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Spatial isolation favours the divergence in microcystin net production by Microcystis in Ugandan freshwater lakes

William Okello1, Veronika Ostermaier, Cyril Portmann

  • 1Austrian Academy of Sciences, Institute for Limnology, Mondseestrasse 9, Mondsee, Austria.

Water Research
|March 12, 2010
PubMed

Insights

Microcystis blooms in East African lakes produce toxins called microcystins (MCs). The proportion of MC-producing Microcystis cells, indicated by the mcyB gene, directly influences MC toxin levels in the water.

Area of Science:

  • Environmental Science
  • Microbiology
  • Ecotoxicology

Background:

  • Cyanobacteria, particularly Microcystis species, are prevalent in East African freshwater lakes.
  • Microcystins (MCs) are potent hepatotoxins produced by certain cyanobacteria, posing risks to aquatic ecosystems and human health.
  • The regulation and variability of MC production within Microcystis populations remain poorly understood, especially in tropical regions.

Purpose of the Study:

  • To investigate the abundance of Microcystis and the prevalence of the mcyB genotype, a marker for MC production, in five Ugandan freshwater lakes.
  • To quantify microcystin concentrations and cell quotas in relation to Microcystis abundance and mcyB genotype proportion.
  • To determine seasonal and lake-specific variations in MC production by Microcystis populations.

Main Methods:

  • Monthly sampling of five Ugandan lakes (crater, shallow, and large lakes) from May 2007 to April 2008.
  • Quantification of Microcystis abundance, mcyB genotype proportion using molecular methods, and microcystin concentrations via analytical chemistry.
  • Statistical analysis to correlate mcyB genotype proportion with MC cell quotas and concentrations.

Main Results:

  • Microcystis consistently dominated phytoplankton across all studied lakes throughout the year.
  • The mcyB genotype proportion varied significantly among lakes, ranging from 1.4% in Lake George to 37% in Lake Saka.
  • MC cell quotas and concentrations were directly and linearly correlated with the proportion of mcyB-containing Microcystis cells, with Lake Saka showing the highest levels and Lake George the lowest.

Conclusions:

  • Microcystis populations exhibit consistent, lake-specific differences in mcyB genotype proportion, independent of season.
  • These genotypic variations lead to distinct population-specific differences in average MC content per cell.
  • The proportion of the mcyB genotype is a key determinant of microcystin production and concentration in these East African freshwater systems.

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