Studies On Blooms And Inhibitory Mechanisms Of Biological Originated Substances To The Algae In Taiyuan Region Of The Fenhe River

Harmful algal blooms(HABs)have become a widespread concern in recent decades due to their frequent occurrence in eutrophic freshwater ecosystems.Cyanobacterial blooms,especially Microcystis aeruginosa blooms,can cause serious water-quality problems in aquatic environments,such as issues of oxygen depletion,aquaculture,drinking water supply and recreational use of the waters.Moreover,toxic secondary metabolites(cyanotoxins)produced by some bloom-forming cyanobacterial species are lethal to aquatic organisms and highly poisonous to human beings and livestock.Shanxi Province suffers water shortages and there are few rivers in there.As the largest and longest river in Shanxi,the Fenhe River plays an immeasurable role in the economic development and local resident life.The Taiyuan region of the Fenhe River lies in the middle of the Fenhe River basin.With the development of industry and the rapid expansion of urban population,the eutrophication of water body is becoming increasingly serious,which leads to the deterioration of water quality.In August 2011,the Taiyuan region of Fenhe River experienced unprecedented HABs dominated by M.aeruginosa over a large area.From then on,cyanobacterial blooms occurred every year in the river.Therefore,how to control the excessive growth and reproduction of planktonic phototrophs in aquatic ecosystems has been a hot environment issue in the management of HABs.From 2012 to 2016,the phytoplankton diversity and dominant species of water bloom were investigated in the Taiyuan region of Fenhe River,and for the first time the toxic and odor producing Microcystis species in Shanxi was discovered in the study.Moreover,the environment-friendly inhibitory action and mechanisms of biological originated substances on M.aeruginosa TY001 were explored and the main results are as following:(1)From 2012 to 2016,a total of 202 phytoplankton species belonging to 72 genera and 7 divisions were identified.Among these,the Chlorophyta,Bacillariophyta and Cyanophyta were the most dominant divisions.With the change of season and year,the total number of phytoplankton species showed a rising trend,especially the Cyanophyta,Chlorophyta and Bacillariophyta.Other new species can be found every year in the monitoring station,and the numbers were on the rise.The cell densities of phytoplankton showed a high degree of spatial heterogeneity and unequal distribution in each quarter.In the five years,the average cell densities of phytoplankton were 68.85×106–74.00×106 cells L-1,194.57×106–226.70×106 cells L-1 and 45.63×106–63.00×106 cells L-1 at the ten monitoring stations in May,July and October,respectively.Meanwhile,the algal cell densities took on a rising trend,in which cyanobacteria took an absolute advantage with the change of year.The dominant species of each sampling site showed a high spatial and temporal heterogeneity with the change of season and year.There were some differences between the dominant species in each quarter of the year.In this study,a total of 19 dominant species belonging to 6 divisions were identified,among which Cyanophyta accounting for 42.11% of the total dominant species,followed by Chlorophyta(21.05%),and less other species.Water bloom of Euglena can be found every year in May in Jifen and Nanzhonghuan Bridge area,and the dominant species was E.sanguinea.From July to September every year,the dominant species became different kinds of Microcystis in different sample sites,including M.aeruginosa,M.novacekii and M.wesenbergii,respectively.(2)This study was the first report of toxic and odor producing Microcystis species in Shanxi.Results showed that five(TY001,FH0003,FH0004,FH0006 and FH0007)of the eight strains contain mcyA,mcyD and mcyE genes,and they were toxin Microcystis.The microcystin concentrations of the five Microcystis strains ranged from 32.09±2.08 to 59.76±5.32 pg/108 cells.Eight purified Microcystis strains were isolated from the samples,and their volatile odorous compounds were determined by gas chromatography and flavor analysis.Among these,six strains(TY001,FH0002,FH0003,FH0004,FH0005 and FH0006)were found to produce the odorous compound ?-cyclocitral.(3)Our results indicated that pyrogallol,like other allelochemicals,has a significant impact on photosynthesis in M.aeruginosa TY001.First,the expression of nblA gene was up-regulated,which may induce an added PBS degradation protein and further reduce the efficiency of photosynthesis.Second,the chlorophyll fluorescence parameters(Fv/Fm,PIABS,PICS)of M.aeruginosa TY001 decreased significantly except at 1 mg L-1 pyrogallol,which indicated that the full photosynthesis performance of cyanobacteria was markedly inhibited.These data suggest that photosynthesis inhibition may be an important mechanism by which pyrogallol acts on M.aeruginosa TY001.In summary,the high concentration of pyrogallol inhibited M.aeruginosa TY001 to the zero or negative growth state after 24 h exposure.Further investigation is warranted to determine more inhibitory mechanism of algae in greater detail to mitigate or eliminate harmful algal blooms in the Taiyuan region of Fenhe River.(4)Our results demonstrated that the expression of prx,ftsH,grpE,fabZ,recA and gyr B genes were up-regulated.Meanwhile,the activities of antioxidant enzymes(SOD,CAT and POD)were increased,and the stress caused MDA levels to change and lipid peroxidation to occur.These results indicate that oxidative damage and DNA damage are important toxicity mechanisms for the observed growth inhibition of the M.aeruginosa TY001.Unexpectedly,the relative transcript abundance of microcystin synthesis gene mcyB and mcyD and the absorption of the nitrogen-related gene ntcA exhibited a significantly increase compared with the control exposure to pyrogallol.These changes were responsible for the increase of microcystin production.More effective allelochemicals and the inhibitory mechanism needs to be discovered in the further research to control or eliminate harmful algal blooms.(5)The allelochemical 5,4'-dihydroxyflavone(5,4'-DHF)exposure significantly inhibited the growth of M.aeruginosa TY001.First,The ROS level and the activities of antioxidant enzymes(SOD,CAT and POD)boosted dramatically.Meanwhile,the expression of peroxidase gene prx was up-regulated.With exposure time extending,the activities of antioxidant enzymes and the expression of prx gene obviously decreased,but the ROS level increased continuously.The stress caused MDA levels to change and lipid peroxidation to occur.Moreover,the expression of fabZ gene was down-regulated.These results indicated that 5,4'-DHF stress caused oxidative damage of M.aeruginosa TY001.Second,the relative transcript abundance of recA,which showed that DNA may be damaged.Finally,a significant decrease in photosynthesis-related gene psb A expression was observed as exposure time extends,which indicated that the full photosynthesis performance of cyanobacteria was markedly inhibited.Encouragingly,the relative transcript abundance of microcystin synthesis gene mcyB exhibited a significant decrease compared with the control exposure to 5,4'-DHF,which suggested that 5,4'-DHF stress may decrease the ability of microcystins synthesis in TY001.In conclusion,oxidative damage,DNA damage and photosynthesis inhibition are the primary mechanisms for the allelopathic effect of 5,4'-DHF on M.aeruginosa TY001.The results demonstrated that the 5,4'-DHF may be a potential candidate as an effective and safe algicide to control cyanobacterial blooms.