Toxicities Of Microcystis Aerusinosa Exudates At Different Growth Phases On Daphnia Magna And The Correlation Between Effects And Metabolites

Blooming cyanobacteria can produce toxicity to other organisms by releasing various compounds,but the composition of its active substances remains to be determined.In recent years,some studies have also found that cyanobacteria in different growth phases have different interferences on other aquatic organisms,which suggests that the active compounds synthesized by cyanobacteria in different growth phases may be different.However,the chemical ecological effects and material basis of cyanobacteria in different growth phases need to be further clarified.In order to explore the toxicity of cyanobacteria in different growth phases to zooplankton and the changes of metabolite components of cyanobacteria,this study selected the microcystin-producing strain and microcystin-free strain of Microcystis aeruginosa,the dominant species in cyanobacteria blooms,and the zooplankton model species Daphnia magna as the test material.Compare the toxicity of D.magna on mitochondrial dysfunction,oxidative stress and lipid peroxidation,filter-feeding action,heart rate and reproductive characteristics exposed to M.aeruginosa exudates(MaE)obtained from exponential growth phase and stationary phase,and combining with metabonomics analysis of MaE,to explore the correlation of toxicity of different growth phases of M.aeruginosa with its metabolites components.The research contents and results are as follows:(1)At the level of mitochondrial function,there was an obvious depolarization of mitochondrial membrane potential under the treatment of MaE.The inhibition effect of stationary phase on mitochondrial function is stronger than exponential growth phase in the same strain,and the microcystin-free strain was stronger than that in the microcystin-producing strain at the same phase.Metabolomics results show that the differential expression of active compounds toxic to mitochondria in MaE,such as 7-Ketocholesterol,may lead to different toxicity of different types of MaE.(2)At the level of oxidative stress and membrane damage,ROS levels in D.magna were significantly increased under MaE treatment,H₂O₂was also significantly increased,indicating that the organism was in a state of oxidative stress;The levels of antioxidant enzymes including SOD,CAT,GPX,GSTs and antioxidant GSH were also significantly increased in the treatment group,indicating that the body is in the state of anti-oxidation and detoxification;The contents of MDA and Lipofuscin increased correspondingly,indicating that MaE exposure caused oxidative damage to D.magna.However,there was no significant difference among all treatment groups,indicating that there was no difference in oxidative stress and peroxidation damage under different treatments of MaE in different strains at different phases,and the different toxic inhibition effects may be produced through other pathways.(3)In the filter feeding action and heartbeat level,with the increase of exposure time,the inhibition effect of MaE on the thoracic limb movement was gradually enhanced,and the inhibition effect of the microcystin-free strain in the stationary phase at 48h was significant,and the inhibition rate was as high as 29.54%.Heart rate was significantly inhibited in each treatment group after 4h,and the inhibition intensity did not change significantly with time.The post-abdominal claw activity had a certain degree of promoting effect,but there was no significant difference.The overall inhibition intensity of filter feeding action and heart rate was stronger in stationary phase than exponential growth phase,and the inhibition effect of microcystin-free strain was stronger than that of microcystin-producing strain at the same phase.Metabolomics results show that the differential expression of the neurotoxic active substances such as retinoids in MaE may lead to the different toxicity of different types of MaE on the filter feeding action and heart rate of D.magna.(4)The total number of pregnant eggs and the total number of young in D.magna were significantly inhibited after 21 days of exposure to MaE at the stationary phase of microcystin-free strain,and the inhibition rates were 24.68%and 38.10%,respectively,which indicated that MaE had obvious reproductive interference toxicity during stationary phase of microcystin-free strain.Metabolomics results show that the active substances with reproductive interference in MaE,may ultimately cause the inhibitory effect of MaE on the reproductive characteristics of D.magna.(5)For different strains at different phases of MaE metabolic points and differences in expression analysis found that metabolites richness of the stationary phase is higher in different MaE,at the same time is found that for metabolites differentially expressed in stationary phase increases more than the number of compounds in the exponential growth phase,with the MaE of stationary phase results are basically in agreement with stronger toxicity.In conclusion,different types of MaE are toxic to zooplankton D.magna,which is related to the toxic compounds in MaE.At the same time,the toxicity showed that the stationary phase was stronger than the exponential growth phase,which was related to the differential up-regulation of toxic compounds in MaE in stationary phase.The effect difference is different from the research results at plant level,which indicates that the species difference of aquatic organisms should also be considered when exploring the hazards of cyanobacteria in different periods.In addition,microcystin-free strains have stronger toxicity than microcystin-producing strains,which indicates that the potential harm of microcystin-free populations can not be ignored while controlling microcystin-producing populations.