The harmful cyanobacterium Microcystis aeruginosa PCC7806 is more resistant to hydrogen peroxide at elevated CO₂

Rising atmospheric CO₂ can intensify harmful cyanobacterial blooms in eutrophic lakes. Worldwide, these blooms are an increasing environmental concern. Low concentrations of hydrogen peroxide (H₂O₂) have been proposed as a short-term but eco-friendly approach to selectively mitigate cyanobacterial blooms. However, sensitivity of cyanobacteria to H₂O₂ can vary depending on the available resources. To find out how cyanobacteria respond to H₂O₂ under elevated CO₂, Microcystis aeruginosa PCC 7806 was cultured in chemostats with nutrient-replete medium under C-limiting and C-replete conditions (150 ppm and 1500 ppm CO₂, respectively). Microcystis chemostats exposed to high CO₂ showed higher cell densities, biovolumes, and microcystin contents, but a lower photosynthetic efficiency and pH compared to the cultures grown under low CO₂. Subsamples of the chemostats were treated with different concentrations of H₂O₂ (0-10 mg·L^-1 H₂O₂) in batch cultures under two different light intensities (15 and 100 μmol photons m^-2·s^-1) and the response in photosynthetic vitality was monitored during 24 h. Results showed that Microcystis was more resistant to H₂O₂ at elevated CO₂ than under carbon-limited conditions. Both low and high CO₂-adapted cells were more sensitive to H₂O₂ at high light than at low light. Microcystins (MCs) leaked out of the cells of cultures exposed to 2-10 mg·L^-1 H₂O₂, while the sum of intra- and extracellular MCs decreased. Although both H₂O₂ and CO₂ concentrations in lakes vary in response to many factors, these results imply that it may become more difficult to suppress cyanobacterial blooms in eutrophic lakes when atmospheric CO₂ concentrations continue to rise.