Strategies adopted by Aphanizomenon flos-aquae in response to phosphorus deficiency and their role on growth
Abstract Background: The N2-fixing cyanobacterium, Aphanizomenon flos-aquae is a globally distributed bloom causing species that degrades water quality of freshwater and ocean worldwide. Overcoming phosphorus (P) deficiency is one of the ecological advantages for bloom forming cyanobacteria. It remains unclear to what extent can A. flos-aquae alleviate P deficiency by regulating P using strategies.Results: Based on in situ observations of extracellular alkaline phosphatase (APase) in A. flos-aquae via enzyme-labeled fluorescence in freshwater bodies in China, Poland and Czechia, we further investigated responses of isolated A. flos-aquae to different P supplies (sufficient inorganic P (Pi) as +DIP, α-glycerophosphate and β-glycerophosphate as +DOPα and +DOPβ, P-free condition as -P). The significantly negative relationships between percentage of APase producing cells and soluble reactive P concentration in fields and cultures respectively suggested that the excretion of APase was regulated by ambient Pi supply. Suffering from extreme P deficiency in the -P treatments, A. flos-aquae showed the highest APase activity reaching a vigorous growth at the early stage, which might also benefit from the formation of polyphosphate body (PPB) and the decrease of cellular P content. In the +DOP treatments, the coordination of dissolved organic P (DOP) mineralization and continuous utilization and formation of PPB might contribute to a maintenance but not reproduction of A. flos-aquae when relying on DOP, since the highest cell density reached only 13.38% of that in +DIP treatments while photochemical efficiency was comparable during the whole experiment. Luxury uptake of phosphate as PPB in the +DIP treatments was consistent to the canonical view of polyphosphate as P storage. Conclusion: A. flos-aquae could achieve an instantaneous growth in response to P deficiency with the coordination of P utilization strategies, while only a long-term sustainable growth could be maintained under DOP condition. Persistent and active reproduction could only be achieved in high Pi supply, which implying that an effective consequence can be expected for combating the bloom of A. flos-aquae when controlling P supply.