Cyanobacteria in eutrophic waters benefit from rising atmospheric CO 2 concentrations.

Rising atmospheric carbon dioxide (CO ₂ ) may stimulate the proliferation of cyanobacteria. To investigate the possible physiological responses of cyanobacteria to elevated CO ₂ at different nutrient levels, Microcystis aeruginosa were exposed to different concentrations of CO ₂ (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 CO ₂ at different nutrient levels. The light utilization efficiency and photoprotection of photosystem II were improved by elevated CO ₂ , 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 CO ₂ , accompanied by high cellular carbon/nitrogen ratios. Besides, cells growing with fewer nutrients were more prone to suffer intracellular acidification with elevated CO ₂ than those growing with abundant nutrients. Nonetheless, alkaline phosphate activity of cyanobacteria was improved by high CO ₂ , provided that reduced pH was in the optimum range for alkaline phosphate activity. Nitrate reductase activity was inhibited by elevated CO ₂ 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 CO ₂ conditions. Overall, cyanobacteria in eutrophic waters may benefit more than in oligotrophic waters from rising atmospheric CO ₂ concentrations, and evaluations of the influence of rising atmospheric CO ₂ on algae should account for the nutrient level of the ecosystem.
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