We studied the frequency and composition of potential microcystin (MC) producers in 70 Finnish lakes with general and genus-specific microcystin synthetase gene E (mcyE) PCR. Potential MC-producing Microcystis, Planktothrix and Anabaena spp. existed in 70%, 63%, and 37% of the lake samples, respectively. Approximately two-thirds of the lake samples contained one or two potential MC producers, while all three genera existed in 24% of the samples. In oligotrophic lakes, the occurrence of only one MC producer was most common. The combination of Microcystis and Planktothrix was slightly more prevalent than others in mesotrophic lakes, and the cooccurrence of all three MC producers was most widespread in both eutrophic and hypertrophic lakes. The proportion of the three-producer lakes increased with the trophic status of the lakes. In correlation analysis, the presence of multiple MC-producing genera was associated with higher cyanobacterial and phytoplankton biomass, pH, chlorophyll a, total nitrogen, and MC concentrations. Total nitrogen, pH, and the surface area of the lake predicted the occurrence probability of mcyE genes, whereas total phosphorus alone accounted for MC concentrations in the samples by logistic and linear regression analyses. In conclusion, the results suggested that eutrophication increased the cooccurrence of potentially MC-producing cyanobacterial genera, raising the risk of toxic-bloom formation.Cyanobacterial mass occurrences are a frequent phenomenon worldwide. A survey of the blooms in freshwaters has shown that on average, 59% contain toxins, with hepatotoxic blooms being more common than neurotoxic blooms (45). Toxic blooms expose water users to health risks and prevent the recreational use of water (19).Microcystins (MCs) are the most prevalent cyanobacterial hepatotoxins in freshwaters, where they are produced mainly by strains of the genera Anabaena, Microcystis, Planktothrix, and occasionally Nostoc (45). The toxicity of MCs is due to the inhibition of eukaryotic protein phosphatases 1 and 2A (11,25) in liver cells, where MCs enter via the bile acid transport system (1). MCs are cyclic heptapeptides with a general structure of cyclo(-D-Ala-X-D-erythro--methylaspartic acid-ZAdda-D-Glu-N-methyldehydroalanine), where X and Z are various L-amino acids and Adda is 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid. D-Glu and Adda form the part of the molecule that interacts with the protein phosphatases and thus are the crucial amino acids for the toxicity of MCs (8).MCs are produced by nonribosomal enzyme complexes. Adda is synthesized and integrated into the MC molecule by the enzymes McyG, McyD, and McyE. McyE also incorporates D-Glu, the other crucial amino acid for toxicity. Microcystin synthetase (mcy) gene clusters that encode these biosynthetic enzymes have now been characterized from all the main MCproducing genera (2,30,43,48). The presence of biosynthetic genes has also been proven a prerequisite for MC production (5). Although intensively studied, only a few st...