Plankton mucus pulls lightweight microplastics into the deep sea, study finds

A mesocosm experiment tank used in the study [Korea Institute of Ocean Science and Technology]

Scientists have found an explanation for why microplastics — which are lighter than water and should float — turn up even in the deepest parts of the ocean. Sticky mucus secreted by marine phytoplankton binds microplastics into dense clumps heavy enough to sink, and the type of phytoplankton present matters far more than how much of it there is: the sinking rate varied by nearly threefold depending on which species dominated.

The Korea Institute of Ocean Science and Technology (KIOST) said Wednesday that a research team led by Dr. Baek Seung-ho of its Ecological Risk Research Division had confirmed through a field experiment — conducted in conditions closely mimicking the real ocean — that the composition of phytoplankton communities significantly affects how much microplastic sinks into the water column.

Until now, research on how microplastics move through the ocean had focused mainly on the physical properties of the particles, such as size and weight. Studies on the relationship between phytoplankton and microplastics had also largely been confined to laboratory experiments using single species.

The research team took the question to the sea itself. Off the coast of Geoje in South Gyeongsang Province, they deployed five cylindrical tanks — each 1 meter in diameter and 2.5 meters deep — filled with seawater and phytoplankton. The setup used a "mesocosm" approach, which recreates near-natural marine conditions to observe ecosystem changes.

Overview of the mesocosm research design [Korea Institute of Ocean Science and Technology]

The team added nutrients — nitrogen, phosphorus and others — at different concentrations to each tank, reproducing the natural variation in nutrient levels found in estuaries and coastal waters depending on rainfall and other factors. Three days into the experiment, diatoms proliferated rapidly in the nutrient-rich tanks, while the nutrient-poor tanks showed slower growth and a different dominant species.

The researchers then introduced 30 million microplastic particles into each tank and measured how many settled to the bottom.

The team had initially expected that more phytoplankton would mean more microplastics sinking. The results told a different story. In the tank with the highest phytoplankton concentration, only 11.6 percent of the microplastics reached the bottom. In a tank with roughly half as much phytoplankton, 34.1 percent settled — a sinking ratio about 2.9 times higher.

The deciding factor was the species composition of the phytoplankton.

As phytoplankton near the end of their life cycle, they secrete mucus. That mucus binds multiple organisms together into clumps denser than water, drawing in surrounding microplastics in the process. The resulting aggregates then sink through the water column.

Tanks dominated by diatoms showed significantly higher microplastic sinking rates. Diatoms reproduce quickly and have short life spans, causing them to secrete large amounts of mucus. Dinoflagellates, which live longer, produced comparatively less mucus and were less effective at binding microplastics into sinking aggregates.

The team also measured the speed of descent. Phytoplankton mucus formed within two days, and the aggregation of microplastics and other particles into clumps took about 10 days. Once formed, the clumps sank at an average rate of 80 meters per day.

"This is the result of confirming through a field experiment how phytoplankton communities — which shift depending on nutrient concentrations — alter the movement of microplastics within the ocean," Baek said.

The findings suggest that predicting where microplastics travel and accumulate requires looking beyond physical particle characteristics to include the biological environment of each sea area.

"Until now, the movement of microplastics has been predicted mainly based on how heavy and small the particles are," Baek said. "Going forward, biological community data for each sea area must also be factored into efforts to predict microplastic pathways and develop management strategies."

He added that the team plans to track how dominant species shift by season and region in order to map where microplastics travel and where they ultimately accumulate.

The research was conducted with support from the Ministry of Oceans and Fisheries under its marine microplastic inflow, generation and environmental behavior research project, and was published in the international journal Water Research.

adastra@heraldcorp.com