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In an attempt to elucidate the effects of different CO2 concentrations (270,380,and 750 μL/L) on the competition of microcystin-producing (MC-producing) and non-MC-producing Microcystis strains during dense cyanobacteria blooms,an in situ simulation experiment was conducted in the Meiliang Bay of Lake Taihu in the summer of 2012.The abundance of total Microcystis and MC-producing Microcystis genotypes was quantified based on the 16S rDNA and mcyD gene using real-time PCR.The results showed that atmospheric CO2 elevation would significantly decrease the pH value and increase the dissolved inorganic carbon (DIC) concentration.Changes in CO2 concentration did not show significant influence on the abundance of total Microcystis population.However,CO2 concentrations may be an important factor in determining the subpopulation structure of Microcystis.The enhancement of CO2 concentrations could largely increase the competitive ability of non-MC-producing over MC-producing Microcystis,resulting in a higher proportion of non-MC-producing subpopulation in treatments using high CO2 concentrations.Concurrently,MC concentration in water declined when CO2 concentrations were elevated.Therefore,we concluded that the increase of CO2 concentrations might decrease potential health risks of MC for human and animals in the future.