Determining the fate of Microcystis aeruginosa cells and microcystin toxins following chloramination

The cyanobacterium Microcystis aeruginosa can produce potent toxins known as microcystins. While many studies have focussed on the chlorination of microcystin toxins, little work has been conducted with respect to the chloramination of the microcystins. In addition, no studies have been reported on the effect of chloramination on intact Microcystis cells. This study was conducted to determine the fate of M. aeruginosa cells and microcystin toxins following chloramination of a drinking water source. Results indicate that monochloramine could effectively oxidise dissolved microcystin-LR (MCLR) provided high CT values were employed, typically greater than 30,000 mg min L⁻¹. The decay of MCLR was demonstrated to be a pseudo first-order reaction with rate constants ranging from 9.3 × 10⁻⁷ to 1.1 × 10⁻⁵ s⁻¹ at pH 8.5. However, in the presence of Microcystis cells, monochloramine was ineffective in oxidising microcystin toxins due to the cells exerting a demand on the oxidant. The doses of monochloramine applied (2.8 and 3.5 mg L⁻¹) were shown to rapidly release intracellular microcystins into the dissolved state. Flow cytometric analysis of the cells determined that the lower monochloramine dose did not compromise the cell membrane integrity, even though microcystins were rapidly released from the cells. In contrast the higher monochloramine dose resulted in cell membrane disruption with up to 90% of the cells shown to be non-viable after the high dose was applied.