Preliminary Study On The Mechanism Of Cyanobacteria Advantage Maintenance In Lake Taihu

Eutrophication is one of the most severe environmental problems, which is caused by the excess phosphorus that serves as a key element limiting primary productivity in aquatic ecosystems. Surface waters in Lake Taihu are increasingly threatened by phosphorus (P)-related eutrophication. In the water of Lake Taihu, the fraction of dissolved reactive phosphorus is very small and half of the total phosphorus can be hydrolyzed as inorganic phosphate to compensate for phosphorus deficiency of algae growth. Many kinds of organic phosphate are characterized by31P NMR. Organically bound P is for the most part not directly available to living organisms. To be taken up, organic P should first be converted to orthophosphate. However, some organic P can’t be utilized by alga. What’s more, many studies about the interaction between bacteria and alga or secreting the growth-promoting factor, which promote the growth of alga. Many studies have focused on the effects of dissolved organic phosphorus (DOP) or bacteria on cyanobacteria separately, while there are relatively few studies about the assisting role of bacteria playing in the utilizing of DOP. In this study, three DOP compounds and five bacterial strains isolated from the Lake Taihu bloom waters were used to study the mechanism of the growth-promoting effects of bacteria on blooming algae in Lake Taihu.In this study, the three DOP compounds (G-6-P, ATP and LEC) were separatly added to each bacterial suspension and incubated. The incubations were harvested and filtered. The supernatants were collected as the digested solutions of DOP [DS(DOP)]. Each of the fifteen DS(DOP) was added to P-free Microcystis aeruginosa (Ma) culture and the algal cell densities were determined and the growth condition was observed. The results demonstrated that six of the fifteen digested solutions had significantly growth-promoting effects on Ma. They were as follows:treatments with digested solutions of G-6-P by bacterium Y4[DSy4(G-6-P)], treatments with digested solutions of ATP by bacterium H9[DSH9(ATP)] or Zf [DSZf(ATP)] and treatments with digested solutions of the three DOP digested by bacterium RI [DSRI(DOP)].The next step was to select two bacteria RI and Y4, determinating dissolved inorganic phosphorus (DIP) and alkaline phosphatase activity (APA) in the Ma culture with digested solutions. The results demonstrated that the DIP of Ma cultures treated with DSRI(DOP) and DSY4(G-6-P) were significantly higher than those treated with control. Meanwhile, the status of DIP was also reflected by changes in APA. APA of Ma cultures treated with DSRI(DOP) and DSY4(G-6-P) were significantly lower than those treated with DOP. The results above demonstrated that with the assistance of the bacteria, the initial unavailable organic phosphorus was decomposed into bioavailable DIP to support the growth of cyanobacteria. So the bacteria-mediated decomposition of organic phosphorus may be one cause of DIP accumulation, which caused the eutrophication of lake.