Interspecific protection against oxidative stress: green algae protect harmful cyanobacteria against hydrogen peroxide.

Oceanographic studies have shown that heterotrophic bacteria can protect marine cyanobacteria against oxidative stress caused by hydrogen peroxide (H2 O2 ). Could a similar interspecific protection play a role in freshwater ecosystems? In a series of laboratory experiments and two lake treatments, we demonstrate that freshwater cyanobacteria are sensitive to H2 O2 but can be protected by less-sensitive species such as green algae. Our laboratory results show that green algae degrade H2 O2 much faster than cyanobacteria. Consequently, the cyanobacterium Microcystis was able to survive at higher H2 O2 concentrations in mixtures with the green alga Chlorella than in monoculture. Interestingly, even the lysate of destructed Chlorella was capable to protect Microcystis, indicating a two-component H2 O2 degradation system in which Chlorella provided antioxidant enzymes and Microcystis the reductants. The level of interspecific protection provided to Microcystis depended on the density of Chlorella. These findings have implications for the mitigation of toxic cyanobacterial blooms, which threaten the water quality of many eutrophic lakes and reservoirs worldwide. In several lakes, H2 O2 has been successfully applied to suppress cyanobacterial blooms. Our results demonstrate that high densities of green algae can interfere with these lake treatments, as they may rapidly degrade the added H2 O2 and thereby protect the bloom-forming cyanobacteria.