Seasonal dynamics of photosynthetic activity, Microcystis genotypes and microcystin production in Lake Taihu, China

In recent years, toxic Microcystis blooms have occurred annually in Lake Taihu, China. In order to elucidate the relationships between photosynthetic activity of Microcystis, Microcystis genotypic composition, microcystin (MC) production and environmental factors, water samples and associated environmental data were collected from January to December 2014 in Lake Taihu. Seasonal variations in photosynthetic activity were measured using a Phyto-PAM Analyzer, abundances of total and toxic Microcystis genotypes were determined by quantitative real-time PCR (qPCR), and MC concentrations were quantified by HPLC. The maximum quantum yield of photosystem II (Fv/Fm) of Microcystis cells changed on a seasonal basis. Fv/Fm was not detectable in the winter, but increased from spring to early summer, after which, it gradually decreased until the winter. The level of non-photochemical quenching (NPQ) increased from March to August and then decreased until December. qPCR data showed that the abundances of total Microcystis genotypes ranged from 1.91 × 105 to 8.64 × 107 copies/mL and toxic Microcystis genotypes ranged from 2.38 × 104 to 5.67 × 107 copies/mL. The toxic proportion varied from 12.5 to 65.6%, with an average value of 27.9%. Correlation analysis revealed that there was a positive correlation between photosynthetic activity, genotypic composition and MC production. Water temperature was the only environmental factor that was positively correlated with Fv/Fm, NPQ, total and toxic Microcystis and intracellular MC. Additionally, total phosphorus was also significantly correlated with intracellular MC. These results indicate that future global warming, in addition to eutrophication, could promote Microcystis blooms in Lake Taihu and that blooms may increase in intensity and toxicity.