The Genetic Diversity Of Cyanophage And Cyanobacteria And The Horizontal Gene Transfer Between Them In Lake Dianchi

With the increasing development of social economy,environmental problems caused by the severity of eutrophication and high frequency of cyanobacterial blooms in marine and freshwater have become a concern all over the world.As continuing outbreak of cyanobacterial blooms during the high temperature season,which could not only break the structure and the balance of hydrophytic ecology in the water but also pose serious health threats and restricts the development of economy due to the production of toxic compounds.Lake Dianchi is the largest freshwater lake in southwest of China,which has the extremely important ecological status and function.However,a serious environmental problems were caused by eutrophication and annual cyanobacterial blooms.Microcystin(MCs)produced by harmful Microcystis blooms had caused great concern to public health and ecosystem.It was known that microcystin were synthesized nonribosomally by an integrated peptide-polyketide synthetase system encoded by the microcystin synthetase(mcy)gene cluster.Microcystis are usually comprised of toxic and non-toxic strains in aquatic environment,which the abundance of toxic Microcystis and toxic proportion are very important factors in determining the microcystin concentration.Cyanophages are defined as viruses that specifically infect cyanobacteria.It has great potentialities in controling cyanobacteria blooms due to its important ecological status and could change structural of the host population.This dissertation take eutrophic Lake Dianchi as research object,focuses on the genetic diversity and horizontal gene transfer of cyanobacteria and cyanophage,the pollution of cyanobacteria blooms and associated MCs,the relationship between environmental factors and microcystins.The main conclusions are listed as follows:(1)The water quality parameters of Lake Dianchi,including water temperature,pH value,total phosphorus(TP),total nitrogen(TN)and chlorophyll-a were analyzed.The result showed that the appropriate temperature,alkaline pH values and higher nutrient concentration within Lake Dianchi were suitable for the growth of cyanobacteria.The content of chlorophyll-a reduced gradually and were seriously influenced by total phosphorus from north to south in Lake Dianchi.Correlation analysis showed that the concentration of chlorophyll-a were significantly related to TP,CODMn,pH,water temperature,but were negatively correlated with SD(P<0.1).(2)According to microscopic observation and algal cell count,the dominant algaes in Lake Dianchi was Microcystis.It included different morphological Microcystis species:Microcystis aeruginosa,Microcystis viridis,Microcystis wesenbergii,Microcystis ichthyoblabe,Microcystis smithii,Microcystis pseudofilamentosa,Microcystis flosaquae.The phylogenetic tree producted from psbA gene sequences showed a high diversity of cyanobacteria species in Lake Dianchi.The different psbA gene sequences of cyanobacteria were belonged to Microcystis,Oscillatoria,Synechocystis,Chlorella,Synechococcus and Microcoleus.The proportion of Microcystis’ psbA gene among the obtained sequences was the highest(60%).(3)The toxic Microcystis and no-toxic Microcystis were co-existed in Lake Dianchi.The distribution of toxic Microcystis among the entire lake was uneven.The abundances of toxic Microcystis and total Microcystis population were relatively higher in the high trophic lake regions and,in parallel,the proportion of toxic Microcystis and total Microcystis population were also higher when the water reached a high trophic level.In addition,toxic Microcystis and total Microcystis population were higher in summer,the proportion of toxic Microcystis was higher in winter.Correlation analysis revealed that chlorophyll a,toxic Microcystis and tatal Microcystis were significantly positive to water temperature,pH,TP and CODM(P<0.1),which were significantly negative to SD(P<0.1).(4)The concentration and compsition of three microcystin variants(MC-RR、MC-YR、MC-LR)were varied significantly in different sampling sites by using the high-pressure liquid chromatography.The main microcystin variants were MC-RR and MC-YR in Lake Dianchi.The total microcystin concentration in bottom was higher than that in surface water.The total microcystin concentration was higher in summer and autumn.Correlation analysis showed that microcystin concentration was significantly positive related to toxic Microcystis,total Microcystis population,chlorophyll a,TP,CODMn,pH,water temperature(P<0.1),but was negatively correlated with SD(P<0.1).(5)The phylogenetic diversity of cyanophage based on psbA gene in Lake Dianchi was studied.Phylogenetic analyses demonstrated that the majority of obtained psbA sequences formed five clusters,with their sequences being more closely related to those from freshwater samples than the sequences obtained from marine waters,suggesting that psbA genes in terrestrial aquatic environments were different from those in marine environments.The sequences from freshwater have an evolutionary history that is distinct from that of their marine counterparts(6)According to the phylogenetic tree producted from psbA gene of cyanobacteria and cyanophage,the horizontal gene transfer and genetic relationship between them were analysised.The result showed that the horizontal gene transfer of psbA occurred frequently between cyanobacteria and cyanophage.The psbA gene transfer was occured in Chlorella sp.,Microcystis sp.,Synechococcus sp.and Microcoleus sp.of cyanobacteria between cyanophage,which has the higher frequency(close to 50%)in Chlorella sp.