THE ROLE OF EXTERNAL CARBONIC-ANHYDRASE IN THE PHOTOSYNTHETIC USE OF INORGANIC CARBON IN THE DEEP-WATER ALGA PHYLLARIOPSIS-PURPURASCENS (LAMINARIALES, PHAEOPHYTA)

Mechanisms of inorganic carbon assimilation were investigated in the d eep-water alga Phyllariopsis purpurascens (C. Agardh) Henry et South ( Laminariales, Phaeophyta). The gross photosynthetic rate as a function of external pH, at a constant concentration of 2 mM dissolved inorgan ic carbon (DIC), decreased sharply from pH 7.0 to 9.0, and was not sub stantially different from 0 above pH 9.0. These data indicate that P. purpurascens is inefficient in the use of external HCO3- as a carbon s ource in photosynthesis. Moreover, the photosynthetic rate as a functi on of external DIC and the highest pH (9.01 +/- 0.07) that this specie s can achieve in a closed system were consistent with a low capacity t o use HCO3-, in comparison to many other species of seaweeds. The role of external carbonic anhydrase (CA; EC 4.2.1.1) on carbon uptake was investigated by measuring both the HCO3--dependent O-2 evolution and t he CO2 uptake, at pH 5.5 and 8.0, and the rate of pH change in the ext ernal medium, in the presence of selected inhibitors of extra- and int racellular CA. Photosynthetic DIG-dependent O-2 evolution was higher a t pH 5.5 (where CO2 is the predominant form of DIC) than at pH 8.0 (wh ere the predominant chemical species is HCO3-). Both intra- and extrac ellular CA activity was detected. Dextran-bound sulfonamide (DBS; a sp ecific inhibitor of extracellular CA) reduced the photosynthetic O-2 e volution and CO2 uptake at pH 8.0, but there was no effect at pH 5.5. The pH-change rate of the medium, under saturating irradiance, was red uced by DBS. Phyllariopsis purpurascens has a low efficiency in the us e of HCO3- as carbon source in photosynthesis; nevertheless, the ion c an be used after dehydration, in the external medium, catalyzed by ext racellular CA. This mechanism could explain why the photosynthetic rat e in situ was higher than that supported solely by the diffusion of CO 2 from seawater.