contribute more to carbon sequestration, illustrating another way that the physical structure of marine snow can affect the global ocean carbon pump

So it’s not just size, ballast minerals, or density, viscosity itself is a microscale physical constraint on the pump.

What they actually measured

1. Viscosity mapping: The companion Nature Communications paper describes the method, molecular rotors that fluoresce differently depending on local viscosity and 3D confocal microscopy. This let them see the “mucoscape” inside a single particle for the first time.
2. Bacterial colonization is patchy: Bacteria latch onto low- viscosity zones near the surface easily but can’t infiltrate high- viscosity cores until they enzymatically break it down.
3. Degradation rate = function of viscosity: Quantifying the “viscosity degradation rate”, they found high- viscosity snow resists fragmentation. In lab and modeling, that translates to deeper sinking and >6 times more carbon reaching deep water vs low- viscosity counterparts, the number Phys.org highlighted.

The paper frames marine snow not as a uniform clump of detritus, but as a structured habitat with accessible vs protected carbon, where gooeyness acts like a physical defense against microbial recycling in the upper 100- 200m.

Journal: Nature Communications, Science (2026)

DOI: 10.1126/science.adx4170, (Science)

DOI: 10.1038/s41467-026-75892-y, (Nature Communications)

Provided: University of California – San Diego

Authors: Bryce G. Inman, Stuart Humphries, and Farooq Azam

Abstract

Bacterial degradation of marine snow aggregates is a major component of global carbon cycling. Whether aggregate carbon is released in the upper ocean or is sequestered at depth depends on its sinking speed and degradation rate. However, little is known about the physical constraints of bacterial colonization and degradation of individual aggregates. Using molecular rotors to measure the nanoscale viscosity field of natural aggregates, we show that aggregates are highly structured microhabitats with less-viscous regions that are accessible for colonization and more-viscous regions that correlate to a barrier to bacterial infiltration. By quantifying the viscosity degradation rate, we demonstrate that more viscous aggregates take longer to degrade, sink farther, and contribute more to carbon sequestration, revealing a microscale physical constraint of the global ocean carbon pump.