Microplastics smaller than five millimetres, found from Arctic sea ice to the deepest ocean trenches, can disrupt the plankton that drive the ocean’s biological carbon pump — potentially weakening a natural process that helps draw CO₂ from the atmosphere and stores some of its carbon in the deep sea for hundreds to thousands of years.

Microplastics can interfere with the small organisms and sinking particles that help move carbon into the deep ocean. Laboratory studies have measured changes in plankton feeding and the descent of carbon-rich material; computer models have explored the possible consequences for ocean carbon storage.

These findings establish plausible mechanisms for weakening the biological carbon pump. They do not establish a measured global decline caused by microplastics. The distinction matters because an effect on an animal or particle must pass through an entire food web and ocean circulation system before it becomes a change in atmospheric carbon dioxide.

Plastic has reached Arctic ice and deep trenches

NOAA defines microplastics as plastic pieces smaller than five millimeters. That category covers a wide range of particles, rather than one uniform pollutant. A fiber shed from a textile and a smooth plastic bead can differ in shape, density and behavior in water.

In a 2018 Nature Communications study, Ilka Peeken and colleagues identified microplastics in Arctic sea-ice cores. The composition varied with the ice’s formation and drift history. Sea ice can trap particles, transport them and release them as it melts.

At the other end of the water column, Alan Jamieson’s team reported microplastics and other human-made particles in deep-sea amphipods in Royal Society Open Science in 2019. Their samples came from six Pacific trenches at depths between 7,000 and 10,890 meters. Some identified fibers were cellulosic materials rather than conventional plastics, an important qualification when interpreting particle counts.

Those surveys document contamination and ingestion. Neither measured a loss of carbon storage.

The biological pump depends on what escapes recycling

In sunlit waters, phytoplankton use photosynthesis to turn dissolved carbon dioxide into organic matter. Their activity helps maintain the conditions under which the ocean can take up CO₂ from the air. Grazing animals, dead cells and sticky aggregates then move some of that carbon downward.

Woods Hole Oceanographic Institution’s explanation of the biological pump follows this transfer from surface life into deeper waters. Falling particles include dead plankton, animal waste and the loose clusters known as marine snow. Animal migration provides another route.

Most organic material is eaten or decomposed before reaching the deep ocean. Carbon that does arrive there can remain separated from the atmosphere for hundreds to thousands of years, depending on depth and circulation. It need not remain inside an intact particle: microbial breakdown can leave it dissolved in deep water.

Our earlier article on photosynthesis, oxygen recycling and carbon burial explains why production alone is not the whole accounting. For this question, the additional distinction is between exporting carbon below the surface and keeping it away from atmospheric exchange for a long time. Permanent burial is only one possible fate.

Laboratory copepod studies show two possible disruptions

Copepods are small crustaceans that graze on plankton and package food residues into fecal pellets. They provide a connection between microscopic producers, larger animals and sinking carbon.

In a 2015 Environmental Science & Technology experiment, Matthew Cole and colleagues exposed Calanus helgolandicus to algae and 20-micrometer polystyrene beads at 75 particles per milliliter for 24 hours. The exposed animals consumed 40 percent less algal carbon biomass. The number of algal cells consumed fell by 11 percent, but that difference was not statistically significant.

The result concerns feeding under a specified laboratory exposure. It does not mean ocean plankton everywhere now consume 40 percent less carbon.

Cole’s team examined another mechanism in a 2016 study in the same journal. At an exposure of 1,000 polystyrene particles per milliliter, copepods produced pellets with lower density, slower sinking and greater fragmentation. A pellet that descends more slowly leaves more time for its contents to be consumed or decomposed at shallower depths.

Laboratory marine snow experiments add a physical pathway

Plastic can enter sinking aggregates without first passing through an animal. In a 2024 Limnology and Oceanography study, Cordelia Roberts and colleagues used rotating tanks to form aggregates from diatoms, a group of phytoplankton, with different concentrations of plastic microfibers.

The fibers encouraged aggregate formation but made the resulting clusters less cohesive and more prone to breaking apart. Incorporating buoyant fibers also reduced the sinking speed of aggregates of a given size. At 100,000 fibers per cubic meter, the team’s calculations indicated an 8–45 percent reduction in potential carbon export.

That range belongs to this experimental calculation.

The authors explicitly noted that their concentrations were several orders of magnitude above those generally reported in open surface waters, while discussing circumstances that could concentrate particles. Their result cannot be applied as an 8–45 percent loss from the global carbon pump. It identifies a process whose importance depends on actual exposure and the kinds of aggregates present.

A model gives a conditional estimate, not a global measurement

Karin Kvale and colleagues modeled slower fecal-pellet sinking in Frontiers in Marine Science in 2023. Under their strong-pollution, strong-climate-change scenario, the model produced a cumulative reduction in ocean carbon uptake of about 4.4 billion metric tons of carbon over 1950–2100, equivalent to 0.24 percent of anthropogenic emissions over that period.

This was a scenario result, not an observed historical loss. The authors emphasized uncertain parameters and simplified model structure. Regional ecological changes also differed, demonstrating why a laboratory effect cannot simply be multiplied by the size of the ocean.

Exposure and carbon transport must be measured together

The evidence spans several separate questions: where plastic occurs, what organisms ingest, how feeding changes, what sinks, and how much carbon remains at depth. In our reading, the strongest next test connects those measurements rather than treating one as a substitute for the others.

Even particle surveys require comparable methods. NOAA notes that standardized collection protocols are needed to compare samples from water and sediment. A count that captures large fragments but misses tiny fibers can describe a different exposure from the one experienced by a feeding copepod.

Field studies need to characterize particle sizes, shapes and concentrations alongside plankton communities and sinking carbon. Experiments can then test those observed conditions, helping determine where microplastics meaningfully alter carbon transport and where the laboratory mechanisms have little effect.