CO₂ Concentrating Mechanism In The Bloom-Forming Cyanobacterium Microcystis Aeruginosa And The Effect Of UV-B Radiation On Its Operation

Many alage require CO₂ concentrating mechanism (CCM) to increase the CO₂ concentration in the proximity of Rubisco and to compensate for the low affinity of Rubisco for CO₂, and this is expecially evident in cyanobacteria. A number of CCM variants have been found among the different species or in the same specie under different grown conditions. CCM modulation may be crucial in the energetic and nutritional budgets of cell, and a multitude of environmental factors can exert regulatory effects on the expression of CCM components. The CCM in cyanobacterium was reviewed in the present paper, and then we studied the CCM in one of the most harmful bloom-forming cyanobacterium Microcystis aeruginosa Kutz 854 and the effects of UV-B on its operation.Air-grown M. aeruginosa 854 could utilize HCO₃^- for its photosynthesis and possessed multiple transporters for CO₂ and HCO3₃^-. Its Carboxysomes were regularly pentagonal or round and the average number of carboxysomes per cell section was 3.41±1.37. After exposed to long-term UV-B, the maximal photosynthetic rate was approximately the same, but the apparent affinity for dissolved inorganic carbon was significantly increased. At 150 umol·L^-1 KHCO₃ and pH8.0, Na⁺-dependent HCO₃^-transporter contributed 43.4-40.2% to the photosynthesis in the control, while it was 34.5-31.9% in UV-B treated cells. However, the contribution of Na⁺-independent HCO₃^- transporter increased from 8.7% in the control to 18.3% in UV-B treated cells. As for CO₂-uptake systems, they were less sensitive than the HCO₃^- transport systems, and the contribution between them showed little difference, respectively 47.9-51.0% in the control and 49.8-47.2% in UV-B treated cells. Thus, the rate of total inorganic carbon uptake was marginally affected although UV-B had a different effect on various inorganic carbon transporters. Carboxysomes in the UV-B treated samples appeared to have normal structure, but their numbers significantly declined to 2.70±1.18. Therefore, CCM can be efficiently regulated to adapt to the long-term UV-B exposure in M. aeruginosa.