Effects of ultrasonic irradiation on organic matter of Microcystis aeruginosa cells

Harmful algal bloom stimulated by eutrophication has gained increasing attention worldwide. This phenomenon deteriorates water quality and destroys the local ecological equilibrium, and thus must be addressed. Ultrasound can effectively inhibit the growth of bloom algae, but its effect on the organic matter of Microcystis aeruginosa cells remains unknown. Moreover, the mechanism of ultrasonic irradiation is unclear. This study investigated algal biomass, zeta potential, organic matter, and protein after treatment with different ultrasonic frequencies and power densities to optimize ultrasonic parameters and reveal the mechanism of the effect of ultrasonic irradiation on algae. Results showed that low-frequency ultrasonic irradiation effectively inhibited blue algae growth. The zeta potential, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) protein gray value, and phycobiliprotein level decreased, whereas the fluorescence peak of fulvic acid and dead cells increased. The damage to the RuBisCO protein in algae treated by ultrasonic frequency of 40 kHz was more severe than that in algae treated by the frequency of 20 and 100 kHz under the power density of 0.0628 W/mL. The high ultrasonic density caused severe damage to the RuBisCO protein and photosynthesis. Under a specific ultrasonic frequency, higher power density leads to more obvious inhibition on algae cells. Thus, the growth of M. aeruginosa can be controlled under the optimal ultrasonic frequency and power density of 40 kHz and 0.0628 W/mL, respectively.