The Mechanism Of Nitric Oxide Signaling Molecules Regulating The Depletion Of Microcystis Aeruginosa Blooms

Eutrophication has caused the problem of harmful algal blooms(HABs),which pose serious harm to water ecosystems and public health security.Aquatic plants play an important role in restoring eutrophic water bodies and constructing stable water ecosystems.Aquatic plants release allelochemical is the main way to control cyanobacteria blooms.Some studies indicated that nitric oxide(NO)is significantly increased in cyanobacteria cells under allelopathic stress.NO signaling molecule plays a vital role in regulating programmed cell death in cells.However,the physiological effects and regulatory mechanisms of NO on cyanobacteria cells remain unclear.In this paper,Microcystis aeruginosa FACHB 905,a common dominant and toxic species of cyanobacteria,was selected as a toxic algal strain.By adding pyrogallic acid(PA),a polyphenol allelopathic substance,the algal cells produced endogenous NO.Sodium nitroprusside(SNP)provides exogenous NO to algal cells,and the mechanism of NO signaling molecules regulating M.aeruginosa blooms was studied.To study the physiological effects of NO on M.aeruginosa under allelopathic stress,the acute toxicity tests were conducted by exposing M.aeruginosa to 0,0.4,0.8,1.6,4,8,16 mg/L PA for 96 h and by adding Hemoglobin(Hb)to remove NO in cells.The results showed that:(1)PA stimulates the growth of M.aeruginosa in the concentration range of0.4～0.8 mg/L.With the increase in PA concentration(1.6～16.0 mg/L),the growth of M.aeruginosa was inhibited.(2)The effect of NO on M.aeruginosa depended on the allelopathic stress of PA.After the addition of NO scavenger hemoglobin,the results showed that in the growth stage stimulated by low dose PA,intracellular NO decreased to regulate ROS levels and maintain intracellular redox homeostasis.In the inhibition phase of high dose PA,intracellular NO,ROS and SOD increased significantly,and S-nitroso mercaptan accumulated in M.aeruginosa,indicating that high concentrations of PA induced nitric oxidative stress in M.aeruginosa.The effect of exogenous NO donor SNP(200 μM)on M.aeruginosa with different densities was studied.And the corresponding relationship between the effect of intracellular NO and stress was analyzed.In the high algal density group,the results showed that the cells were under little stress,SOD and esterase activity increased,and intracellular NO and ROS decreased in M.aeruginosa.These phenomena indicated that a small amount of NO entering cells could stimulate the antioxidant system and regulate the decrease of stress.The algal cells in the low algal density group were subjected to high stress,and intracellular NO accumulation,and ROS decline were observed.These phenomena indicated that NO accumulation led to severe nitric oxidative stress in cells,which aggravated cell damage.At the same time,caspase-3 activity of algal cells in the low algal density group was increased under the influence of SNP,indicating that NO signaling molecules mediated the occurrence of PCD in M.aeruginosa under stress.It has been proved that the stress of M.aeruginosa cells is the key factor affecting the action of NO in the cells.In this paper,the effects of low dose SNP(20,40 μM)on M.aeruginosa,Lemna minor,and Spriodela polyrrhiza were compared.The results showed that the dose of SNP could induce severe oxidative stress and PCD in M.aeruginosa.The same dose of SNP had no significant negative effect on the duckweed and even alleviated the phenomenon of chlorophyll content reduction.The experimental results show that when the concentration of NO donor is appropriate,the NO signaling molecules released by the donor can control M.aeruginosa blooms without negative effects on other aquatic plants.This conclusion has important practical significance for cyanobacteria bloom management.