The effects of light, temperature, and coculture on the intracellular microcystin-LR(MCLR) quota of M icrocystis aeruginosa were evaluated based on coculture experiments with nontoxic Dolichospermum( Anabaena) fl os- aquae. The MC-LR quota and transcription of m cy B and m cy D genes encoding MC synthetases in M. aeruginosa were evaluated on the basis of cell counts, high-performance liquid chromatography, and reverse-transcription quantitative real-time PCR. The MC-LR quotas of M. aeruginosa in coculture with a 1/1 ratio of inoculum of the two species were signifi cantly lower relative to monocultures 6-d after inoculation. Decreased MC-LR quotas under coculture conditions were enhanced by increasing the D. fl os- aquae to M. aeruginosa ratio in the inoculum and by environmental factors, such as temperature and light intensity. Moreover, the transcriptional concentrations of mcy B and mcy D genes in M. aeruginosa were signifi cantly inhibited by D. fl os- aquae competition in coculture(P <0.01), lowered to 20% of initial concentrations within 8 days. These data suggested that coculture effects by D. fl os- aquae not only reduced M. aeruginosa 's intracellular MC-LR quota via inhibition of genes encoding MC synthetases, but also that this effect was regulated by environmental factors, including temperature and light intensities.
