Effects Of Heterotrophic Bacteria On Cyanobacterial Growth And A Potential Controlling Measure

With the rapid development of human society, most of freshwaters are recently threatened by eutrophication around the world due to the excessive nitrogen and phosphorus loading, as well as other nutrient inputs. Eutrophication has become one of the most serious environmental problems globally, which takes negative impacts on the structure and function of aquatic ecosystem. Present studies show that nitrogen is the primary factor to regulate algal growth during cyanobacterial bloom seasons and dissolved organic nitrogen accounts for a considerable proportion of nitrogen pollution sources, but most of them cannot be directly used by algae. Previous studies report that heterotrophic bacteria are thriving among blooms. However, the relationship between these bacteria, dissolved organic nitrogen as well as the occurrence of cyanobacterial blooms is less known. In the present study,5 heterotrophic bacterial strains(Pseudomonas sp. A3CT; Pseudomonas sp. A3CB; Alcaligenes sp. Zfl; Acinetobacter sp. Y3 and Gordonia sp. R1) were isolated and screened from Meiliang Bay of Taihu Lake during occurrences of cyanobacterial bloom to explore the potential effects and mechanism of heterotrophic bacteria on maintenance of cyanobacterial advantage in Taihu Lake.1. Stimulating effects of Pseudomonas sp. A3CT on Microcystis aeruginosa in eutrophic water. The decomposed DON solutions of six compounds (four free amino acids and two combined amino acids) were used to test the stimulatory effect of nutrient regeneration by the bacterium. The results indicated that M. aeruginosa grew much better under the six decomposed DON solutions than the corresponding undigested DON forms. Especially, the decomposed L-lysine solution, not only avoided the inhibiting effect of lysine on M. aeruginosa, but instead significantly promoted the cyanobacterial growth. Further chemical tests indicated that Pseudomonas sp. A3CT transformed DON into NH4+, which contributed to the fast growth of M. aeruginosa. This phenomenon revealed the promoting effects and the mechanism of heterotrophic bacteria on cyanobacterial growth.2. Different effects of Pseudomonas sp. A3CT on M. flos-aquae and M. aeruginosa in eutrophic water. Growth of the two Microcystis strains was assessed in co-cultured with Pseudomonas sp. A3CT and lysine solution. Results indicated that Pseudomonas sp. A3CT promoted M. aeruginosa growth but inhibited M. flos-aquae growth. Further chemical tests indicated that growth stimulation of M. aeruginosa by Pseudomonas sp. A3CT was significantly related to NH4+ concentration. While cadaverine possibly acted as a growth inhibitor to M. flos-aquae. Cadaverine addition tests also showed that Pseudomonas sp. A3CT promoted M. aeruginosa growth but inhibited M. flos-aquae growth. The different effects of cadaverine on growth of the two Microcystis strains suggested that Pseudomonas sp. A3CT might play a role in intrageneric succession patterns observed during Microcystis blooms in Taihu Lake.3. Impacts of heterotrophic bacteria on M. aeruginosa along different gradients of organic carbon and nitrogen. M. aeruginosa were cultured in prepared microcosms with or without a freshwater bacterial community under 9 gradients of glucose and nitrate. Results indicated that bacteria were superior competitors for N compared to M. aeruginosa. Additionally, it was interesting to note that the abundance of M. aeruginosa and the bacteria, with the largest species number, were nearly equal in the treatment with 300 ?mol C L-1 and 150 ?mol N L-1 additions.The results suggested that this glucose:nitrate ratio may be suitable for the balance of M. aeruginosa and the bacterial community as well as the maintenance of bacterial diversity. Therefore, availability of dissolved organic carbon and nitrate may fundamentally affect the structure of pelagic ecosystem.4. Degradation activity and mechanism of white-rot fungus Trametes versicolor F21a on M. aeruginosa. The above research indicated that aquatic microorganisms might play important roles in the maintenance of cyanobacterial advantage. There are a number of studies conducted on controlling cyanobacteria by microorganism. The controlling measures generally focus on lytic bacteria with ability of inhibiting blooms and degrading cyanotoxins. However, such capabilities are rarely reported on fungi. The white-rot fungal strain Trametes versicolor F21a were tested its degradation ability on M. aeruginosa as well as its enzyme activities. After T. versicolor F21a and M. aeruginosa were co-incubated for 60 h,> 96% of Microcystis spp. cells were degraded by T. versicolor F21a. The activities of extracellular enzymes showed that cellulase, ?-glucosidase, protease, and laccase were vital to M. aeruginosa degradation in the early stage (0 h to 24 h), while ?-glucosidase, protease, laccase, and manganese peroxidase in the late stage (24 h to 60 h). The positive and significant correlation of the degradation rate with these enzyme activities indicated that these enzymes were differently involved in the degradation of M. aeruginosa cells at different phases. Therefore, the fungal strain can be used to develop a new measure for microorganism control cyanobacterial blooms.