How warming impacts the photosynthetic physiology of the bloom-forming cyanobacterium, Microcystis aeruginosa, under UV exposure.

Microcystis aeruginosa is a common cyanobacterium leading to algal blooms. Coupled effects of temperature increase and UV radiation increase will affect its photosynthesis performance, which may in turn will affect its proliferation and distribution, and change the environmental health of the water body. In this study, M. aeruginosa FACHB 469 was incubated at 25 °C and 30 °C and subjected to photosynthetically active radiation (PAR) and UV radiation (PAR + UVR) to monitor the relevant physiological responses. Exposure to both PAR and PAR + UVR resulted in a decline in PSII maximum quantum yield of M. aeruginosa, with UVR having more significant inhibitory effect. Meanwhile, UVR significantly increased the PSII photoinactivation rate constant (K_pi) and decreased the PSII repair rate constant (K_rec), whereas the warming did not have a significant effect on it, and no significant interaction effect between warming and UVR was observed. Further analysis of the strategies of algal cells to cope with UVR at different temperatures revealed that at 25 °C, algal cells mainly relied on the repair cycle of PSII, and reduced the content of phycocyanin to lower light energy capture, and increased superoxide dismutase (SOD) and catalase (CAT) activities to alleviate the damage of UVR; whereas under warming conditions, algal cells, while relying on PSII repair, mainly photoprotect by strengthening the NPQ mechanism, thus improving their tolerance to UVR. These findings suggest that the differential strategies employed by M. aeruginosa to cope with UVR under varying temperature conditions may influence the resilience of cyanobacterial blooms to environmental stressors in the future.