Two New Strains of <i>Microcystis</i> Cyanobacteria from Lake Baikal, Russia: Ecology and Toxigenic Potential

<i>Microcystis</i>, a potentially toxigenic cyanobacterium known to form extensive blooms in eutrophic lakes globally, was investigated in the cold oligotrophic Lake Baikal. We report the isolation of two <i>Microcystis</i> strains, <i>Microcystis aeruginosa</i> and <i>M. novacekii</i>, and document the presence of the latter species in Lake Baikal for the first time. In <i>M. aeruginosa</i> strain BN23, we detected the microcystin synthetase gene <i>mcy</i>E. Liquid chromatography-mass spectrometry revealed the presence of two microcystin variants in BN23, with microcystin-LR, a highly potent toxin, being the dominant form. The concentration of MC-LR reached 540 µg/g dry weight. In contrast, <i>M. novacekii</i> strain BT23 lacked both microcystin synthesis genes and detectable toxins. The habitat waters were characterized as oligotrophic with minor elements of mesotrophy, exhibiting low phytoplankton biomass dominated by the chrysophyte <i>Dinobryon cylindricum</i> (76–77% of biomass), with cyanobacteria contributing 8–10%. The contribution of <i>Microcystis</i> spp. to the total phytoplankton biomass could not be quantified as they were exclusively found in net samples. The water temperature at both sampling stations was ~19 °C, which is considerably lower than optimal for <i>Microcystis</i> spp. and potentially conducive to enhanced microcystin production in toxigenic genotypes.