The Effects From The Degradation Of Ulva Prolifera On The Aquaculture Environment,Intestinal Microbiota And Transcriptome Of Japanese Flounder Paralichthys Olivaceus

Green tide in the Yellow Sea refer to the proliferation and aggregation of a dominant species Ulva prolifera in suitable environment and climate conditions,has broken out for 15years.Every year,a large number of algal bodies settle and rot in the coastal area which poses a serious threat to the ecological balance and the survival of marine organisms.Japanese flounder（Paralichthys olivaceus）is an important benthic fish and it is also a major economic species.Its natural living sea area and aquaculture water are polluted by the degradation of alage.As a benthic fish model,Japanese flounder has been widely used in environmental pollution and toxicology studies.In this study,juvenile Japanese flounder was used as the experimental subject,the decomposition effluent of U.prolifera and fresh U.prolifera as exposure factors,conducted short-term exposure experiment for 14 days.The effects on the environment and bentic fish from the degradation of the alage were investigated by means of environmental chemistry,microbiome,transcriptome and histopathology.The main results are as follows:（1）The DOC concentrations of the decomposition effluent of U.prolifera group（DU）,fresh U.prolifera group（FU）and control group（CO）were significantly different,the degradation degree of alage and the transformation and utilization of bacteria led to this difference.The concentrations of nutritive salts were as well,The concentrations of NH₄⁺、NO₃^-and PO₄^-in DU and FU groups were significantly higher than those in CO group.The degradation caused the re-release of nitrogen,phosphorus and other substances absorbed in the process of proliferation,and significantly increased the concentration of nutrients.（2）The composition and structure of intestinal microbiota obviously changed with exposure time and groups.The abundance and diversity of microbiota in DU group decreased significantly,while that in FU group decreased first and then increased.The dominant microbiota of different experimental groups was different at each time point.The relativeabundancesofPolaribacter＿4,NS3a＿marine＿group,[Polaribacter]＿huanghehaensis and Olleya in the samples of DU 14d were significantly increased.The potential pathogenic bacteria in intestinal microbiota increased significantly,especially Vibrio.Through the prediction of microbial metabolic function,nitrogen and phosphorus metabolic pathways of water and intestinal microbiota were significantly different.（3）The degradation of U.prolifera affected the expression of genes related to immunity,metabolism and tissue structure in intestine.Immunorelated genes such as pik3r5（phosphatidylinositol-3 kinase receptor 5）,ifn-alpha/beta R2-like and toll-like receptor 9toll-like receptor 9 were significantly down-regulated.Metabolic related genes such as apolipoprotein a4,abcg 5 and glulb（glutamine synthase gene）were significantly different.The structure-related gene claudin 15-like was significantly down-regulated,and bacterial defense,immune response and metabolism-related signaling pathways were significantly different with increasing exposure time.（4）After the degradation of U.prolifera,the intestinal tissue showed pathological damage,such as villi rupture,loose structure and goblet cell increase.With the increase of exposure time,the pathological degree and mortality increased obviously.In conclusion,the degradadtion of U.prolifera not only damages the living environment of Japanese flounder,deteriorates water quality and disturds the balance of microbiota,but also damages intestinal microbiota and intestinal tissue structure,affects intestinal immune and metabolic functions,and threatens the survival of fishes.