Responses of bloom-forming Microcystis aeruginosa to polystyrene microplastics exposure: Growth and photosynthesis

Microplastics were often detected in water environment, however, their effects on algal blooms were unclear. In this study, the growth and photosynthesis capacity of bloom-forming algae Microcystis aeruginosa growing with 10–100 mg/L polystyrene microplastics (MPs) (5 μm) under low (105 cells/mL) and high (106 cells/mL) initial algal density were investigated. The results showed that microplastics had a U-shaped effect on the growth of microalgae under both initial algal densities. In the early (1 d) and end (7 d) stages of cultivation, the promotion effect was obvious, and the maximum promotion rate was (133.33 ± 14.43)%, while the promotion was weakened or even inhibited in the middle stage (2–5 d), and the maximum inhibition rate was (30.00 ± 14.14)%. Under low initial algal density, the contents of water-soluble pigments (phycoerythrin, phycocyanin and allophycocyanin) and fat-soluble pigments (chlorophyll a and carotenoids) in 50 and 100 mg/L group were inhibited at 4 d, then recovered at 7 d. Under high initial algal density, water-soluble pigment contents decreased from 4 d to 7 d, while the fat-soluble pigments continued to recover to the control. The chlorophyll fluorescence parameters of 50 and 100 mg/L groups were suppressed in the early and late stages of microalgae growth. In summary, high concentrations of polystyrene microplastics (50 and 100 mg/L) would cause significant fluctuations in the growth and photosynthesis of M. aeruginosa, and growth-promoting effects in the early and late stages of the experiment indicated that MPs had the potential to exacerbate M. aeruginosa blooms.