The Operation Of CO₂ Concentrating Mechanism Of Two Model Cyanobacteria And Their Roles In Dissipating Excess Light Energy

The CO₂ concentrating mechanism in cyanobacteria is the most efficient of all photosynthetic organisms and it could elevate CO2 concentration around the active site of ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) to compensate for the low affinity of Rubisco for CO₂. So far, there are at least 5 distinct modes of active Ci uptake in cyanobacteria, however, the physiological data available on the relative contribution of these Ci uptake systems to the photosynthesis and their operation under low or high external Ci concentrations are rather fragmentary. The operation of CCM requires light energy. It has been suggested that the operation of CCM could diminish photodynamic damage by dissipating excess light energy, which has recently been established in the edible cyanobacterium Ge-Xian-Mi (Nostoc). However, no data is available for other cyanobacterial species. It is important to establish whether this is a unique mechanism of Ge-Xian-Mi or a widespread mechanism for most of other cyanobacteria. Thus, I choose the model organisms Synechococcus sp. 805 (also known as Synechococcus PCC 7942) and Synechocystis sp. 898 (also known as Synechocystis PCC 6803) to investigate on inorganic carbon transports and its accumulation within the cell and their roles in alleviating photoinhibiton by the means of photosynthetic oxygen evolution, chlorophyll fluorescence quenching analysis and fluorescence rise kinetics.Multiple transporters for CO₂ or HCO₃^- operated in air-grown Synechococcus sp. 805 and Synechocystis sp. 898, and Na⁺-dependent HCO₃^- transport was their primary mode of active Ci uptake. At 250 μmol L^-1 KHCO₃ and pH 8.0, Na⁺-dependent HCO₃^-transport, CO₂-uptake systems and Na⁺-independent HCO₃^- transport could contribute 47.6-58.9%, 33.8-45.1% and about 7.3% to photosynthesis of Synechococcus sp. 805 respectively, and about 57.0%, 26.5-43.4% and about 16.5% to Synechocystis sp. 898. The inhibition of oxygen evolution of Synechococcus sp. 805 by lack of Na⁺ and addition of EZ (ethoxyzolamide) was reduced at higher KHCO₃ concentration. And the inhibition of Na⁺ lack on photosynthesis of Synechocystis sp. 898 could be relieved when samples were added with 10000 μmol L^-1 KHCO₃. These are probably due to the increased contribution of CO₂ diffusion to total inorganic carbon acquisition or the reduced CO₂ leakage. The photosynthetic oxygen evolution and maximalphotosynthetic efficiency of Synechococcus sp. 805 with high light treatment at 100 ^umol L"1 KHCO3 was much higher than that at 10000 /imol L"1 KHCO3. Results in these studies clearly showed that the operation of the CCM in Synechococcus sp. 805 and Synechocystis sp. 898 served as means of diminishing photodynamic damage by dissipating excess light energy and higher external DIC in the range of 100-10000 fimol L"1 KHCO3 was associated with more severe photoinhibition under high irradiance. When Synechocystis sp. 898 samples added with 10000 /imol L*1 KHCO3 were exposed to high light of 1400 fimol photons m'2 s"1, both Fv/Fm and ETJRC decreased rapidly and the decreasing extent of samples with Na+ addition was lower than those without Na+ addition. Besides, ABS/RC and DIq/RC increased significantly after strong light treatment and the increasing extent of samples with Na+ addition was lower than those without Na+ addition. All these indicate that the operation of Na+-dependent HCO3" transport in Synechocystis sp. 898 could play a role in alleviating photoinhibition.