Ecophysiology Research In Growth And Microcystins Production Of Toxic Microcystis Population

With the acceleration of industrialization and urbanization,aquatic ecosystems are receiving excessive nitrogen and phosphorus loads derived from intensive anthropogenic activities,resulting in the global occurrence of eutrophication and cyanobacterial blooms.The main concern regarding the presence of cyanobacterial blooms is the production of cyanotoxins.Among the diverse cyanotoxins,highly prevalent hepatotoxic microcystins(MCs)can seriously threaten the aquatic ecosystem health and water system functions.In this study,Lake Taihu and Yanghe Reservoir were chosen as study areas.Samples of water and phytoplankton were taken in Lake Taihu and Yanghe Reservoir during the bloom season of 2015.The concentrations of MC-RR,MC-LR and MC-YR were analyzed using high-pressure liquid chromatograph.And the abundance of Microcystis and microcystin-producing Microcystis were subsequently detected by real-time quantitative polymerase chain reaction,targeting cpcBA and mcyB genes,respectively.The relations between MCs-producing Microcystis,MCs production,and environmental variables were investigated,aiming to identify the critical factors influencing MCs concentrations in the two lakes.Furthermore,an axenic Microcystis aeruginosa(M.aeruginosa)strain was selected as a model organism to discover the physiological role of different nitrogen and phosphorus sources in regulating the growth and MCs production of Microcystis.A series of physiological changes in M.aeruginosa,such as growth status,photo synthetic activity and MCs production were monitored,along with the transcription of key regulator genes,under different bioavailable nitrogenous or phosphorous compounds supplies.The main contributions of the study are as follows:(1)Toxigenic cells contributed to 8.94%-75.68%and 7.87%-58.69%of the total Microcystis in Lake Taihu and Lake Yanghe,respectively.The total MCs concentration was 0.78-14.69 ?g/L in Lake Taihu,and was 0.24-10.05 p.g/L in Lake Yanghe,with MC-RR being the most abundant variants in both lakes.The dynamics of toxigenic cells and MCs production were positively associated with ammonium concentration in Lake Taihu,while positively associated with the concentrations of total phosphorus,total dissolved phosphorus and orthophosphate in Lake Yanghe.The significant relationship between TLR eq(total MCs after transformation of MC-RR and MC-YR into MC-LR)and chlorophyll-a(chl-a)concentration implied that chl-a could be an alternative measure to predict MCs risk in the two lakes,and the safe threshold value of chl-a was proposed as 25.38 and 31.06 ?g/L in Lake Taihu and Lake Yanghe,respectively.(2)Increases in nitrate concentration significantly enhanced M.aeruginosa growth.Furthermore,although ammonium was more beneficial to MCs synthesis than nitrate or urea,it exhibited toxicity at higher concentrations(7 and 15 mg-N/L)thereby suppressed cell growth.Urea could stimulate M.aeruginosa growth,but it had little effect on MCs synthesis.In addition,the higher levels of MCs component amino acids,especially leucine in cells growing under high concentrations of ammonium(7 and 15 mg-N/L),led to remarkably higher intracellular MCs(IMCs)content in these cells,with the correlation between IMCs content and MCs component amino acids reaching 0.626(p<0.001).Results further showed that nitrogen affected MCs release via regulating M.aeruginosa cell density.(3)Increases in inorganic phosphorus(dipotassium hydrogen phosphate)concentration significantly enhanced M.aeruginosa growth,while high concentration of organic phosphorus(?-sodium glycerophosphate)was unfavorable for M.aeruginosa growth.M.aeruginosa grown on organic phosphorus demonstrated high alkaline phosphatase activity,hydrolysing the organophosphorus compounds to cope with phosphorus deficiency.The expression of genes involved in phosphorus acquisition,including sphX,pstS,and phoX was strongly upregulated under low phosphorus conditions.M.aeruginosa adapt well to conditions of phosphorus deficiency by enhancing phosphorus uptake and hydrolyzing organic phosphorus.IMCs concentration and transcription of mcyA and mcyD gene exhibited no significant differnences under the different phosphorus concentrations,no matter which phosphorous compound was supplied,indicating that changes in phosphorus concentration had no effect on MCs synthesis.However,inorganic phosphorus affected MCs release via regulating M.aeruginosa cell density.This study elucidated the effects of nitrogen and phosphorus on apparent growth changes in M.aeruginosa,and revealed the regulation mechanisms of nitrogen and phosphorus on the production and release of microcystins from ecophysiological aspects.