Inhibitory Effect Of Oocystis Borgei On Microcystis Aeruginosa

In the middle and late period of aquaculture ponds,the pond environment is eutrophicated seriously,and harmful cyanobacteria are easy to multiply,which can secrete toxic substances,which seriously threatens the economic aquaculture biological health,and is one of the main problems faced by aquaculture industry.Microalgae directed culture can improve the culture environment and inhibit the growth of harmful cyanobacteria by optimizing microalgae community structure.It has the advantages of simple operation,simple method and low cost.Oocystis borgei can maintain dominance in water for a long time,and can effectively absorb ammonia nitrogen in water,inhibit the growth of Vibrio,maintain water stability and protect the health of aquaculture organisms.In this study,Microcystis aeruginosa,a common harmful cyanobacteria,was used as the research object to explore the competition relationship between it and Oocystis borgei,and the influence of different initial culture concentrations on the competition.M.aeruginosa was cultured by collecting the extracellular filtrate of O.borgei at different growth stages（exponential phase and decay phase）and the O.Borgei-M.aeruginosa mixed growth extracellular filtrate to explore the influence of growth stage and mixed culture on the inhibition effect.In filtrate culture experiments,M.aeruginosa were cultured under the filtrate,and the number of its cells and esterase activity,PSⅡphotosynthetic efficiency,cell membrane integrity,SOD activity and MDA content,and observe it’s submicroscopic structure change were measured.Transciptome sequencing of M.aeruginosa cultured in BG11（control group）and O.borgei extracellular filtrate（experimental group）was performed by Illumina platform.The differently expressed genes（DEGs）were screened and the pathway enrichment analysis was carried out,and some DEGs were verified by q RT-PCR.The transcriptional inhibition mechanism of O.borgei against M.aeruginosa was studied.The main research results are as follows:1.M.aeruginosa was significantly inhibited when the V_OB:V_MAwas 1:1 and 1:4,respectively（P<0.05）,the growth inflection point was earlier,and the maximum capacity decreased significantly.When V_OB:V_MA=1:1,the growth of M.aeruginosa could be effectively inhibited within 14 days,but the growth of M.aeruginosa could still be inhibited in the late culture period（>14 days）grew exponentially.When V_OB:V_MA=1:4,the inhibition effect of M.aeruginosa was more significant,and the inhibition effect was maintained until the end of the experiment（18 days）.In the V_OB:V_MA=1:1 group,O.borgei could grow normally within 14 days,and the inhibition rate was only 26.0%on the 18th day.On the 18th day,in the V_OB:V_MA=1:4 group,the concentration of O.borgei was higher than that of the control group,and the inhibition rate was-63.1%.O.borgei was not inhibited.In conclusion,when the initial culture proportion is high enough,O.borgei can significantly inhibit the growth of M.aeruginosa,and its influence on M.aeruginosa is small.In order to control the outbreak of cyanobacteria,it is a feasible method for biological control of cyanobacteria by expanding O.borgei in advance to a higher concentration to occupy the ecological dominant position.2.In the extracellular filtrate culture experiment,the O.borgei extracellular filtrate in the exponential phase had better inhibitory effect on the growth and various physiological indexes of M.aeruginosa than the extracellular filtrate in the decay phase.The O.Borgei-M.aeruginosa mixed growth extracellular filtrate was less effective than that of O.Borgei.it has proved that O.Borgei can secrete effective inhibitory substances naturally with its growth,and it does not need the presence of M.aeruginosa to stimulate it.The analysis of M.aeruginosa treated with the extracellular filtrate of O.borgei showed that in the first 3 hours of culture,the proportion of the population with excited esterase activity was significantly higher than that of the control group（P<0.05）,after 1 day,the proportion of lipase activity of inhibited state in the population was significantly higher than that in the control group（P<0.05）,which is a common allelopathic stress induced change.SOD activity on day 2 was significantly higher than that in the control group（P<0.05）and significantly decreased on day 6（P<0.05）,MDA continued to accumulate in 6 days,indicating that the antioxidant enzyme system of M.aeruginosa was damaged and lipid peroxidation was aggravated.On day 4,the cells of M.aeruginosa were observed with transmission electron microscopy（TEM）,showing folds and obvious plasma-wall separation.On day 5,the proportion of damaged cells peaked at 58%.PSⅡmaximum light energy conversion efficiency（Fv/Fm）,the actual photochemical quantum yield（ΦPSII）change trend is consistent,are declining in 3 hours after treatment.Based on the analysis of various physiological indicators,the inhibition mechanism of O.Borgei filtrate on M.aeruginosa may be induced by excessive production of large ROS,aggravation of lipid peroxidation,damage of cell membrane,damage of photosynthetic system,decrease of esterase activity and inhibition of growth.In the water body where there is no M.aeruginosa,the targeted cultivation of O.borgei in advance can effectively inhibit the M.aeruginosa bloom by allowing it to naturally accumulate inhibitory substances in the water body.3.Compared with the control group（BG11）,1483 genes of M.aeruginosa were differentially expressed after filtrate treatment,among which 788 genes were up-regulated and 695 genes were down-regulated.GO（Gene Ontology）functional classification divided all DEGs into 35 categories,including catalytic activity,binding,metabolic process,cellular process,membrane,membrane part and so on.In KEGG pathway analysis,all the DEGs were clustered in 108 categories,among which 84DEGs were annotated in metabolism,and the oxidative phosphorylation pathway was enriched most significantly.The expression of photosynthesis-related genes was mainly up-regulated（26 DEGs,20 up and 6 down）.The q RT-PCR validation were consistent with the expression trend of transcriptomic sequencing results.O.borgei caused the up-regulated expression of genes in the oxidative phosphorylation pathway of M.aeruginosa,increased the synthesis efficiency of ATP,and caused the cell damage with the production of excessive oxygen free radicals,It was consistent with the results of physiological index analysis.