Limited Potential of Polystyrene Microplastic as a Vector of Microcystin-LR in Diluted Lysate of Microcystis aeruginosa Strain MASH01-A05 in Laboratory Freshwater and Brackish Water Conditions

Microplastics (MPs) and microcystins (MCs) frequently occur together in eutrophic environments. However, their interaction in aquatic systems is poorly understood. This study aimed to examine how MP particle size and salinity influence the adsorption behaviour of the cyanotoxin MC-LR onto polystyrene MPs (PS-MPs). Two particle size groups (180–500 µm and 700–1000 µm diameter) were mixed with a microcystin-LR (MC-LR) producing Microcystis aeruginosa lysate in either freshwater (salinity ≤ 0.05 g L−1) or brackish water (salinity 16.00 g L−1) and incubated at 25 °C in an orbital shaker for 48 h. MC-LR bound to PS-MPs was extracted and measured using triple quadrupole LC-MS/MS. The MC-LR adsorption rate exhibited a degree of oscillation throughout time, with peak adsorption observed for the smaller-sized PS-MPs at 1.60% in freshwater after 4 h and 4.60% in brackish water after 6 h. For the larger particle size of PS-MPs, peak adsorption occurred after 4 h, reaching 0.1% in freshwater and 1.3% in brackish water. This study provides evidence that PS-MPs have limited potential as vectors of MC-LR in eutrophic freshwater and brackish environments.