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.