Influence of polystyrene microplastics on the growth, photosynthetic efficiency and aggregation of freshwater microalgae Chlamydomonas reinhardtii

Microplastics are ubiquitous in aquatic ecosystems worldwide, but knowledge on their impacts on phytoplankton, especially freshwater microalgae, is still limited. To investigate this issue, microalgae Chlamydomonas reinhardtii was exposed to polystyrene (PS) microplastics with 4 concentration gradients (5, 25, 50 and 100 mg/L), and the growth, chlorophyll a fluorescence, photosynthetic activities (Fv/Fm), the contents of malondialdehydes (MDA), soluble proteins, extracellular polymeric substances (EPS) and settlement rate were accordingly measured. Results showed that the density of microalgae decreased as the increase of PS microplastics concentrations, and the highest inhibitory rate (IR) was 45.8% on the 7th day under the concentration of 100 mg/L. The high concentration (100 mg/L) of microplastics evidently inhibited the content of EPS released by microalgae into the solution. PS under all dosages tested could reduce both the chlorophyll a fluorescence yields and photosynthetic activities. The scanning electron microscope (SEM) images demonstrated that microplastic beads were wrapped on the surface of microalgae and damaged their membranes, which could suggest the reduction of photosynthetic activities and the increase of soluble proteins and MDA content. The results also showed that PS microplastics could inhibit the settlement of microalgae at the later stage, which also indicated the recovery of microalgae from the toxic environment. Our findings will contribute to understanding the effects of microplastics on freshwater microalgae, as well as evaluating the possible influences of microplastics on aquatic ecosystems.