Effect Of Cell Concentration And Environmental Factors On The Production Microcystin Microcystis Aeruginosa

In recent years, eutrophication has affected more and more freshwater lakes in China and cyanobacteria blooms have become one of the severe environmental problems. A variety of cyanobacteria genera produce toxins, among which microcystin (MC) is the main type, threatening public health and socio-economic development. But its physiological function to itself remains unclear and it was considered as secondary metabolite in the past. The production of many bacterial secondary metabolites is controled by quorum sensing. However, there is very limited research on the effect of cell concentration on microcystin production. In this study, we used Microcystis aeruginosa PCC7806 which was a widely studied strain in the past to detect the change of cellular quotas of MC (QMC) under different cell concentration by high and low inoculation. The result showed that QMC kept the same level in the whole culture process, except a slightly increasing during the initial period after inoculation. There was not a cell concentration threshold to suddenly promote QMC or quorum sensing effect. We speculated that the production of MC is constitutive. The synthesis process began at the early phage of culture, rather than only in the logarithmic phage as a number of secondary metabolites. This work is the first to evaluate whether there is quorum sensing effect on MCs production by Microcystis aeruginosa.Dianshan Lake is a main drinking water source for Shanghai city and surrounding area. Since two decades ago, it has become eutrophic and Microcystis aeruginosa is the dominant species. In this study, Microcystis aeruginosa FACHB-911 which was isolated from Dianshan Lake was used to study the influence of environmental factors on QMC by statistical methodology., including factorial design, steepest ascent method and central composite design (CCD). The result of factorial design showed that light intensity, iron and phosphate were significant environmental factors on QMC. By steepest ascent method and CCD, a quadratic model was built to show the relationship of environmental factors and QMC: QMC=6.68+2.09×x1-15.56×x2+0.477×x3-0.215x1×x2-0.023x1×x3+2.22×x22 (x1:light, x2:iron,x3:phosphate)According to the model, the interactive factor effects on QMC were found between light and iron as well as light and phosphate, which might explain the conflict conclusions in many studies in the past. If iron and phosphate kept in constant level, and when Fe+0.107×P<9.72, QMC would increase with increasing light intensity; when Fe+0.107×P>9.72, light would suppress MCs production. If only considered the impact of light, when levels of P and light were constant, if Fe-0.048×light>3.5, Fe had positive effect to QMC; if Fe-0.048×light<3.5, higher Fe concentration brought lower QMC. If Fe and light were unchanging, QMC would increase with increasing phosphate level if light intensity was less than 21.02μmol m-2 s-1, but would decrease with increasing phosphate level if light intensity was higher than 21.02μmol m-2 s-1.This work is for the first time to use statistical methodology to study the interactive factor effects on QMC of a strain of MC-producing cyanobacteria isolated from Dianshan Lake.