Effects of Algal Utilization of Dissolved Organic Phosphorus by Microcystis Aeruginosa on Its Adaptation Capability to Ambient Ultraviolet Radiation

Abstract

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.