Studies On Competitive Advantages And Toxical Effects Of Toxic And Non-toxic Microcystis Aeruginosa Strains

Microcystis blooms in natural freshwater system are usually caused by the combination of toxic and non-toxic Microcystis species.Furthermore,climate warming was thought to lead to the expansion of these blooms.To know competitive advantages between toxic and non-toxic Microcystis aeruginosa strains under the background of global warming,and the toxicity effects as well as ecological risks of Microcystis blooms generating and vanishing process on the coexisting plankton,the intraspecific competitive capability of toxic M.aeruginosa FACHB-905 and non-toxic M.aeruginosa FACHB-469 at different temperatures,the allelopathic effects of toxic M.aeruginosa FACHB-905 and non-toxic M.aeruginosa of on C.vulgaris under the background of climate warming,the allelopathic effects of Microcystin-LR(MC-LR),cell homogenates and filtrate of toxic M.aeruginosa 905 on the growth and photosynthesis of C.vulgaris,the impacts of toxic M.aeruginosa FACHB-905 of different cell concentration on the population dynamics major groups offreshwater zooplankton(cladocerans,copepods and rotifers)were studied.The main results of the study were as follows:1.The average specific growth rate of both non-toxic M.aeruginosa FACHB-469 and toxic M.aeruginosa FACHB-905 increased with increased temperatures ranged from 20 to 30 oC,and then began to decrease when temperature was further increased to 35 oC after they had been cultured for 10 days at different temperatures.Moreover,the specific growth rate of toxic M.aeruginosa FACHB-905 was slightly higher than that of non-toxic M.aeruginosa FACHB-469 at temperatures ranged from20 to 35 oC though the difference was not statistically significant.However,the maximal photosynthetic rate(Pmax)and dark respiration(Rd)of toxic M.aeruginosa FACHB-905 were significantly higher than non-toxic M.aeruginosa FACHB-469.The maximal photochemical efficiency(Fv/Fm)of both strains decreased significantly when they were exposed to stressful irradiance(600 μmol·photons·m-2·s-1)for 4 h,however,the Fv/Fm of toxic M.aeruginosa FACHB-905 was significantly higher than that of non-toxic M.aeruginosa FACHB-469 at 30 and 35 oC.The toxic M.aeruginosa FACHB-905 showed higher growth rate,photosynthetic capacity and resistance to stressful irradiance compared with non-toxic M.aeruginosa FACHB-469 after they had been acclimated to temperatures ranged from 20 to 35 oC.2.The allelopathic effects of two M.aeruginosa strains on thegrowth and photosynthesis of C.vulgaris were both temperature-and species-dependent when they co-cultured with C.vulgaris.The growth and photosynthesis of C.vulgaris,co-cultured with the toxic or non-toxic M.aeruginosa strains was promoted at 20 oC,but with the rise of temperature the inhibition increases gradually and its inhibition rate was reached 100%at 35 oC.Furthermore,toxic M.aeruginosa FACHB-905 induced more inhibition toward C.vulgaris than non-toxic M.aeruginosa FACHB-469.C.vulgaris dominated over non-toxic M.aeruginosa FACHB-469 but toxic M.aeruginosa FACHB-905 overcame C.vulgaris when they were co-cultured in mesocosms with the temperatures ranged from 20 to 25 oC.3.Significantly inhibitory effects of Microcystin-LR(MC-LR)on growth of C.vulgaris when its concentration exceeded 200μg/L.Furthermore,the inhibitory effects of MC-LR on growth and photosynthesis of C.vulgaris increased with enhanced concentration and resulted in declined photochemical efficiency of C.vulgaris.Compared with the inhibitory effects of MC-LR on C.vulgaris,the cell homogenates of toxic M.aeruginosa FACHB-905,which could contained the same concentration of MC-LR,showed stronger inhibition on growth and photosynthesis of C.vulgaris.While the filtrate of toxic M.aeruginosa 905 cells showed insignificant inhibitory effects on the growth and photosynthesis of C.Vulgaris the same temperature(25oC).4.Abundance of the cladocera and rotifers decreased but that of the copepods increased in the earlier days then raised with prolonged time when the zooplankton species collected from the eutrophic river were co-cultivated with toxic M.aeruginosa FACHB-905.However,the abundances of all the three above mentioned zooplankton groups increased in the earlier days then slowly decreased with prolonged time when C.vulgaris was added.Furthermore,the lethal effects of toxic M.aeruginosa FACHB-905 on zooplankton species were alleviated and the bigger individual size could be reached when the ratios between C.vulgaris and M.aeruginosa FACHB-905 increased(C.vulgaris/ M.aeruginosa=1/10,1/1 and 10/1,respectively).The above results indicate that global warming will further expanding the Microcystis blooms,and change the blooms usually caused by toxic and non-toxic M.aeruginosa strains to the ones mainly triggered by toxic M.aeruginosa strains.Microcystin produced by toxic M.aeruginosa strains have deleterious allelopathic effects toward the coexistent phytoplankton and zooplankton.Therefore,global warming may favor the predomination of toxic Microcystis species in natural waters,which further aggravate the ecological risk of cyanobacterial blooms.