Autumnal increase in proportion of toxic Microcystis in Lake Taihu depends more on temperature adaptability than on effects of toxigenicity.

Abstract

Toxic Microcystis strains have selective advantages over nontoxic strains under certain conditions. This study was undertaken to analyse whether fluctuations of toxic/nontoxic ratio in Microcystis population are based on effects of toxigenicity. Annual Microcystis population dynamics was investigated by sequencing of gvpAC-IGS libraries and qPCR analysis of mcyA. gvpAC-IGS type 1 dominated the Microcystis population in Lake Taihu in seasons except autumn, while types 6, 9 and 10 and mcyA copies in total Microcystis rapidly increased in autumn and decreased in winter. Isolated type 6 was found to be toxic, while types 1, 9 and 10 were nontoxic. Physiological characterization and co-cultures indicated that type 1 was favoured at lower temperatures when competing with types 6, 9 and 10. The rapid increase in proportion of toxic Microcystis was probably based on the same mechanism as that for other nontoxic strains to arise in total Microcystis. Fluctuations of toxic/nontoxic ratio are not necessarily due to effects of toxigenicity. This is the first report that the dynamics of toxic/nontoxic Microcystis was investigated in combination with characterizations of multiple isolated strains. The results are useful in understanding Microcystis genotype succession.