Cyanobacterial blooms occur increasingly often and cause ecological,economic and human health problems worldwide.Microcystins(MCs)are the dominant toxins produced by cyanobacteria and are implicated in epidemic disease and environmental problems.Extensive research has been reported on the various regulating factors,e.g.,light,temperature,nutrients such as nitrogen and phosphorus,p H,iron,xenobiotics,and predators,that influence microcystin(MC)synthesis,but little is known about the effects of cyanobacteria-associated bacteria on MC synthesis.A considerable number of studies have focused on interactions between Microcystis species and their associated bacteria.In this study,we evaluated the effects of MC-degrading bacteria(MCDB)on MC synthesis gene mcy D expression and MC synthesis in axenic strain PCC7806,non-axenic strain FACHB905,and colony strain FACHB1325 of Microcystis by quantitative real-time polymerase chain reaction(RT-PCR)assay and enzyme-linked immunosorbent assay(ELISA).We demonstrate for the first time that MCDB can induce and up-regulate the MC production and transcriptional response of the mcy D gene of toxic Microcystis.On day 4 of the culturing experiment,the intracellular MC concentration and transcriptional response of mcy D of FACHB1325 were up-regulated 1.9 and 5.3-fold over that of the control,and for FACHB905 were up-regulated 1.8 and 4.2-fold over that of the control,respectively.On day 10,the transcriptional response of mcy D was up-regulated 21.3-fold in PCC7806.These results indicate that there are interactions between toxic Microcystis and MCDB,and MCDB may play a role in regulating mcy D expression in toxic Microcystis.
An annual investigation on phytoplankton communities was conducted to reveal the effects of nutrients on phytoplankton assemblages in Lake Taihu,East China. A total of 78 phytoplankton taxa were identified. Phytoplankton biomass was higher in the northern part of the lake than in the southern part. Cyanobacteria and Bacillariophyta alternated dominance in the northern area,where algal blooms often appear,and co-dominated in the southern area. In the northern part,the proportions of cyanobacteria and Bacillariophyta varied significantly in total biovolume,both along the phosphorus(P) gradient,and between total nitrogen levels(≤3 mg/L and >3 mg/L TN). The proportions of cyanobacteria and Bacillariophyta had no signif icant variations in total biovolume along P and N(nitrogen) gradients in the southern part. Correlation analysis and CCA results revealed that P was the key factor regulating phytoplankton community structure. Nitrogen was also important for the phytoplankton distribution pattern. It was concluded that nutrient structure was heterogeneous in space and shaped the distribution pattern of phytoplankton in the lake. Both exogenous P and internally sourced Prelease needs to be considered. N reduction should be considered simultaneously with P control to efficiently reduce eutrophication and algal blooms.
