In a groundbreaking study published in the April 2025 issue of the Journal of Hazardous Materials, researchers from the Korea Institute of Ocean Science & Technology (KIOST) have uncovered a critical mechanism by which marine microalgae contribute to the fate of buoyant microplastics (MPs) in oceanic environments. Led by Dr. Seung Ho Baek and Dr. Young Kyun Lim, the team focused on the role of Heterosigma akashiwo, a microalga notorious for causing red tide events along the Korean coast during summer months. Their findings reveal that this microorganism not only interacts with plastic particles but also significantly alters their density, promoting the aggregation and sinking of plastics that would otherwise float indefinitely.
The study’s central discovery highlights how the secretions of extracellular polymeric substances (EPS) by Heterosigma akashiwo adhere to buoyant microplastics such as polyethylene (PE) and polypropylene (PP). These EPS coatings increase the effective density of plastic particles, leading to the formation of dense aggregates heavier than seawater. Such aggregates overcome flotation forces, resulting in sinking from the surface ocean to the seabed. This process fundamentally shifts long-held assumptions about the environmental behavior of floating microplastics, which are often considered persistent migrants predominantly at or near the surface.
To quantify sinking dynamics, the research team conducted an extensive experimental investigation using two of the most prevalent plastic polymers worldwide: polyethylene, known for its relative density near 1.0 g/cm³, and polypropylene, with a lighter density of approximately 0.91 g/cm³. More than 5,000 PE microplastic aggregates ranging between 10 and 20 micrometers were analyzed, yielding an astonishing 28% settlement rate within 20 days. In contrast, roughly 1,250 larger but lighter PP aggregates (45-75 micrometers) exhibited only 1.8% sinking over the same duration. These comparative results underscore the decisive role played by polymer density and particle size in modulating the influence of microalgal EPS, highlighting that denser plastics are more susceptible to bio-induced sedimentation.
Intriguingly, despite size and density differences, the average vertical sinking velocity of MP aggregates hovered around 63 meters per day for both PE and PP aggregates. This uniformity in settling rates suggests that the EPS-mediated aggregates adopt sedimentation dynamics largely governed by biofilm characteristics and aggregate morphology rather than polymer intrinsic properties alone. Such findings illuminate the complex interplay between biological secretions and abiotic factors, revealing how microbial colonization transforms microplastic behaviors from passive drifters to active components of benthic sediment pools.
Further experimentation simulated the cold, dark conditions characteristic of benthic ocean floors where these microplastic aggregates eventually come to rest. The team sought to determine whether microbial degradation processes, particularly bacterial colonization, might disaggregate or otherwise remobilize settled plastics back into the water column. Notably, while copious bacterial populations were observed on aggregate surfaces, no measurable resuspension of buoyant MPs was detected. This lack of upward flux indicates a predominantly one-way transport mechanism, suggesting that microalgal aggregation facilitates long-term sequestration of microplastics within seabed sediments rather than promoting their recycling to surface waters.
The ecological significance of these findings cannot be overstated. Given the ubiquity of Heterosigma akashiwo blooms coinciding with peak plastic pollution inputs in temperate coastal regions, this biological pathway provides critical insight into natural remediation processes. By mediating microplastic sinking, this microalga influences spatial distribution and potential exposure risks for benthic organisms, altering contaminant transport and ecosystem dynamics. These processes likely affect carbon cycling as well, since sinking plastics coupled with biofilms can become vectors for organic carbon export into the deep ocean.
This research marks an important leap in marine ecology and pollution science, offering a carefully quantified, mechanistic understanding of how microbial interactions shape the environmental fate of marine microplastics. The experimental design, focusing on realistic polymer types and particle sizes, strengthens the ecological relevance of the results, bridging laboratory observations to in situ oceanic phenomena. The study sets a new standard for incorporating biological factors into predictive models of microplastic behavior, a critical advancement toward comprehensive marine pollution management.
Looking ahead, KIOST plans to deepen its investigations by developing advanced detection technologies and predictive analytical tools aimed at monitoring the influx, formation, and ecological impacts of microplastics throughout marine ecosystems. These innovations are expected to refine risk assessments of plastic pollution and inform sustainable mitigation strategies, particularly in regions prone to both elevated plastic loadings and frequent harmful algal blooms.
From a methodological standpoint, the study’s experimental approach stands out for its rigorous simulation of natural parameters, including temperature gradients, light availability, and microbial community interactions. Such realism ensures that findings accurately reflect the multifaceted conditions governing microplastic fate in coastal marine systems. The integration of microbiological assays with polymer science effectively captures the interdisciplinary essence required to tackle oceanic pollution challenges.
This study also expands the scientific discourse on microplastic sedimentation by emphasizing the role of microalgal secretions rather than relying solely on physical aggregation or abiotic settling processes. This nuance recalibrates previous models that underestimated the biological complexity underpinning microplastic transport, paving the way for subsequent research into other microorganism-plastic interactions throughout diverse marine habitats.
Ultimately, the KIOST findings signify a promising natural attenuation mechanism wherein marine microalgae, notorious for their harmful red tides, inadvertently facilitate the downward migration and sequestration of plastic pollutants. This dual role highlights the need to consider ecological trade-offs when evaluating algal bloom impacts on ocean health. By elucidating this novel bio-physical interaction, the study contributes both to theoretical frameworks and practical policy development addressing one of the ocean’s most pressing environmental crises.
This research was generously supported by the Korea Institute of Marine Science & Technology Promotion (KIMST) under the project “Land/Sea-based input and fate of microplastics in the marine environment,” funded by the Ministry of Oceans and Fisheries of the Republic of Korea (RS-2022-KS221604). The collaborative efforts of marine biologists, polymer chemists, and oceanographers at KIOST epitomize the integrative research necessary to decode and manage the complex patterns of marine plastic pollution in the 21st century.
Subject of Research:
Not applicable
Article Title:
KIOST Reveals Process by which Marine Microalga Heterosigma akashiwo Causes Buoyant Microplastics to Sink
News Publication Date:
10-Apr-2025
Web References:
http://dx.doi.org/10.1016/j.jhazmat.2025.137242
References:
Baek, S. H., Lim, Y. K., et al. (2025). Impact of Heterosigma akashiwo on the environmental behavior of microplastics: Aggregation, sinking, and resuspension dynamics. Journal of Hazardous Materials.
Keywords:
Marine ecosystems, Microplastics, Heterosigma akashiwo, Extracellular polymeric substances, Microalgae, Plastic sinking dynamics, Red tide, Marine pollution, Polyethylene, Polypropylene, Sedimentation, Oceanography
