Photosynthetic use of exogenous inorganic carbon in the agarophyte Gracilaria lemaneiformis (Rhodophyta)

Abstract

Oxygen evolution characteristics of the agarophyte Gracilaria lemaneiformis (Bory) Weber-van Bosse (Rhodophyta) were examined to establish the mechanism of photosynthetic use of exogenous inorganic carbon (Ci). Oxygen evolution in seawater of pH 8.0 was sharply reduced with the increase of pH. However, the rates of O 2 evolution at pH 8.0 and above exceeded the theoretical calculated maximum rates supported solely by the CO 2 supply from the spontaneous dehydration of HCO 3 − in the bulk seawater. In addition, G. lemaneiformis exhibited a high pH compensation point (9.58). These data indicate that G. lemaneiformis was able to use HCO 3 − as a source of Ci for photosynthesis. Both external and internal carbonic anhydrase (CA) activities, which could be detected potentiometrically, were essential for photosynthesis at alkaline pH values (8.0 and 9.0), because O 2 evolution was inhibited with the addition of CA inhibitors acetazolamide (AZ) and ethoxyzolamide (EZ). By contrast, the anion exchanger inhibitor, 4,4′-diisothiocyano-stilbene-2,2′-disulfonate (DIDS) had no inhibitory effects on photosynthetic O 2 evolution at neither acidic nor alkaline pH values. Therefore, the experiments with CA inhibitors would suggest that the mechanism of extracellular dehydration of HCO 3 − into CO 2 mediated by external CA activity was involved in HCO 3 − use by G. lemaneiformis. The relationship between photosynthetic O 2 evolution rates and Ci concentrations showed that the normal Ci composition in natural seawater could fully saturate photosynthesis of G. lemaneiformis at pH 8.0 or lower. However, the affinity for Ci and the carbon-saturated maximum photosynthesis were dramatically reduced at pH 9.0.