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Photosynthesis and inorganic carbon acquisition in the cyanobacterium Chlorogloeopsis sp. ATCC 27193

1997; Wiley; Volume: 99; Issue: 1 Linguagem: Inglês

10.1111/j.1399-3054.1997.tb03434.x

1399-3054

Robert S. Skleryk, Pascal N. Tyrrell, George S. Espie,

Photoreceptor and optogenetics research

The ability of the morphologically complex cyanobacterium Chlorogloeopsis sp. ATCC 27193 to actively transport and accumulate inorganic carbon (C 1 = CO 2 + HCO 3 − + CO 3 2− ) for photosynthetic CO 2 fixation was investigated. Mass‐spectrometric assays revealed that Chlorogloeopsis cells grown under C 1 limitation rapidly took up CO 2 from the medium in a light‐dependent reaction which was independent of CO 2 fixation. Ethoxyzolamide, a carbonic anhydrase (CA) inhibitor, inhibited CO 2 transport. Since electrometric and mass‐spectrometric assays did not detect the presence of a periplasmic CA, it is suggested that CO 2 transport was mediated by a CA‐like activity which converted CO 2 to HCO 3 − during passage across the membrane. Radiochemical assays, using H 14 CO 3 as substrate, showed that C 3 ‐limited cells also had a high affinity (K 0.5 HCO 3 − = 37 μ M ), Na + ‐independent HCO 3 − uptake mechanism. HCO 3 − uptake was light dependent and occurred against its electrochemical potential indicating a carrier‐mediated, active transport process. The rate of Na + ‐independent HCO 3 − transport was sufficient to account for the steady state rate of CO 2 fixation. Although not absolutely required. Na + did specifically enhance the rate of HCO 3 − transport by up to 2‐fold, but had no effect on the apparent affinity of the transport system for HCO 3 − Combined CO 2 and HCO 3 − transport resulted in C 1 accumulation as high as 25 m M and in excess of 300 times the external concentration. The C 1 pool was the source of CO 2 for photo‐synthetic fixation and was generated, presumably, by the dehydration of HCO 3 − catalyzed by an intracellular CA. The collective evidence indicates that Chlorogloeopsis has a physiologically functional CO 2 ‐concentrating mechanism which is essential for photosynthesis.