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The Korea Institute of Ocean Science & Technology (KIOST) has revealed that marine microalgae are responsible for ocean aggregates of buoyant microplastics (MPs), increasing the density of MP aggregates and causing them to sink.

The findings were in the Journal of Hazardous Materials in a paper titled "Impact of Heterosigma akashiwo on the environmental behavior of microplastics: Aggregation, sinking, and resuspension dynamics."

The KIOST research team, led by Dr. Seung Ho Baek and Dr. Young Kyun Lim estimated that marine microalgae significantly influence the sinking of buoyant microplastics (MPs) in the ocean. They analyzed the effects of Heterosigma akashiwo, which is associated with frequent red tide events along the Korean coast during summer, on the environmental behavior of buoyant MPs.

The study revealed that Heterosigma akashiwo secretes extracellular polymeric substances (EPS) that attach to buoyant MPs, increasing MP density. This process results in the formation of MP aggregates that become heavier than seawater, causing them to sink to the seafloor.

The research team selected polyethylene (PE) and polypropylene (PP), two predominant plastic polymers accounting for a significant portion of global plastic production, to examine sinking rates based on size and density.

An analysis of 5,000 polyethylene (PE) MP aggregates, characterized by their relatively small size (10–20 μm) but higher density (1.0 g/cm³), showed that 28% sank within 20 days.

In contrast, an analysis of 1,250 polypropylene (PP) MP aggregates that are larger in size (45–75 μm) but less dense (0.91 g/cm³) indicated that only 1.8% sank over the same period. These results demonstrate that MP aggregates formed by Heterosigma akashiwo are more likely to sink when composed of denser plastic materials.

The sinking velocity of the settled PE and PP MP aggregates was also measured, resulting in an average of 63 m/day, with no significant differences observed based on MP size or density.

The research team also simulated the cold, dark conditions of the seafloor, where MP aggregates settle, to investigate whether bacterial decomposition could degrade these aggregates and cause the buoyant MPs to resuspend and rise to the sea surface again.

They found that numerous bacteria were on the surface of the settled MP aggregates but did not detect any resuspension. This suggests that continuous sinking of buoyant MPs could lead to their accumulation on the seafloor over the long-term.

This study is significant for its quantitative analysis and academic clarification of the process by which marine microalgae aggregate and sink buoyant MPs. KIOST plans to continue research to develop technologies for accurately assessing and predicting the influx, generation, and behavior of MPs in marine ecosystems.