The effect of pH on the release of metabolites by cyanobacteria in conventional water treatment processes

Cyanobacteria and their metabolites are recognised as serious water quality issues for water suppliers around the world. Water shortages not only increase the risk of the proliferation of cyanobacteria but also necessitate the development of more effective treatment regimes. Conventional water treatment processes are considered an effective initial barrier in removing cyanobacterial cells intact and without damage. However, the cells can be exposed to a range of pH conditions during these processes. The impact of various pH conditions typical of the water treatment processes was evaluated on common bloom species, Microcystis aeruginosa, Cylindrospermopsis raciborskii and Anabaena circinalis. By using cyanobacterial metabolite analysis and cell viability assays, it was shown that low pH stress (pH below 5), which can occur during coagulation upon the application of metal hydroxide coagulants, occurs quickly and results in considerable cell lysis and metabolite release. It is demonstrated that it is critical to keep the pH of water above 5 (locally) at all times in order to reduce the risk of water contamination with problem metabolites.