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

Abstract

Surface samples of the 2007 Microcystis bloom occurring in Copco Reservoir on the Klamath River inNorthern California were analyzed genetically by sequencing clone libraries made with amplicons at three loci: the internal transcribed spacer of the rRNA operon (ITS), cpcBA, and mcyA. Samples were taken between June and October, during which time two cell count peaks occurred, in mid-July and early September. The ITS and cpcBA loci could be classified into four or five allele groups, which provided a convenient means for describing the Microcystis 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 and cpcBA loci. Different ITS and cpcBA 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 the mcyB copy number fell further as the bloom declined.Toxic freshwater cyanobacterial blooms, commonly caused by Microcystis, are of current concern in many parts of the world because of their effects on drinking water, water-based recreation, and watershed ecology (5, 7). Microcystis cells are able to produce microcystin, a nonribosomally synthesized cyclic heptapeptide hepatotoxin with potent inhibitory activity against mammalian protein phosphatases (27) whose synthesis is directed by the 55-kb mcy gene cluster (25). The Microcystis genus exhibits worldwide occurrence, although the extent of genetic differentiation between or within geographical regions is currently uncertain due to a relatively sparse database, in spite of a growing number of studies (1,2,9,11,26,28,29).Only a few studies to date have used gene-specific tools to investigate the changes in the Microcystis population structure throughout the development of a bloom season. In some instances, there has been little indication of major population changes. Thus, the proportion of toxigenic (mcyB ϩ ) Microcystis was stable over the course of two consecutive bloom seasons in Lake Wannsee (Berlin, Germany) (17). The internal transcribed spacer of the rRNA operon (ITS) genotype, as assessed by denaturing gradient gel electrophoresis (DGGE) and sequencing, was also stable in Lake Volkerak (Netherlands) during 2001 (15). In contrast, studies of other lakes have observed changes in the Microcystis genotypes and in the proportion of potentially toxigenic cells during a bloom season (3,15,21,31,32). A better understanding of the population changes that occur during the development of toxic blooms is important in understanding their ecology and in assessing whether it might be feasible to manage Microcystis blooms in order to minimize toxicity.

An overview of diversity, occurrence, genetics and toxin production of bloom-forming Dolichospermum (Anabaena) species

“…90). Genotype compositions of natural populations have more widely been studied during Microcystis blooms (Briand et al, 2009;Bozarth et al, 2010;Xu et al, 2011;Zhu et al, 2012), and rarely studied in Dolichospermum blooms (Liu et al, 2014 regression analyses showed that the succession of the two dominant genotypes was significantly correlated with water temperature. These two dominant Dolichospermum genotypes displayed striking seasonal pattern and temperature-associated features, implying that these are major ecotypes during Dolichospermum blooms in Lake Taihu.…”

Genetic analysis on Dolichospermum (Cyanobacteria; sensu Anabaena) populations based on the culture-independent clone libraries revealed the dominant genotypes existing in Lake Taihu, China

“…This finding added a better understanding the spatiotemporal dynamics of cyanobacterial blooms dominated by filamentous cyanobacteria in freshwater ecosystems. Comparative results from ITS sequences between Microcystis and Dolichospermum blooms in Lake Taihu, demonstrated that genetic diversity (represented as values of genotype number/ITS sequence number) from Microcystis blooms was higher than that from Dolichospermum blooms (Bozarth et al, 2010;Sabart et al, 2009). These studies on genotypes of Microcystis dominated blooms, from river-based reservoirs with large surface areas and certain degrees of water flow and pond systems, indicated that strong hydrodynamics influence may trigger the selection of the main genotypes.…”

“…Briand et al (2009) revealed 306 different genotypes from 784 ITS sequences in a France Reservoir, and they further found the marked changes in the ITS genotype composition of Microcystis aeruginosa populations during the development of the bloom. Bozarth et al (2010) described 61 ITS genotypes in the 155 obtained sequences of Microcystis populations in the Copco Reservoir of Klamath River, US, and they indicated that different ITS and cpcBA genotypes dominated at the two population peaks in the reservoir. Zhu et al (2012) found relatively higher genetic diversity from ITS genotypes of Microcystis dominated blooms in a pond system, compared with the studies from the reservoirs.…”

Vertical Heterogeneity of Genotypic Structure and Toxic Potential within Populations of the Harmful Cyanobacterium Microcystis aeruginosa

“…Moreover, temporal shifts of the genotypic structure at a given sampling point can be accompanied by significant variations of the toxic potential of a bloom and can occur within two weeks [8,27]. Understanding the regulation of this temporal genotypic succession is thereafter of major importance.…”