The Response Of Bacterial Community To A Cyanobacteria Bloom In Dianchi Lake And Algicidal Characteristics Of Bacillus Siamensis Sp37

The water eutrophication is a worldwide environmental problem,a serious consequence is the outbreak of algal blooms.Bloom can cause severe problem,e.g.,endanger aquatic ecosystems,reduce the utilizability of water resources,and directly threaten human health as well as other organisms by algal toxins.Besides algae,bacteria are important part and play important roles in aquatic ecosystems,and they they has close relationship with algae.The utilization of bacteria in the biocontrol of bloom has received a lot attention.Dianchi,the largest plateau lakes in China with high economic and ecological importance,has been facing serious cyanobacterial bloom problem from the past two decades.In this study,we analyzed the dynamics of bacteria community response to the cyanobacteria bloom in Dianchi,isolated the bloom-causing algae and multiple algicidal strains,and investigated the algae-lys ing mechanism of one algicidal bacterium on Microcystis aeruginosa DCM4.The research can provide theoretical guidance and resource of algicidal bacteria for the biocontrol of algal blooms.The main results are listed as follows:1.Dynamics of bacteria community response to the cyanobacteria bloom.Dianchi Lake was in an eutrophic state,and the bloom of Dianchi Lake in summer of 2015 was mainly caused by Microcystis aeruginosa.Among the environmental parameters,only the pH was significant different between bloom and control area,and the overall environmental conditions in the bloom area was more similar than in the control area indicated by PCA analysis.This bloom significantly increased bacterial dens ity,decreased bacterial diversity and significantly changed the bacterial community structure.The environmental factors that bacterial community sensitive to varied in bloom and control area:pH,NO₃^-,and COD significantly correlated with bacterial community in bloom s ites and SS,TN,NH₄⁺,NO₃^-and COD significantly correlated with bacterial community in control sites.pH,COD and NO₃^-were the main factors responsible for the environmental differences between the bloom area and control area,and they were also important environmental factors that attributed the differences in bacterial communities in these two areas.Among the bacteria?113?associated with Microcystis aeruginosa,94.7%?107?were negatively correlated with Microcystis aeruginosa.Mainly of these bacteria were from the bacterial communities with lower abundance in the severe blooms such as Actinobacteria,Proteobacteria and Bacteroidetes.2.A total of 13 algal strains were isolated from Dianchi Lake in different seasons:2Chlorophyta?Monoraphidium and Mychonastes?and 11 Cyanobacteria?Microcystis?.Among Cyanobacteria,7 strains are Microcystis aeruginosa,3 strains are Microcystis flos-aquae,1 strain is Microcystis wesenbergii.159 bacteria strains were isolated which were dominant by Bacillus?26.4%?,Pseudomonas?16.4%?.Among the total bacteria,35 strains were algicidal bacteria of M.aeruginosa DCM4 and mainly of them were Bacillus?57.1%?.The number of cultivable bacteria isolated in spring?66 strains?was the largest with the highest number of algae-lysing bacteria?18strains?.Some algicidal bacteria isolated in Dianchi Lake were identified as Chryseobacterium,Flavobacterium and Porphyrobacter,which were found negatively correlated with M.aeruginosa during the analysis about the OTUs correlated with M.aeruginosa.3.Strain Bacillus siamensis Sp37 showed a strong algicidal activity against M.aeruginosa DCM4?algicidal rate was 92.4±1.5%on day 4?,and it also can kill M.flos-aquae and M.wesenbergii of Cyanophyta,but not Monoraphidium and Mychonastes of Chlorophyta,indicates that Sp37 can kill Microcystis selectively without damaging other algae.This selectivity can provide ecological security at a certain degree in the control of Microcystis blooms with Sp37.Algicidal activity of Sp37 mainly relies on releasing algicides that were heat-tolerant and pH-resistant,suggesting that the algicidal substances are not proteins or peptides,and it can provide direction for the purification of the algicidal compounds.4.Study on the primary algicidal mechanism of Sp37 cell-free filtrates.The content of CAT?catalase?in algal cells of DCM4 increased significantly?p<0.05?after treated with the cell-free filtrate of Sp37 for 18h;the content of GSH?reduced glutathione?increased in initial stage and then declined;the content of MDA?malonaldehyde?increased significantly?p<0.01?after treated for18h,indicating the antioxidant system of algal cells responded rapidly when treated with Sp37cell-free filtrate.The algicidal substance of strain Sp37 severely affected the transcriptional expression of some important genes of M.aeruginosa DCM4.prx,fabZ,recA and psbA1 genes play important roles in algal cells.prx is a gene encoding for peroxiredoxin?Prx?which belongs to the thiol oxidation system.fabZ encodes?-hydroxyacyl acyl carrier protein?ACP?dehydratase,it is correlated with repair of cell membranes.recA is a gene encoding RecA,which forms filaments on the single-stranded DNA.psbA1 encodes D1 protein,a key subunit of Photosystem II?PS II?.The expression of prx was up regulated?1.15 fold relative to the untreated control?,the expression of fabZ was down regulated?0.13 fold?,the expression of recA was up regulated?3.25 fold?,and the expression of psbA1 was up regulated?b23.64 fold?when treated after 18 hours.According to the above results,substance secreted by Sp37 can bring oxidative stress to the algal cells,and cause lipid peroxidation of the algal cell membrane,damage cell membrane integrity,and cause DNA damage,affect the normal photosynthetic system operation of the algae cells and eventually leading to the death of algal cells.The relationship between algal and bacterial communities was investigated during a bloom in Dianchi Lake,and the dominant algal species and a large number of algicidal bacteria were isolated,and the algicidal mechanism of algicidal bacterium Sp37 against M.aeruginosa DCM4 were investigated.This study can provide a large number of algicidal bacteria resources and theoretical basis for the microbial control of cyanobacteria bloom.