Plastic-algae interactions in freshwater and marine systems: mechanisms, evidence and prospects for bioremediation.

Plastic pollution is among this century's defining environmental problems, and aquatic systems carry a disproportionate share. As polymers fragment into microplastics and nanoplastics they disperse through rivers, lakes and oceans, harming organisms and entering food webs. Algae have attracted growing attention as a biological complement to physical and chemical clean-up. This review synthesises peer-reviewed studies published between 2015 and 2026 on interactions between plastics and freshwater and marine algae. We examine eukaryotic microalgae from freshwater (Chlorella, Scenedesmus, Coccomyxa, Uronema) and marine systems (Navicula, Phaeodactylum, Skeletonema, Dunaliella); cyanobacteria (Phormidium, Oscillatoria, Anabaena, Nostoc, Arthrospira), treated separately as prokaryotes; and the macroalga Sargassum. Reported outcomes range widely from a few percent of low-density polyethylene surface-altered by cyanobacteria, to roughly a quarter of mixed-polymer film attributed to Coccomyxa, and up to ~ 27% of high-density polyethylene by a freshwater epiplastic consortium, along with many studies that document surface colonisation, hetero-aggregation and physiological stress rather than complete breakdown. We explain the mechanisms involved: extracellular enzymes proposed to cleave polymer chains, the extracellular polymeric substances that anchor cells to plastic, reactive-oxygen-driven oxidation and the staged formation of the plastisphere. The supporting evidence is graded from gene prediction to demonstration by a purified enzyme. Most outcomes prove to be physical capture or surface modification, and no study reviewed demonstrates mineralisation. We assess engineered diatoms and green algae expressing bacterial PET hydrolases and identify what field application would require: synthetic consortia, omics-based enzyme mining, photobioreactor integration, machine-learning-guided enzyme design, and standardised protocols focused on mineralisation.