Evaluation of the mechanisms of the effect of ultrasound on Microcystis aeruginosa at different ultrasonic frequencies.

Blooms of cyanobacteria are now considered to be a common environmental issue. They are hazardous to both domestic and wild animals and humans. Current treatments are unable to effectively control such blooms as they become tolerant to biocides and it is difficult to degrade cyanobacterial toxins in water. Alternative methods for control are currently under investigation. One potential effective method is ultrasonic irradiation. Ultrasound inactivates algal and cyanobacteria cells through cavitation by generating extreme conditions, resulting in a number of physical, mechanical and chemical effects. The aim of this study was to investigate the effect of ultrasound at different frequencies on Microcystis aeruginosa. Flow cytometry was used to measure cyanobacterial metabolic cell viability in addition to the more commonly used haemocytometry, optical density and fluorimetry. Results indicate low frequency 20 kHz ultrasound with high intensity (0.0403 W cm(-3)) is effective for the inactivation of cyanobacterial cells. Higher frequencies of 580 kHz (0.0041 W cm(-3)) also resulted in an inactivation effect, but 1146 kHz (0.0018 W cm(-3)) showed a declumping effect as evidenced by flow cytometry. Ultrasonic treatment over time under different sonication conditions demonstrates the following: 1. Acoustic cavitation via mechanical effects can induce sufficient shear forces to directly rupture cyanobacteria cells. 2. At higher ultrasonic frequencies the mechanical energy of cavitation is less but a larger proportion of free radicals are produced from the ultrasonic degradation of water, which chemically attacks and weakens the cyanobacteria cell walls. 3. At higher frequencies free radicals also damage chlorophyll a leading to a loss in photosynthetic cell viability. 4. At low powers ultrasonic energy results in declumping of cyanobacteria.