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

Abstract

ABSTRACTThe present work investigated the inorganic carbon (Ci) uptake, fluorescence quenching and photo‐inhibition of the edible cyanobacterium Ge‐Xian‐Mi (Nostoc) to obtain an insight into the role of CO2 concentrating mechanism (CCM) operation in alleviating photo‐inhibition. Ge‐Xian‐Mi used HCO3– in addition to CO2 for its photosynthesis and oxygen evolution was greater than the theoretical rates of CO2 production derived from uncatalysed dehydration of HCO3–. Multiple transporters for CO2 and HCO3– operated in air‐grown Ge‐Xian‐Mi. Na+‐dependent HCO3– transport was the primary mode of active Ci uptake and contributed 53–62% of net photosynthetic activity at 250 µmol L−1 KHCO3 and pH 8.0. However, the CO2‐uptake systems and Na+‐independent HCO3– 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 Ci. 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 CO2 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 HCO3–. 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 KHCO3 was associated with more severe photo‐inhibition under strong irradiance.