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.

Understanding phosphorus (P) cycling is essential to reducing algal blooms and other negative effects in water. However, few studies have investigated the contributions of alkaline phosphatase (APase) or alkaline phosphatase activity (APA) to P cycling in natural riparian zones. Here, seasonal and spatial development of plankton, P fractions, APA fractions, and fluorescence labeled enzyme activity were investigated in a natural riparian zone in the main diversion channel for water transfer into Lake Taihu. Plankton greatly benefited from APase, and APA production was their important advantage for P acquisition and proliferation. Fluorescence labeling varied greatly between species and seasons, indicating different adaptive strategies for dissolved organic P acquisition. Phytoplankton experienced severe P stress when they were the main APA producers in spring and summer, but bacteria were superior P competitors. During autumn and winter, free APA dominated, phytoplankton APA (phyt-APA) deceased because of low biomass or high dissolved reactive P (DRP) and bacterial APA (bact-APA) proportions were high. Bulk APA and DRP were negatively correlated for the entire data-set and during the productive seasons. Significant negative correlation was observed between phyt-APA and DRP in the lacustrine and wetland zones, but the situation was complicated for bact-APA, probably owing to organic carbon limitation.

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.

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 was treated with 5-h of ambient irradiation in the culture medium with different availability of dissolved organic P (DOP) and dissolved organic matter (DOM), to explore algal responses. Compared to photosynthetically active radiation (PAR), ambient UV-A and UV-B radiation exerted oxidative stresses and had inhibitive effects on the growth and photosynthesis of M. aeruginosa in DOP-rich medium. However, M. aeruginosa had a strong adaptation capability and the negative effects of UV radiation were eliminated with DOM addition (5 mg L^− 1 humic acid [HA]). Combining dissolved inorganic P (DIP) and DOP contents in the culture medium, adaptation of M. aeruginosa to UV radiation in the DOP-rich waters could be mainly achieved through the hydrolysis of DOP and metabolism of DIP, and the DOP utilization behaviors of M. aeruginosa greatly affected its UV adaptation capability. In DOP-rich medium without DOM, APase inactivation after UV radiation negatively affected DOP utilization by M. aeruginosa, and algal P demand for UV adaptation could not be fully met. DOM could act as an antioxidant and greatly decreased APase inactivation after UV radiation, when M. aeruginosa utilized DOP faster to produce more DIP and to increase algal carotenoid, phycocyanin production and its superoxide dismutase (SOD) activity in the DOP-rich medium, resulting in a strong UV adaptation capability. Consequently, M. aeruginosa showed the lowest adaptation capability to UV radiation in the DOP-free medium. Overall, our findings indicated the close relationship of algal DOP utilization and its adaptation to ambient UV radiation of typical cyanobacteria in DIP-limited and DOP-enriched natural waters.

Phosphorus (P) has an important role in eutrophication and it is essential to explore the processes and mechanisms of P mobility in natural waters. In this study, laboratory experiments were conducted to simulate the SW system (sediment and water) and SAW system (sediment, algae and water) under four hydrodynamic intensity conditions (static control, 50 rpm, 125 rpm and 200 rpm treatments), to investigate P mobility. Results in SW system showed that, sediment was an important source of P for overlying water, and the released total P (TP) increased with stronger hydrodynamic intensity, when P associated with metal pools (redox-sensitive P [BD-P] and meta-oxides bound P [NaOH-P]) were the most unstable and easier to migrate into the overlying water. Stronger hydrodynamic disturbances could enhance the processes including sediment resuspension, dissolution of particles, and release of P, when P mobility had a close relationship with redox conditions near sediment-water interface (SWI). Therefore, the release of TP, BD-P and NaOH-P from sediment increased and decreased in the control and 50-200 rpm treatments over time. In SAW system, the release of TP significantly increased from sediment comparing to SW system, and the growth of Microcystis aeruginosa could selectively enhance the release of BD-P, NaOH-P, and organic P (OP). Meanwhile, the released P from sediment was quickly accumulated by algal cells. The maximum accumulation ability of P by cells, the highest photosynthetic efficiency and the best growth of M. aeruginosa were observed in 125 rpm treatment. But with excessively strong hydrodynamic intensity (200 rpm treatment), the accumulation ability of P and alkaline phosphatase activity (APA) of M. aeruginosa was suppressed, which might hinder algal utilization of P and inhibit algal growth. Overall, our findings demonstrated the patterns of P mobility in natural ecosystems and could contribute to the understanding of P cycling.