Colonial morphology weakens the response of different inorganic carbon uptake systems to CO levels in Microcystis population.

Rising atmospheric CO concentration negatively impacts aquatic ecosystems and may induce evolutionary changes in the CO-concentrating mechanism (CCM) of cyanobacteria. As the most notorious freshwater cyanobacteria, Microcystis strains have high phenotypic plasticity to form colonies and blooms in lakes and reservoirs worldwide. However, phenotypic plasticity of Microcystis responses to elevated CO is still a major open question. Here, we studied how Microcystis strains with two genotype of inorganic carbon uptake systems, bicA and sbtA, and different colonial morphology response to 200 ppm, 400 ppm, and 800 ppm CO levels. The results revealed that sbtA genotypes showed significantly higher specific growth rates, Chl a concentration, and photosynthetic efficiency at 200 ppm CO, whereas higher specific growth rates, Chl a concentration, and photosynthetic efficiency were found in bicA genotype at 800 ppm CO. The highest values of specific growth rates, Chl a concentration, F/F, and maximal net photosynthesis (Pm) were observed in unicellular morphology, followed by small colony and large colonial morphology at all CO levels. The values of K (DIC), K (CO), and K (HCO) in the large colonials increased with rising CO levels, but these values significantly decreased in the unicellular and small colonials. ANOSIM analysis indicated that colonial morphology reduced significantly inter-group differences between bicA and sbtA genotypes at all CO treatments. These results suggest that colonial morphology of Microcystis can weakens the response of different inorganic carbon uptake systems to CO levels. Moreover, phenotypic and genotypic plasticity is likely to broaden strongly the fitness of Microcystis from rising atmospheric CO.