Phosphorus strategy in bloom-forming cyanobacteria (Dolichospermum and Microcystis) and its role in their succession

Dolichospermum (formerly Anabaena) and Microcystis cause harmful cyanobacterial blooms in freshwater ecosystems worldwide. Input reduction of both nitrogen (N) and phosphorus (P) are commonly recognized as basic ways of controlling blooms, but little is known about the roles of nutrients and their using strategy among cyanobacteria in triggering the succession of diazotrophic to non-diazotrophic cyanobacteria. In this study, we investigated in situ responses of cyanobactria to ambient P status during the transition from Dolichospermum flos-aquae to Microcystis spp. in Lake Taihu and Lake Chaohu. While dominant in phytoplankton community, D. fibsaquae experienced P deficiency as evidenced by qualitative detection of extracellular phosphatase via enzyme labeled fluorescence (ELF). The percentage of ELF-labelled D. fios-aquae cells was 33% when it dominated the phytoplankton community, and was 78% when it co-dominated with Microcystis spp., indicating an increase in P deficiency. Meanwhile, no ELF-labelled Microcystis cells were observed while polyphosphate body (PPB) were present, suggesting that Microcystis spp. were not P deficient. Additionally, the percentages of Microcystis cells containing PPB showed an inverted "U-shaped" relationship with concentrations on soluble reactive phosphorus (SRP). To validate the field observation, a laboratory study of the monocultures of the dominant cyanobacteria was conducted. Extracellular alkaline phosphatase activity (APA) and PPB accumulation were regulated by P availability in monocultures of D. flos-aquae. Interestingly, no cell bound extracellular phosphatase was found on Microcystis aeruginasa even in the culture without P supply. Consistently, the expressions of phosphatase encoding gene phoX showed no differences among the treatments. The way in which PPB accumulation occurred in Microcystis spp. in response to P availability in the cultures was similar to that observed in the field, demonstrating a strategy of energy conservation over P accumulation. The competitive advantage of Microcystis spp. was displayed at low P concentrations: where it could rapidly uptake and store inorganic P, which also increased the P deficiency of the coexisting phytoplankton species. Responses of P-transport gene pstS confirmed this hypothesis. The physiological and molecular mechanisms mentioned above enable Microcystis to survive and proliferate in environment with low available P supply more efficiently. In conclusion, different cyanobacterial species have distinct ways of responding to P availability, suggesting that the control of cyanobacterial blooms by targeted nutrient reduction is largely dependent upon the dominant species. P reduction is more effective in controlling diazotrophic cyanobacteria than non-diazotrophic cyanobacteria.