Chemical control of hepatotoxic phytoplankton blooms: Implications for human health

Chemicals used to control phytoplankton blooms induce the release of phytotoxins that increase the potential health risks in drinking water supplies. To test this hypothesis, the effects of six chemical treatments on the release of the cyanobacterial toxin, microcystin-LR (MCLR; 182–837 μg g −1 dry wt) from freshly collected phytoplankton were examined in laboratory experiments. In addition, the integrity of a chemically-treated culture of Microcystis aeruginosa was examined by both a scanning electron microscope and a transmission electron microscope. Chemicals which control cyanobacterial blooms through inhibition of cell functions (e.g. Reglone A, potassium permanganate, chlorine, and Simazine) appeared to induce cell lysis and subsequently increased dissolved MCLR concentration in the surrounding water. In contrast, both lime and alum treatment (within pH 6–10) controlled the cyanobacterial blooms mainly by cell-coagulation and sedimentation, without any (lime) or only little (alum) increase in dissolved MCLR concentration in the water. The estimated half-life of released MCLR from these dense cyanobacterial blooms ranged from 0.5 (± 0.1) to 1.6 (± 0.0) d. In contrast, × ⩾39% of the MCLR remained in decaying phytoplankton for up to 26 d, therefore it is likely that MCLR would persist and decay inside the lime or alum coagulated Microcystis cells, before being released into the surrounding water phase. For these reasons, lime or to a lesser extent alum, appears to be more suitable than either algicides or chlorine for the control of microcystin-containing cyanobacterial blooms in drinking water.