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.