| Microcystis aeruginosa（M.aeruginosa）is one common cyanobacteria in harmful algal blooms in fresh water.Systematic investigation of algal growth,toxin production and degradation of M.aeruginosa has representative and guiding significance for prevention and management of harmful algal blooms.In this paper,algal growth,changes of water quality parameters,and toxin production of M.aeruginosa under different illumination intensities,temperatures and nitrogen to phosphorus（TN/TP）ratios have been investigated,as well as their inner relationships.Meanwhile,impacts of lake sediment,illumination intensity,organic matter and temperature on degradation of microcystins in crude cyanobacterial extracts solution have also been studied.Grinding method for intracellular microcystins has been developed.Results show that,a suitable illumination intensity for growth of M.aeruginosa was 500-2000 lux;when illumination intensity was≥2500 lux,photooxidative death may occur which resulted in mass algal death,and dramatic increase of extracellular microcystins.The highest growth rate and toxicity occurred at different temperatures.M.aeruginosa had higher growth rate at 25?C and 32?C,while higher intracellular microcystins were monitored at 10?C and 15?C.Nutrient level could result in different algal sizes,while had little influence in intracellular microcystins content.A certain TN/TP ratio for nitrogen-or phosphorus-limiting may not exist.Death of algal cells and degradation of microcystins could lead to the increase of nitrogen and phosphorus content in water.Water pH value was expected to be an indicator in monitoring and prevention of algal blooms for water resources.When algal density reached 1.6×107 individual/L,extracellular microcystins increased rapidly.Intracellular microcystins were mainly dependent on algal density.Intracellular microcystin content for an individual cell was calculated for first time in this paper,specifically,19.06×10-3-56.98×10-3 pg for MC-RR,3.75×10-3-16.79×10-3 pg for MC-LR,and 4.64×10-3-11.98×10-3 pg for MC-YR.In crude cyanobacterial extracts solution of M.aeruginosa,degradation of microcystins had no lag time with existence of lake sediment,following a zero-order kinetic model.Degradation rate for MC-RR was the biggest,and half-time for MC-RR,-LR and-YR was 60-84 h.Lake sediment and illumination intensity had little influence in degradation of microcystins,degradation rate was slow at low organic matter content and low temperature.Degradation of microcystins had a lag time of 72-96 h under different illumination intensities,organic matters and temperatures.Three microcystins had a lag time of 96-156 h at higher organic matter content and 25-32?C.Main mechanism in degradation of microcystins by lake sediment included adsorption and microbial degradation.No lag time during degradation of microcystins may be due to abundant existence of microorganisms in lake sediment.With existence of algal pigments（such as chlorophyll-a）and other organic matters,microcystins could be degraded under visible light irradiation.And degradation rate was faster with more organic matters.In this paper,important effects of temperature and illumination on growth of M.aeruginosa,and the strong adaptability of M.aeruginosa to different nutrient levels were figured out.Here are suggestions:first,strengthen preventive measures to guarantee drinking water safety during strong illumination and high temperature times（for example,summer）;second,waterbody（including sediment）and algal contents could promote degradation of algal toxins,and this characteristics could be used in prevention and management of algal bloom;last,also the most important,reduce nutrient input.Only by this way,could we control eutrophic waterbody and suppress algal bloom. |
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