Damaging effects of ultrasonic treatment on the photosynthetic system of Microcystis aeruginosa

Harmful algal blooms have become a global environmental problem. Ultrasonic technology was found to effectively inhibit the algal growth and reproduction of cyanobacteria in eutrophic water. Given that cyanobacterial blooms are difficult to treat, and treatment methods may cause problems leading to secondary pollution, a preventive control was proposed. In this research, algal cell sedimentation was performed to analyze the effects of ultrasonic technology on algae. The photosynthetic system of algae and the optimal combination of the ultrasonic removal of Microcystis aeruginosa were also studied. Low-frequency ultrasound (20, 40 and 100 kHz) was selected to analyze the changes in phycobiliprotein content, the expression level of the key gene in photosynthesis, and the settlement index after treatment with different ultrasonic frequencies and power densities. Results showed that the phycobiliprotein content and gene expression level decreased after ultrasonic treatment, and a large quantity of cells settled. The inhibition effectiveness at 40 kHz was much better than that at 20 and 100 kHz. When the power density of 0.02 W/mL decreased and was applied at a frequency of 40 kHz, the relative content of phycobiliprotein decreased significantly, and the phycobiliprotein level increased by 44%-45% compared with that of the control group after 10 d. Relative quantifications (RQs) of the rbcL gene (encoding RUBISCO protein) and cpc gene (encoding phycocyanin protein) were inhibited with a frequency of 40 kHz. The RQ value decreased by more than 40%. When the power density remained constant and was applied with a frequency of 40 kHz, both the algal cell sedimentation index and the inhibitory effect on algal growth increased. Thus, ultrasound with frequency of 40 kHz and intervals of 4-6 d is suggested when dealing with algal cells to prevent the algal biomass below the level of algal blooms from achieving preventive restraint, which is the ability of algal cells to self-repair and reproliferate.