The CO2-concentrating mechanism in the bloom-forming cyanobacterium Microcystis aeruginosa (Cyanophyceae) and effects of UVB radiation on its operation

The bloom-forming cyanobacterium Microcystis aeruginosa (Kutz.) Kutz. 854 was cultured with 1.05 W . m(-2) ultraviolet-B radiation (UVBR) for 3 h every day, and the CO2-concentrating mechanism (CCM) within this species as well as effects of UVBR on its operation were investigated. Microcystis aeruginosa 854 possessed at least three inorganic carbon transport systems and could utilize external HCO3- and CO2 for its photosynthesis. The maximum photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly decreased from 74.7 mu mol . L-1 in the control to 34.7 mu mol . L-1 in UVBR-treated cells. At 150 mu mol . L-1 KHCO3 and pH 8.0, Na+-dependent HCO3- transport contributed 43.4%-40.2% to the photosynthesis in the control and 34.5%-31.9% in UVBR-treated cells. However, the contribution of Na+-independent HCO3- transport increased from 8.7% in the control to 18.3% in UVBR-treated cells. The contribution of CO2-uptake systems showed little difference: 47.9%-51.0% in the control and 49.8%-47.2% in UVBR-treated cells. Thus, the rate of total inorganic carbon uptake was only marginally affected, although UVBR had a differential effect on various inorganic carbon transporters. However, the number of carboxysomes in UVBR-treated cells was significantly decreased compared to that in the control.