SEOUL, July 29 (AJP) - The algal blooms that turn rivers and lakes green every summer do more than cloud the water. They also make discarded plastic crumble faster, accelerating the point at which a plastic bag or wrapper starts shedding microplastics, according to research from Korea Advanced Institute of Science and Technology.
The finding matters because the two problems have been managed as separate ones. Water authorities treat blooms as a nutrient pollution issue and plastic waste as a solid waste issue, and the research shows the first speeds up the damage caused by the second. Blooms are also becoming more frequent worldwide as warming water and fertilizer runoff push more nutrients into freshwater systems.
Korea Advanced Institute of Science and Technology (KAIST) announced the results Wednesday. The work was led by Myung Jae-wook, a professor in the Department of Civil and Environmental Engineering, with doctoral student Kim Young-ju as first author, and was carried out with three researchers at Hong Kong University of Science and Technology.
Plastic that ends up in a river does not sit there inertly. Within days, bacteria and algae colonize its surface and build a living film, an environment researchers call the plastisphere. Scientists already knew this film influences how pathogens spread and how microplastics travel through water. What nobody had established was what happens to that film when a serious bloom hits.
To find out, the team built miniature ecosystems in the laboratory using freshwater collected from a pond on the KAIST campus in Daejeon, then added low-density polyethylene film, the flexible plastic used in shopping bags. Blooms were induced by seeding the water with Microcystis aeruginosa, a cyanobacterium commonly used as a model for harmful freshwater blooms, and by adjusting light and nutrient levels. The team then tracked the plastic surfaces for 42 days, analyzing the microbial film, the makeup of the community living in it and the genes those organisms carried.
Under bloom conditions, cyanobacteria and a wide range of ordinary bacteria multiplied together and produced a thicker, more complex film than in normal water. Organisms that secrete exopolysaccharides, the sticky substances that bind microbes to each other and to surfaces, increased sharply. The film held on tighter and grew heavier.
That shift changed what the plastic itself went through. Bacteria carrying genes for enzymes involved in oxidizing and breaking down plastic, including alkane hydroxylase, copper oxidase and esterase, became more abundant under bloom conditions. Infrared spectroscopy showed a rise in oxidized chemical groups on the plastic surface, and scanning electron microscopy revealed a denser pattern of fine cracks. Both are signs that the material was becoming brittle, the stage before it fragments into microplastic particles.
No single plastic-eating microbe was responsible. The researchers attributed the acceleration to the whole community, photosynthetic bacteria and ordinary bacteria working as a layered network, which widens the picture of how plastic degrades in real water bodies rather than in controlled tests with one strain.
The study also compared polyethylene with polylactic acid, a plant-based plastic marketed as biodegradable, and found different bacterial groups dominating on each surface.
"This research is meaningful in that it looked at plastic pollution and algal blooms not as separate problems but as one environmental problem in which each affects the other," Myung said. "As climate change makes blooms more frequent, I expect this will serve as important evidence for building integrated environmental management strategies that consider water quality management and waste plastic management together."
The paper appeared in the journal Water Research on May 25.
(Reference Information)
Journal: Water Research
Title: Eutrophication drives taxonomic and functional trajectories in plastic-associated biofilms
DOI: 10.1016/j.watres.2026.126183
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