Comparative growth and cellular responses of toxigenic Microcystis exposed to different types of microplastics at various doses

Microplastics (MPs) pollution frequently co-occur with Microcystis-dominated blooms in freshwaters, but MPs effects on toxigenic Microcystis growth and effect mechanisms remained poorly understood. This study used 0.5 μm-size polyethylene (PE) and polyvinyl chloride (PVC) to explore dose- and time-dependent effects of single and combined MPs (i.e., PE + PVC) on toxigenic Microcystis growth and cellular responses during 16 day-test. Results showed that Microcystis growth and cellular responses depended on exposure time, MPs dose and type. MPs elicited hormesis effect in early stage at low dose (5 mg/L), while increasingly inhibited growth with rising PVC or PE + PVC dose but declining PE dose (5, 10, 50 mg/L) in mid-late stage, with stress intensity of PE + PVC > PVC > PE. Further analyses revealed unobvious cell damage under MPs stress, largely because antioxidases were increasingly activated as MPs stress enhanced. Unicellular MCs release ability during mid stage almost coincided with total/bound amount and each fraction of ex-poly and ex-pro trends under MPs stress. Significant positive relationship existed between MCs release ability and ex-poly/ex-pro fractions and total amount of Microcystis cells along mid-late stage under MPs stress, validating that ex-poly/ex-pro production was regulated as a result of MCs release. Besides, unicellular MCs production ability was generally positively correlated with soluble, tightly-bound and total ex-poly and ex-pro at late stage. These suggested that cellular antioxidants, MCs production/release ability and ex-poly/ex-pro production of Microcystis could be coupled to exert integrated defense against MPs stress to protect surviving cells in Microcystis population. These findings are crucial for acquiring the fate of Microcystis-dominated blooms co-occurring with MPs pollution, and reasonably assessing and managing involved eco-risks.