THE DEVELOPMENT OF A CO2-CONCENTRATING MECHANISM IN EMILIANIA-HUXLEYI

The development of an inorganic carbon concentrating mechanism (CCM) i n relation to the expression of extracellular carbonic anhydrase (CA(e xt)) in cells from the stationary phase of Emiliamia huxleyi cultures was investigated. Unlike exponential phase cells, those in the station ary phase showed a high affinity for CO2: the K-0.5[CO2] (i.e. the CO2 concentration required for half-maximal rate of photosynthesis) was 1 .6 mu M at pH 8.3. Measurement of the internal inorganic carbon concen tration by the silicone oil centrifugation technique showed that, with 1 mM external dissolved inorganic carbon (DIC), the intracellular ino rganic carbon concentration was sixfold greater at pH 7.5 and twofold greater at pH 8.3, than the external concentration. The potent CA(ext) inhibitor, dextran-bound sulphonamide (DBS), gave an 80% inhibition o f DIC-dependent photosynthetic oxygen evolution. Similarly, DBS decrea sed the intracellular DIC concentration and inhibited photosynthetic ( CO2)-C-14 fixation. The lipid-permeable CA inhibitor, ethoxyzolamide ( EZ), increased the intracellular DIC concentration but inhibited photo synthetic (CO2)-C-14 fixation. The observed intracellular DIC concentr ations indicated that the stationary phase cells have the capacity to transport CO2 actively against a concentration gradient.