| In recent years,frequent outbreaks of cyanobacterial blooms are still one of the toughest environment problem in Taihu Lake.One of its hazards is that it can produce highly toxic cyanobacterial toxins,posing a serious threat to the aquatic life of Taihu Lake and the lives and property of surrounding residents.Microcystins(MCs)is the most common and most studied cyanobacterial toxin.MCs has obvious liver toxicity,can inhibit protein phosphatase activity in organisms,and cause endoplasmic reticulum oxidative stress,DNA damage and cell apoptosis.In areas where cyanobacterial blooms are more serious,the existence of high concentration of MCs is bound to have serious toxic effects on aquatic organisms,and can endanger human health through the transmission of drinking water and food chain.As a common dominant species among the benthic animals in Lake Taihu,Bellamya Aeruginosa are of great significance in improving the water quality,increasing the species abundance of eutrophic water bodies and providing aquaculture feed.Due to the long-term life in the bottom of the water and the direct contact with MCs,the study of the MCs accumulation patterns,the possible MCs source and the relationship with environmental factors of Bellamya aeruginosa in the cyanobacteria bloom period is important for deepening the understanding of migration and transformation of MCs in lake ecosystem and providing the health risk assessment of MCs.In this article,from May to November in 2020,the water bodies,sediments and Bellamya aeruginosa in Zhushan Bay where the cyanobacteria bloom is severe in Taihu Lake were sampled to grasp the MCs level in the snails and habitat medium and the food composition of the snails.The changes of various environmental factors(p H,DO,temperature)were also observed.Subsequently,by determining the concentration of MCs in snail tissues,the changes of antioxidant and immune system-related enzyme indexes,metabolic rate,etc.,the factors that affect the accumulation of MCs and the physiological processes of the snails were screened.Finally,we set up different algae composition in indoor experiment system and controlled the ratio of toxic(Microcystis.aeruginosa)and non-toxic algae(Chlorella)to explore the snail feeding preference,accumulation and removal patterns of MCs,and the physiological response of the antioxidant and immune systems.The influence of different algae composition on MCs accumulation,physiological processes of Bellamya aeruginosa were analyzed.The main conclusions are as follows:(1)During the cyanobacteria bloom,the MCs accumulation of B.aeruginosa was in high level and the content of MCs in the hepatopancreas was higher than the edible part.The accumulation of MCs in the snails was negatively correlated with the shell height and was significantly positively correlated with the content of intracellular MCs in the surface water and sediment overlying water.The aquatic plant detritus SC is the main food source of the snails in spring(36.36%),while the suspended particulate organic matter POM is the main food source of the snails in summer and autumn(39.99%,38.49%).The contribution rate of different food sources is due to the richness of organic matter changes in different seasons.The main way of MCs accumulation in the snails is by ingesting suspended organic particulate matter POM and sediment organic matter SOM.It can be inferred from the food composition that the toxinproducing algae in phytoplankton is the main source of MCs in the snails.As the residents around Taihu Lake have the habit of eating snails,and the negligible MCs accumulation in edible part may pose a potential health risk to the residents.(2)During the Taihu cyanobacteria outbreak,CAT activity did not change significantly,while SOD enzyme activity first decreased and then increased.GST enzyme activity did not change significantly in the early stage but increased later.The MDA content decreased from May to July and remained stable in later sampling time.The activity of lysozyme increased first and then decreased,indicating that various stress during the cyanobacteria outbreak period can also stimulate the antioxidant defense and innate immune response of the snails,but long-term exposure may also damage the immune system.During the cyanobacterial blooms,the oxygen consumption rate of B.aeruginosa showed a decreasing trend in fluctuation.The ammonia excretion rate was relatively high in August and September,but was low in the rest of the months.The O:N ratio decreased sharply after July.Snails mainly use carbohydrates and fats as their main metabolic energy sources,and protein is more involved in the metabolic process after July.Except that SOD enzyme activity is significantly negatively correlated with MCs content in sediments and intracellular MCs concentration in surface water,the other antioxidant and immune system-related enzyme activities are not related to physical and chemical factors,and MCs content in snails and habitat media,as well as biology indexes.We speculate that the antioxidant response and oxidative damage of the snails are caused by the combined effects of the physical and chemical factors of the habitat and MCs.The specific impact mechanism needs further laboratory experiments.(3)In the indoor experiment,under the control of different algae composition conditions(including the non-toxic algae group,the toxic algae group and the mixed algae group),the filtration rate of B.aeruginosa first decreased and then increased as the proportion of Microcystis increased.The filter feeding rate of Microcystis decreased firstly and then increased as its proportion increased,which indicates that the snail has a certain selectivity to food under different algae composition.In the MCs accumulation and removal experiment under different algae composition,the MCs content in the toxic algae group was higher than the mixed algae group in the accumulation stage,and the MCs content in both the two groups showed a trend of first increasing and then decreasing;During the purification stage,the MCs content of hepatopancreas in the two groups both showed a fluctuating downward trend.Under different algae composition conditions,the antioxidant and immune system-related enzyme activities both increased first and then decreased during the accumulation stage while there was no significant difference in the enzyme activities between the different treatment groups.MDA decreased significantly in the non-toxic algae group.However,the MDA of the two groups with toxic algae did not change significantly during the experiment.This may indicate that the presence of toxic algae will cause the oxidative stress and immune response of the snails to reduce the damage of the body,but the change in the proportion of toxic algae has no significant effect on the antioxidant and immune-related enzyme activities. |
