Effects Of Saltwater Intrusion On Microcystis Aeruginosa Growth And Microcystin Production Mechanism In Estuarine Reservoir

Qingcaosha?QCS?Reservoir is located in the Yangtze River estuary in China,which is the largest water source in Shanghai.Like other estuarine reservoirs in the world,QCS Reservoir is facing the problem of saltwater intrusion.After the water of Yangtze River estuary flows into QCS Reservoir,the current slows down and the water stays longer,which make QCS Reservoir more vulnerable to eutrophication and algal blooms.Microcystis aeruginosa is both a common cyanobacteria and a major primary producer,playing a key role in aquatic environment.Microcystis aeruginosa produces toxin called microcystin?MC?that may act as a defense mechanism against environmental changes.MC not only causes health hazards to aquatic organism but also affects the municipal safety water supply.In this paper,we monitored the QCS Reservoir and analyzed the the concentration of ions and chlorophyll a?chl-a?.Moreover,we studied the effects o f saltwater intrusion on Microcystis aeruginosa from celluar level and molecular level,and further analyzed the influence mechanism of saltwater intrusion on algal growth and microcystin production.Below is our major conclusion.?1?The ion inhibited algal growth in QCS Reservoir,and stimulated the production and release of MC.We found a negative correlation betwee chl-a and kation(SO₄^2-and Ca²⁺).There is a strong positive correlation between soluble MC and ions(Cl^-,SO₄^2-,Ca²⁺,Mg²⁺,Na⁺and K⁺)in QCS Reservoir.?2?Cl^-,SO₄^2-,Ca²⁺and Mg²⁺inhibited the growth of Microcystis aeruginosa in adose-dependentmanner.Excessive Ca²⁺and Cl^-inhibited the related genes expression of DNA replication?dnaB,dnaX,dnaE and polA?,increased the related genes expression of cell division?secF,secY and tatA?,led to cell death,and finally decreased the cell number.?3?Cl^-,SO₄^2-,Ca²⁺and Mg²⁺caused oxidative stress.The ions increased the intracellular ROS level,enhanced the activities of antioxidant enzyme?SOD,GPx and GR?and the content of antioxidant?GSH?,affected the related genes expression?sod and gsh?,and then damaged the balance between oxidation and antioxidation,led to the lipid peroxidation,and finally cuased oxidative damage.?4?Cl^-and Ca²⁺caused globally changed and adjusted of carbohydrate metabolism in Microcystis aeruginosa.The results of transcriptome analysis showed that ions affected glycolysis,gluconeogenesis,pentose phosphate pathway and citric acid cycle,which affected the synthesis and metabolism of carbohydrate.?5?Cl^-,SO₄^2-,Ca²⁺and Mg²⁺decreased pigment content?chl-a,carotenoid and phycocyanin?,damaged their ability to absorb and pass the light energy to chl-a,affected electron transport rate,inhibited the assimilation and fixation of CO₂,and finally caused the abnormality of photosynthesis and energy metabolism.?6?Cl^-,SO₄^2-,Ca²⁺and Mg²⁺not only affected the transcription and translation of MC,but also affected the MC concentration from cellular level.The ions influenced the production and release of MC from cellular level,affected the related genes expression at transcription level.Cl^-,SO₄^2-,Ca²⁺and Mg²⁺also controlled the synthesis of 50 S,30 S and ammonia acyl-tRNA synthetase,facilitated the secretion of new peptide chain,and finally influenced the MC production at translation level.In summary,Cl^-,SO₄^2-,Ca²⁺and Mg²⁺inhibited the growth of Microcystis aeruginosa,decreased the photosynthes,led to oxidative stress,affected the carbohydrate metabolism,inhibited DNA replication,influenced polypeptide translation,facilitated translation,caused the death of Microcystis aeruginosa,making the algae produce and release amount of MC.