Immediate and long-term impacts of UV-C irradiation on photosynthetic capacity, survival and microcystin-LR release risk of Microcystis aeruginosa

In this study, the immediate and long-term impacts of shortwave ultraviolet (UV-C) irradiation on photosynthetic capacity, survival, and recovery of Microcystis aeruginosa were investigated. The risk of microcystin-LR (MC-LR) release during irradiation was also estimated. The cell density was determined by a flow cytometry, and typical chlorophyll fluorescence parameters, including the effective quantum yield, photosynthetic efficiency and maximal electron transport rate, were measured by a pulse amplitude modulated (PAM) fluorometer. Under various UV-C dosages (140-4200mJ cm^-2), photosynthetic capacities were reduced, to different degrees, accompanied by slight cytoclasis and complete degradation of extracellular MC-LR immediately after irradiation. In a 6-d cultivation following UV-C irradiation, cell density and extracellular MC-LR in the samples treated by 140mJcm^-2 UV-C irradiation increased from 4.0×10⁶cellsmL^-1 and 8μgL^-1 to 5.1×10⁶cellsmL^-1 and 20μgL^-1, respectively. Significant M. aeruginosa cytoclasis (cell density from 4.0×10⁶ to 1.0×10⁶cellsmL^-1) and MC-LR release (2-25μgL^-1) occurred when the UV-C dosage reached 350mJcm^-2. Cell cytoclasis and MC-LR release were enhanced in the cultivated samples under higher UV-C dosages. Results revealed that photosynthetic parameters were useful tools to predict the recovery profiles of M. aeruginosa cells, and the MC-LR release risk should be considered after UV-C inactivation.