Biocidal H 2 O 2 treatment emphasizes the crucial role of cyanobacterial extracellular polysaccharides against external strong oxidative stress.

A biocidal level of hydrogen peroxide (H 2 O 2 ), far beyond the natural level, is widely used to control bloom-forming cyanobacteria in freshwater. The extracellular polymeric substance of these cyanobacteria is a key factor in determining the applied H 2 O 2 dosage. The exopolysaccharide (EPS) in the extracellular polymer shows H 2 O 2 scavenging capability. However, the scavenging capabilities of EPSs from other cyanobacteria against biocidal levels of H 2 O 2 as well as their protective roles against cyanobacterial cells are not well known. In this study, we used four nonbloom-forming cyanobacteria as target organisms, two with rich EPS envelopes (EPS-rich strains) and two with thin EPS envelopes (EPS-thin strains), to assess the roles of EPS. It was found that the two EPS-rich strains were much more tolerant to a high dose of exogenous H 2 O 2 than the two EPS-thin strains. The EPSs extracted from the four strains exhibited similar but rapid H 2 O 2 scavenging activity. Additionally, the EPSs from the EPS-rich strains could improve the tolerance of the EPS-thin strains to H 2 O 2 stress, implying potentially nonselective protection against oxidative stress. In addition, all the cell lysates of the four strains showed H 2 O 2 decomposition ability, with the efficiency being slightly different between the two types of strains. This study suggests that cyanobacterial EPS plays a generally crucial role against external strong oxidative stress and may provide a useful reference for the application of H 2 O 2 in environmental management.