Suppressing Cyanobacteria with Hydrogen Peroxide Is More Effective at High Light Intensities

Hydrogen peroxide (H(2)O(2)) can be used as an emergency method to selectively suppress cyanobacterial blooms in lakes and drinking water reservoirs. However, it is largely unknown how environmental parameters alter the effectiveness of H(2)O(2) treatments. In this study, the toxic cyanobacterial strain Microcystis aeruginosa PCC 7806 was treated with a range of H(2)O(2) concentrations (0 to 10 mg/L), while being exposed to different light intensities and light colors. H(2)O(2) treatments caused a stronger decline of the photosynthetic yield in high light than in low light or in the dark, and also a stronger decline in orange than in blue light. Our results are consistent with the hypothesis that H(2)O(2) causes major damage at photosystem II (PSII) and interferes with PSII repair, which makes cells more sensitive to photoinhibition. Furthermore, H(2)O(2) treatments caused a decrease in cell size and an increase in extracellular microcystin concentrations, indicative of leakage from disrupted cells. Our findings imply that even low H(2)O(2) concentrations of 1–2 mg/L can be highly effective, if cyanobacteria are exposed to high light intensities. We therefore recommend performing lake treatments during sunny days, when a low H(2)O(2) dosage is sufficient to suppress cyanobacteria, and may help to minimize impacts on non-target organisms.