Seasonal dynamics of photosynthetic activity of Microcystis, genotype abundances and microcystin concentrations in Meiliang Bay, Lake Taihu

Lake Taihu is the third largest freshwater lake (surface area: 2338 km2, mean depth: 1.9 m) in eastern China, and it is also the primary drinking water source for 30 million residents in the lake basin. In recent decades, Lake Taihu has annually experienced lake-wide cyanobacterial blooms. However, the seasonal changes of photosynthetic activity of Micricystis, genotype abundances and microcystin and their relationship with environmental factors are still poorly understood. In the present study, surface water samples in Meiliang Bay were collected monthly from January to December in the year of 2014, at the same time, we investigated the photosynthetic activity of Micricystis, the abundances of total and toxic Microcystis genotypes, and microcystins (MCs) concentrations using pulse amplitude modulation (PAM), quantitative real-time PCR (qPCR), and high performance liquid chromatography (HPLC) techniques, respectively. It was found that the maximum and effective quantum yields both had an obvious seasonal patterns. In winter season, photosynthetic activity were not detectable, Then, it increased fast from spring to summer, at the last, it gradually decreased from fall to winter. The level of non-photochemical quenching (NPQ) increased from March to August, when it showed a maximum, decreasing thereafter toward December. The abundances of total and toxic Microcystis genotypes ranged from 1.91 ± 0.15 × 105 to 8.64 ± 0.56 × 107, from 2.38 ± 0.25 × 104 to 5.67 ± 0.43 × 106 copies/mL, respectively, meanwhile, the toxic proportion ranged from 12.5 ± 1.3 to 65.6 ± 3.8%, and they all showed similar change patterns with increasing first and then decreasing later. The extacellular and intracellular MCs concentrations varied from 0.17 ± 0.05 to 1.82 ± 0.35 μg/L, from 0.59 ± 0.18 to 14.36 ± 1.62 μg/L, respectively. The correlation analysis suggested that photosynthetic capacity and NPQ of Microcystis had positive correlation with water temperature. Meanwhile, there were strong positive correlation among the total and toxic Microcystis genotypes, toxic proportion and intracellular MCs, and they were significant positively correlated with water temperature. In addition, intracellular MCs also have strong positive correlation with total phosphorus. In conclusion, the photosynthetic activity of Microcystis was corresponded to the dynamics of Microcystis bloom, and water temperature may be a key environmental factor which determined Microcystis photosynthetic activity, total and toxic Microcystis genotypes abundances as well as intracellular MCs in Lake Taihu.