| Summary: | The North American Great Lakes are a vital source on a global scale, as they hold ~18 % of the potable water resources on our planet. Cyanobacteria of the genus Microcystis are commonly found in fresh water environments around the world, and since the mid-1990s also in Lake Erie. The reasons for the success for these potentially toxic cyanobacteria in Lake Erie are not completely understood. In this study we have applied modern molecular tools to analyze field samples to provide an insight into the genotypic composition and diversity of the Microcystis community in the past and present day Lake Erie. We have also analyzed a three-year data set to identify specific environmental factors that contribute to the abundance of Microcystis genotypes and microcystin production. In addition, in a laboratory-based study we examined the effect of nutrients on transcriptional activity of the microcystin synthetase gene mcyD.
The results of this study suggest that, although toxic Microcystis form < 10 % of the total cyanobacterial population in Lake Erie, the toxin-producing Microcystis community in Lake Erie is diverse, and that these populations are stabile on a time scale of decades. Sediments acting as a reservoir of Microcystis are likely contributing to the persistence of the population. Although Microcystis is the dominant microcystin producer in the lake, other microcystin-producing cyanobacteria were also found in spatially isolated regions of the lake. While microcystin concentration in Lake Erie is correlated positively with total phosphorus (P<0.001) and surface reactive phosphorus (P<0.001), and negatively with the molar ratio total nitrogen to total phosphorus (P<0.001); toxic Microcystis abundance correlates negatively with NO3 concentration (P=0.04) and positively with surface water temperatures (ranging from 20.8 °C to 27.4 °C) (P=0.03). These observations, along with findings from culture based experiments, suggest decoupling of the factors governing proliferation of toxic cells and toxin production. Culture based experiments also suggested that the chemical form of phosphorus may be an important factor in regulating microcystin biosynthesis in Microcystis based on monitoring relative transcriptional activity of the mcyD gene. The transcriptional activity of mcyD was higher (P=0.118) in cells grown in BG11-medium containing 2.3 μM organic phosphorus (glycerol 2-phosphate disodium salt hydrate) than in cells grown in BG11-medium containing 2.3 μM inorganic phosphorus (K2HPO4).
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