Effect of microplastics and microplastic-metal combinations on growth and chlorophyll a concentration of Chlorella vulgaris

Microplastic pollution has become a global problem and it is threatening soil and water bodies around the world. In this study, i) the sole effect of 0.5 μm sized polystyrene microplastics under different concentrations (1, 5, 50, 100, and 1000 mg/L) and ii) the combined effect of the microplastic-metal combination (Cu, Zn, Mn) were investigated on the growth and chlorophyll a content of the freshwater microalgae; Chlorella vulgaris. Results showed that lower concentrations of microplastics (1, 5 mg/L) had no impact while higher concentrations (50, 100, 1000 mg/L) significantly reduced growth and chlorophyll a content of Chlorella vulgaris. Highest inhibition ratios were found between 15.71 and 28.86% on growth and 9.2-21.3% on chlorophyll a concentration for 50-1000 mg/L of microplastic. Microplastic-metal combinations showed greater inhibition on growth and chlorophyll a concentration. It has been shown that single metal-microplastic combinations reduced growth by 47.83-49.57% and chlorophyll a concentration by 44.75-50.25%. However, a much higher impact was observed for the triple metal-microplastic combination by 70.43% and 64.09% on growth and chlorophyll a content, respectively. SEM images provided insight into the probable cause of the toxic effect as the microplastic particles were adsorbed and embedded on the algal cell. Negative effects were observed generally at the early stages of exposure and weakened over time.