The effect of low frequency ultrasonic treatment on the release of extracellular organic matter of Microcystis aeruginosa

Abstract

The effect of low frequency ultrasonic treatment on the growth of cyanobacteria and the behavior of extracellular organic matter (EOM) were investigated in this study. The growth of cyanobacteria can be controlled effectively by low frequency ultrasonic treatment, and the inhibitory effect of ultrasound on the photosynthetic ability of cells was more significant than on the cells density (the minimum values of cells density and chlorophyll-a was obtained at day 3 and day 1 respectively). Significant rupture of cells was observed when irradiated by a higher intensity ultrasound (ultrasonic dosage ≥ 1.5 J/mL). Tyrosine-like components were mainly distributed in the soluble EOM and loosely bound EOM fractions; both of the tryptophan-like components and polyaromatic humic-like components were distributed in the soluble EOM, loosely bound EOM, and tightly bound EOM fractions; and polycarboxylate humic-like components were mainly distributed in soluble EOM fraction. Given that the production of hydroxyl radicals was measured, negligible due to the limit of ultrasonic treatment duration (≤1 min). On the other hand, intensive shock waves and shear forces of collapsing acoustic cavitation bubbles were considered as the main reason for the rupture of cells and the changes of the behavior of EOM. A higher intensity ultrasonic treatment resulted in a greater release of EOM, including bound EOM (ultrasonic dosage ≥ 0.1 J/mL), and intracellular organic matter (ultrasonic dosage ≥ 1.5 J/mL). Moreover, the determination of protein and polysaccharide suggested that the bonding of polysaccharides to cells surface is stronger than that of proteins to cells surface, and a portion of the polysaccharides in other fractions was transferred to the tightly bound state during the first serval days after ultrasonic treatment.