Population Turnover in a Microcystis Bloom Results in Predominantly Nontoxigenic Variants Late in the Season

Surface samples of the 2007Microcystis bloom occurring in Copco Reservoir on the Klamath River in Northern California were analyzed genetically by sequencing clone libraries made with amplicons at three loci: the internal transcribed spacer of the rRNA operon (ITS),cpcBA , andmcyA . Samples were taken between June and October, during which time two cell count peaks occurred, in mid-July and early September. The ITS andcpcBA loci could be classified into four or five allele groups, which provided a convenient means for describing theMicrocystis population and its changes over time. Each group was numerically dominated by a single, highly represented sequence. Other members of each group varied by changes at 1 to 3 nucleotide positions, while groups were separated by up to 30 nucleotide differences. As deduced by a partial sampling of the clone libraries, there were marked population turnovers during the season, indicated by changes in allele composition at both the ITS andcpcBA loci. Different ITS andcpcBA genotypes appeared to be dominant at the two population peaks. Toxicity (amount of microcystin per cell) and toxigenic potential (mcyB copy number) were lower during the second peak, and themcyB copy number fell further as the bloom declined.