KIOST Reveals Sedimentation Process of Marine Microalga Heterosigma akashiwo with Suspended Microplastics

Published 26 Mar.2025 08:58(KST)

Updated 31 Jul.2025 18:17(KST)

The Korea Institute of Ocean Science and Technology (President Lee Heeseung, KIOST) has revealed that marine microalgae responsible for red tides in the ocean aggregate suspended microplastics, increasing their density and causing these suspended microplastics to sink. The results have been published in an international academic journal.

The research was published in the international journal, Journal of Hazardous Materials, on April 5, 2025, by KIOST researchers Baek Seungho and Lim Younggyun. The paper, titled "Impact of Heterosigma akashiwo on the environmental behavior of microplastics: Aggregation, sinking, and resuspension dynamics," discusses the effects of Heterosigma akashiwo on the environmental behavior (aggregation/sinking/resuspension) of microplastics.

Schematic diagram of the effect of Heterosigma akashiwo on microplastic behavior.

The research team led by Baek Seungho and Lim Younggyun at KIOST hypothesized that marine microalgae would have a significant impact on the sinking of suspended microplastics in the ocean. They analyzed the effect of Heterosigma akashiwo, one of the main causes of summer red tide outbreaks along the Korean coast, on the behavior of suspended microplastics.

Marine microalgae are unicellular organisms ranging from 10 to 200 micrometers in size. Through photosynthesis, they absorb carbon dioxide and generate oxygen, acting as marine microorganisms.

The study found that the extracellular polymeric substances (EPS) secreted by Heterosigma akashiwo adhere to suspended microplastics, increasing their density. As a result, the microplastic aggregates become heavier than seawater and sink to the ocean floor.

In particular, the research team selected polyethylene (PE) and polypropylene (PP)?the most representative plastics accounting for the majority of global plastic production?to analyze the sinking rates according to differences in size and density.

Analysis of 5,000 PE microplastic aggregates, which are relatively small (10?20 micrometers) but heavy (density: 1.0g/cm³), showed that 28% sank within 20 days. In contrast, analysis of 1,250 PP microplastic aggregates, which are larger (45?75 micrometers) but lighter (density: 0.91g/cm³), showed that only 1.8% sank during the same period.

This demonstrates that microplastic aggregates formed by Heterosigma akashiwo sink more easily to the ocean floor when composed of heavier plastic materials.

Furthermore, measurements of the sinking speed of the sunken PE and PP microplastic aggregates showed an average sinking rate of 63 meters per day, with no significant difference according to the size and density of the microplastics.

The research team also simulated the cold and dark environment of the seabed where microplastic aggregates settle, to determine whether sunken microplastic aggregates could be decomposed by bacteria present in seawater and resurface as suspended microplastics.

The results showed that although numerous bacteria were present on the surface of the sunken microplastic aggregates, resuspension due to decomposition was not observed. This suggests that if suspended microplastics continue to sink, there is a possibility of long-term accumulation.

Experimental results of settling velocity of microplastic aggregates (left) and resuspension analysis results with fluorescent microscope images of aggregates (right).

This study is significant in that it quantitatively analyzed and academically identified the process by which marine microalgae aggregate and sink suspended microplastics. KIOST plans to continue research to develop related technologies, enabling accurate assessment and prediction of the influx, occurrence, and behavior of microplastics in the marine environment.

This research was conducted as part of the Ministry of Oceans and Fisheries' project on "Research on the Influx, Occurrence, and Environmental Behavior of Marine Microplastics."