PHOTOSYNTHESIS AND INORGANIC CARBON ACQUISITION IN THE CYANOBACTERIUMCHLOROGLOEOPSIS SP ATCC-27193

The ability of the morphologically complex cyanobacterium Chlorogloeop sis sp. ATCC 27193 to actively transport and accumulate inorganic carb on (C-i = CO2 + HCO3- + CO32- for photosynthetic CO2 fixation was inve stigated. Mass-spectrometric assays revealed that Chlorogloeopsis cell s grown under C-i limitation rapidly took up CO2 from the medium in a light-dependent reaction which was independent of CO2 fixation. Ethoxy zolamide, a carbonic anhydrase (CA) inhibitor, inhibited CO2 transport . Since electrometric and mass-spectrometric assays did not detect the presence of a periplasmic CA, it is suggested that CO2 transport was mediated by a CA-like activity which converted CO2 to HCO3- during pas sage across the membrane. Radiochemical assays, using (HCO3-)-C-14 as substrate, showed that C-i-limited cells also had a high affinity (K-0 .5 HCO3- = 37 mu M), Na+-independent HCO3- uptake mechanism. HCO3- upt ake was light dependent and occurred against its electrochemical poten tial indicating a carrier-mediated, active transport process. The rate of Na+-independent HCO3- transport was sufficient to account for the steady state rate of CO2 fixation. Although not absolutely required, N a+ did specifically enhance the rate of HCO3- transport by up to 2-fol d, but had no effect on the apparent affinity of the transport system for HCO3-. Combined CO2 and HCO3- transport resulted in C-i accumulati on as high as 25 mM and in excess of 300 times the external concentrat ion. The C-i pool was the source of CO2 for photosynthetic fixation an d was generated, presumably, by the dehydration of HCO3- catalyzed by an intracellular CA, The collective evidence indicates that Chlorogloe opsis has a physiologically functional CO2-concentrating mechanism whi ch is essential for photosynthesis.