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Utilization of inorganic carbon in the edible cyanobacterium Ge‐Xian‐Mi (Nostoc) and its role in alleviating photo‐inhibition

The present work investigated the inorganic carbon (C;) uptake, fluorescence quenching and photo-inhibition of the edible cyanobacterium Ge-Xian-Mi (Nostoc) to obtain an insight into the role of CO 2 concentrating mechanism (CCM) operation in alleviating photo-inhibition. Ge-Xian-Mi used HCO 3 - in addition to CO 2 for its photosynthesis and oxygen evolution was greater than the theoretical rates of CO 2 production derived from uncatalysed dehydration of HCO 3 - . Multiple transporters for CO 2 and HCO 3 - operated in air-grown Ge-Xian-Mi. Na + -dependent HCO 3 - transport was the primary mode of active C i uptake and contributed 53-62% of net photosynthetic activity at 250 μmol L -1 KHCO 3 and pH 8.0. However, the CO 2 -uptake systems and Na + -independent HCO 3 - transport played minor roles in Ge-Xian-Mi and supported, respectively, 39 and 8% of net photosynthetic activity. The steady-state fluorescence decreased and the photochemical quenching increased in response to the transport-mediated accumulation of intracellular C i . Inorganic carbon transport was a major factor in facilitating quenching during the initial stage and the initial rate of fluorescence quenching in the presence of iodoacetamide, an inhibitor of CO 2 fixation, was 88% of control. Both the initial rate and extent of fluorescence quenching increased with increasing external dissolved inorganic carbon (DIC) and saturated at higher than 200 μmol L -1 HCO 3 - . The operation of the CCM in Ge-Xian-Mi served as a means of diminishing photodynamic damage by dissipating excess light energy and higher external DIC in the range of 100-10000 μmol L -1 KHCO 3 was associated with more severe photo-inhibition under strong irradiance.