Frequent outbreaks of harmful algal blooms (HABs) represent one of the most serious outcomes of eutrophication, and light radiation plays a critical role in the succession of species. Therefore, a better understanding of the impact of light radiation is essential for mitigating HABs. In this study, Chlorella pyrenoidosa and non-toxic and toxic Microcystis aeruginosa were mono-cultured and co-cultured to explore algal responses under different nutrient regimes. Comparisons were made according to photosynthetically active radiation (PAR), UV-B radiation exerted oxidative stresses, and negative effects on the photosynthesis and growth of three species under normal growth conditions, and algal adaptive responses included extracellular polymeric substance (EPS) production, the regulation of superoxide dismutase (SOD) activity, photosynthetic pigments synthesis, etc. Three species had strain-specific responses to UV-B radiation and toxic M. aeruginosa was more tolerant and showed a higher adaptation capability to UV-B in the mono-cultures, including the lower sensitivity and better self-repair efficiency. In addition to stable μ_max in PAR ad UV-B treatments, higher EPS production and enhanced production of photosynthetic pigments under UV-B radiation, toxic M. aeruginosa showed a better recovery of its photosynthetic efficiency. Nutrient enrichment alleviated the negative effects of UV-B radiation on three species, and the growth of toxic M. aeruginosa was comparable between PAR and UV-B treatment. In the co-cultures with nutrient enrichment, M. aeruginosa gradually outcompeted C. pyrenoidosa in the PAR treatment and UV-B treatment enhanced the growth advantages of M. aeruginosa, when toxic M. aeruginosa showed a greater competitiveness. Overall, our study indicated the adaptation of typical algal species to ambient UV-B radiation and the stronger competitive ability of toxic M. aeruginosa in the UV-radiated waters with severer eutrophication.

Phosphorus (P) plays an important role in eutrophication and algal adaptation to environmental stresses; therefore, a better understanding of the impact of P is essential to control cyanobacterial bloom. In this study, Microcystis aeruginosa is treated with 5 h of ambient irradiation in the culture medium with different availabilities of dissolved organic P (DOP) and dissolved organic matter (DOM) to explore algal physiological responses. Compared to photosynthetically active radiation (PAR), ambient UV-A and UV-B radiation exerted oxidative stresses and has inhibitive effects on the growth and photosynthesis of M. aeruginosa. However, M. aeruginosa had a strong adaptation capability, and the negative effects of UV radiation can be alleviated with DOM addition in the DOP-rich medium. The adaptation of M. aeruginosa to UV radiation in the DOP-rich waters can be mainly achieved through hydrolysis of DOP and metabolism of dissolved inorganic P (DIP), and the DOP utilization behaviors of M. aeruginosa might greatly affect algal UV adaptation capability. In the DOP-rich medium without DOM, the great inactivation of algal alkaline phosphatase (APase) after UV radiation could result in worse affinity for DOP, slower DOP uptake and lower cellular P quota. Consequently, the P demand of M. aeruginosa could not be satisfied for UV adaptation processes, including decreasing UV-induced damages and promoting self-repair. However, DOM could act as an antioxidant and significantly decrease APase inactivation of UV-radiated M. aeruginosa. In the DOP-rich medium with DOM, DOP utilization by M.aeruginosa in the UV-A and UV-B treatments is promoted and algal demand for P is satisfied for its adaptation, such as enhanced production of photosynthetic pigments, increased superoxide dismutase (SOD) activity, recovery of photosynthetic efficiency, etc. Overall, our findings indicate the close relationship between algal DOP utilization and the adaptation to ambient UV radiation of typical cyanobacteria in DIP-limited and DOP-enriched natural waters.