Strains of the Harmful Cyanobacterium Microcystis aeruginosa Differ in Gene Expression and Activity of Inorganic Carbon Uptake Systems at Elevated CO2 Levels

摘要

Cyanobacteria are generally assumed to be effective competitors at low CO_(2) levels because of their efficient CO_(2)-concentrating mechanism (CCM), and yet how bloom-forming cyanobacteria respond to rising CO_(2) concentrations is less clear. Here, we investigate changes in CCM gene expression at ambient CO_(2) (400 ppm) and elevated CO_(2) (1,100 ppm) in six strains of the harmful cyanobacterium Microcystis . All strains downregulated cmpA encoding the high-affinity bicarbonate uptake system BCT1, whereas both the low- and high-affinity CO_(2) uptake genes were expressed constitutively. Four strains downregulated the bicarbonate uptake genes bicA and/or sbtA , whereas two strains showed constitutive expression of the bicA - sbtA operon. In one of the latter strains, a transposon insert in bicA caused low bicA and sbtA transcript levels, which made this strain solely dependent on BCT1 for bicarbonate uptake. Activity measurements of the inorganic carbon (C_(i)) uptake systems confirmed the CCM gene expression results. Interestingly, genes encoding the RuBisCO enzyme, structural carboxysome components, and carbonic anhydrases were not regulated. Hence, Microcystis mainly regulates the initial uptake of inorganic carbon, which might be an effective strategy for a species experiencing strongly fluctuating C_(i) concentrations. Our results show that CCM gene regulation of Microcystis varies among strains. The observed genetic and phenotypic variation in CCM responses may offer an important template for natural selection, leading to major changes in the genetic composition of harmful cyanobacterial blooms at elevated CO_(2).