Influence of CO2 concentrating mechanism on photoinhibition in Synechococcus sp PCC7942 (Cyanophyceae)

In this study the effects of inorganic carbon transport in the cyanobacterium Synechococcus sp. PCC7942 on its photoinhibition were investigated. The photosynthetic oxygen evolution of samples under a strong irradiance treatment at 100 mu M KHCO3 was much higher than that at 10,000 mu M KHCO3. After strong irradiance treatment, the abundance of the sbtA transcript encoding a Na+-dependent HCO3- transporter was significantly higher under treatment at 100 mu M KHCO3 than at 10,000 mu M KHCO3. Lincomycin had little effect on the difference of photosystem II (PSII) activity decline under these two treatments, which suggests that CCM activity reduces photodamage but does not boost the rate of PSII repair. The maximal photosynthetic efficiency was decreased significantly during the strong irradiance treatment, and its decrease was enhanced when the external DIC concentration was raised from 100 to 10,000 mu M KHCO3. The JIP test showed that there were more active reaction centres under low C-i conditions after high light treatment. Studies on 77-K fluorescence emission spectra showed that the fluorescence peaks F-692 (nm) and F-710 nm in 10,000-mu M KHCO3 treated samples both decreased after strong irradiance treatment and were significantly smaller than those of 100-mu M KHCO3 treated samples. Samples treated with 100 mu M KHCO3, compared to 10,000 mu M KHCO3 had a higher possibility of transmitting an exciton to a closed reaction centre rather than to an open reaction centre and thus to reduce the overexcitation of the reaction centres. These results indicated that higher external inorganic carbon concentration was associated with more severe photoinhibition under high irradiance and that the operation of CO2 concentrating mechanism might reduce the overexcitation of reaction centres to reduce photodamage.