Chiang Mai Journal of Science

Surface bloom of Microcystis aeruginosa and M. wesenbergii occurs frequently in reservoirs and aquaculture ponds in Thailand. Previous studies showed the effectiveness of ultrasonic irradiation at sinking M. aeruginosa cells. This study 1) compared the efficiency of ultrasonic treatment at sinking M. aeruginosa and M. wesenbergii, 2) investigated the ultrastructure of Microcystis cells using TEM before and after ultrasonic treatment and 3) determined the re-flotation behavior of sonicated M. aeruginosa. Initial chlorophyll-a concentrations of M. wesenbergii and M. aeruginosa were 4.674 ± 0.107 mg/L and 4.447 ± 0.024 mg/L, respectively. M. wesenbergii exhibited less efficient algal sedimentation than that of M. aeruginosa after sonication for 120-600 seconds (p<0.05). This result implied that the species of Microcystis which comprise the surface bloom affect ultrasonic treatment efficiency. Transmission electron micrographs confirmed the mechanical damage brought to the cell vacuoles and structures by ultrasonic cavitation. Moreover, after sonication for 30 seconds, M. aeruginosa cells (initial chlorophyll a of 4.560 ± 0.040 mg/L) could not re-float to the surface over 10 observation days.