The Survival Of Microcystis Under The Dark/Anoxic And Nitrogen-limited Conditions And The Changes Of Intracellular Material

Cyanobacterial bloom caused by eutrophication has become a global environmental problem.Microcystis was influenced by environmental factors.However,the environmental factors sometimes would become limiting factors for Microcystis.Microcystis is a common cyanobacteria causing bloom in China.During the bloom,a portion of the biomass would sink to the sediment-water interface and suffer from the dark/anoxic condition.Due to the assimilation of a large number of algae,the water would be lack of nitrogen,especially at the end of the bloom in early autumn.The dark/anoxic and nitrogen-limited condition will affect the survival and physiological process of Microcystis,and change the metabolism and release of intracellular material,thus affecting the biogeochemical cycle of water ecosystem.However,less research has been done.To investigate the survival of Microcystis under the dark/anoxic and nitrogen-limited conditions and the changes of intracellular material,a series of experiments were conducted with Microcystis.The study could help better understand survival mechanism of Microcystis under these stress condition,and provide theory support for the feedback mechanism of Microcystis.Our study focus on three aspects in following:1)To investigate the survival of Microcystis and its secretion under the dark/anoxic condition;2)To explore the increment potential and metabolites of Microcystis under the nitrogen-limited conditions;3)To study the proliferation potential of Microcystis under the nutrient-diluted condition.The results are as follows:?1?Microcystis could survive for at least 6 days under the dark/anoxic condition.Meanwhile,Dissolved organic carbon?DOC?secreted by Microcystis reached up to1.86 mg C mg/Chl a.The Microcystis in 55 d group could survive longer than other groups,and 28 d group had the maximum quantity of secreted DOC.Our study suggested that Microcystis could survive for several days under the dark/anoxic condition,and could secrete DOC during this period.These results were related to the growth period of Microcystis.?2?Under the nitrogen-limited condition,the growth of Microcystis was inhibited.After 15 days of experiment,the Chl a concentration in 25 mg NO₃^--N/L group was only 3.00 mg/L,while the concentration of Chl a in 100 mg NO₃^--N/L group reached 10.80 mg/L.However,the study found that under the nitrogen-limited condition,Microcystis can more efficiently use nitrate,and has high C/N ratio in cells?12:1?,suggesting that cell C/N ratio of Microcystis is variable,and can be affected by nitrogen limitation.The metabolites of nitrogen-limited culturing Microcystis were significantly different from the control.There were 3 differentially expressed carbohydrates,and the fold change ratios of L-Fucose was 3.89×10⁴.There were 9differentially expressed lipids,accounting for 16.7%of the total amount of lipid identified.These 9 different lipids were all metabolites for up-regulated differences.Three amino acids had distinct differences,accounting for 11.5%of the total number of amino acids identified.The fold change ratios of L-?-?-Cystine was 1.02×10^-5,which was down-regulated differences.?3?Under the condition of nitrogen and phosphorus reduction,the biomass trends of Microcystis from Lake Erie was consistent with water inlet in Taipu River.The value of?rank as dilution control group>N cut group>P cut group>NP cut group>control group.The results shows similarity between the two water bodies.The yield of cyanobacterial toxin was increased after the experiment,but the nutrient dilution and toxin production did not show correlation.