In the ocean, it's snowing microplastics

As long as there has been marine life, there has been marine snow — a ceaseless drizzle of death and waste sinking from the surface into the depths of the sea.

The snow begins as motes, which aggregate into dense, flocculent flakes that gradually sink and drift past the mouths (and mouthlike apparatuses) of scavengers farther down. But even marine snow that is devoured will most likely be snowfall once more; a squid's guts are just a rest stop on this long passage to the deep.

Although the term may suggest wintry whites, marine snow is mostly brownish or grayish, comprising mostly dead things. For eons, the debris has contained the same things — flecks from plant and animal carcasses, feces, mucus, dust, microbes, viruses — and transported the ocean's carbon to be stored on the seafloor. Increasingly, however, marine snowfall is being infiltrated by microplastics: fibers and fragments of polyamide, polyethylene and polyethylene terephthalate. And this fauxfall appears to be altering our planet's ancient cooling process.

Every year, tens of millions of tons of plastic enter Earth's oceans. Scientists initially assumed that the material was destined to float in garbage patches and gyres, but surface surveys have accounted for only about 1% of the ocean's estimated plastic. A recent model found that 99.8% of plastic that entered the ocean since 1950 had sunk below the first few hundred feet of the ocean. Scientists have found 10,000 times more microplastics on the seafloor than in contaminated surface waters.

Marine snow, one of the primary pathways connecting the surface and the deep, appears to be helping the plastics sink. And scientists have only begun to untangle how these materials interfere with deep-sea food webs and the ocean's natural carbon cycles.

"It's not just that marine snow transports plastics or aggregates with plastic," said Luisa Galgani, a researcher at Florida Atlantic University. "It's that they can help each other get to the deep ocean."

The sunlit surface of the sea blooms with phytoplankton, zooplankton, algae, bacteria and other minuscule life, all feeding on sunbeams or one another. As these microbes metabolize, some produce polysaccharides that can form a sticky gel that attracts the lifeless bodies of tiny organisms, small shreds of larger carcasses, shells from foraminifera and pteropods, sand and microplastics, which stick together to form larger flakes. "They are the glue that keeps together all the components of marine snow," Galgani said.

Marine snowflakes fall at different rates. Smaller ones have a more languid descent — "as slow as a meter a day," said Anela Choy, a biological oceanographer at Scripps Institution of Oceanography at the University of California, San Diego.

Bigger particles, such as dense fecal pellets, can sink quicker. "It just skyrockets to the bottom of the ocean," said Tracy Mincer, a researcher at Florida Atlantic University.

Plastic in the ocean is constantly being degraded; even something as big and buoyant as a milk jug will eventually shed and splinter into microplastics. These plastics develop biofilms of distinct microbial communities — the "plastisphere," said Linda Amaral-Zettler, a scientist at the Royal Netherlands Institute for Sea Research, who coined the term.

"We sort of think about plastic as being inert," Amaral-Zettler said. "Once it enters the environment, it's rapidly colonized by microbes."

Microplastics can host so many microbial hitchhikers that they counteract the natural buoyancy of the plastic, causing their raft to sink. But if the biofilms then degrade on the way down, the plastic could float back up, potentially leading to a yo-yoing purgatory of microplastics in the water column. Marine snow is anything but stable; as flakes free-fall into the abyss, they are constantly congealing and falling apart, rent by waves or predators.

"It's not as simple as: Everything's falling all the time," said Adam Porter, a marine ecologist at the University of Exeter in England. "It's a black box in the middle of the ocean, because we can't stay down there long enough to work out what's going on."

To explore how marine snow and plastics are distributed in the water column, Mincer has begun to sample deeper waters with a dishwasher-size pump full of filters that dangles on a wire from a research boat. The filters are arranged from big mesh to small to filter out fish and plankton. Running these pumps for 10 hours at a stretch has revealed nylon fibers and other microplastics distributed throughout the water column below the South Atlantic subtropical gyre.

But even with a research boat and its expensive and unwieldy equipment, an individual piece of marine snow is not easily retrieved from deep water in the actual ocean. The pumps often disturb the snow and scatter fecal pellets. And the flakes alone offer little insight into how fast some snows are sinking, which is vital to understanding how long the plastics linger, yo-yo or sink in the water column before settling on the seafloor.

"Is it decades?" Mincer asked. "Is it hundreds of years? Then we can understand what we're in here for, and what kind of problem this really is."

To answer these questions, and work within a budget, some scientists have made and manipulated their own marine snow in the lab.

In Exeter, Porter collected buckets of seawater from a nearby estuary and loaded the water into continuously rolling bottles. He then sprinkled in microplastics, including polyethylene beads and polypropylene fibers. The constant churning, and a squirt of sticky hyaluronic acid, encouraged particles to collide and stick together into snow.

"We obviously don't have 300 meters of a tube to make it sink," Porter said. "By rolling it, what you're doing is you're creating a never-ending water column for the particles to fall through."

After the bottles rolled for three days, he pipetted out the snow and analyzed the number of microplastics in each flake. His team found that every type of microplastic they tested aggregated into marine snow, and that microplastics such as polypropylene and polyethylene — normally too buoyant to sink on their own — readily sank once incorporated into marine snow. And all the marine snow contaminated with microplastics sank significantly faster than the natural marine snow.

Porter suggested that this potential change of the speed of the snow could have vast implications for how the ocean captures and stores carbon: Faster snowfalls could store more microplastics in the deep ocean, whereas slower snowfalls could make the plastic-laden particles more available to predators, potentially starving food webs deeper down. "The plastics are a diet pill for these animals," said Karin Kvale, a carbon cycle scientist at GNS Science in New Zealand.