Cyanobacteria in eutrophic waters benefit from rising atmospheric CO₂ concentrations

Rising atmospheric carbon dioxide (CO₂) may stimulate the proliferation of cyanobacteria. To investigate the possible physiological responses of cyanobacteria to elevated CO₂ at different nutrient levels, Microcystis aeruginosa were exposed to different concentrations of CO₂ (400, 1100, and 2200 ppm) under two nutrient regimes (i.e., in nutrient-rich and nutrient-poor media). The results indicated that M. aeruginosa differed in its responses to elevated atmospheric CO₂ at different nutrient levels. The light utilization efficiency and photoprotection of photosystem II were improved by elevated CO₂, particularly when cells were supplied with abundant nutrients. In nutrient-poor media, both total organic carbon and the polysaccharide/protein ratio of the extracellular polymeric substance increased with elevated CO₂, accompanied by high cellular carbon/nitrogen ratios. Besides, cells growing with fewer nutrients were more prone to suffer intracellular acidification with elevated CO₂ than those growing with abundant nutrients. Nonetheless, alkaline phosphate activity of cyanobacteria was improved by high CO₂, provided that reduced pH was in the optimum range for alkaline phosphate activity. Nitrate reductase activity was inhibited by elevated CO₂ regardless of nutrient levels, leading to a reduced nitrate uptake. These changes indicate that the biogeochemical cycling of nutrients would be affected by higher atmospheric CO₂ conditions. Overall, cyanobacteria in eutrophic waters may benefit more than in oligotrophic waters from rising atmospheric CO₂ concentrations, and evaluations of the influence of rising atmospheric CO₂ on algae should account for the nutrient level of the ecosystem.