External carbonic anhydrase and affinity for inorganic carbon in intertidal macroalgae

Sixteen intertidal macroalga species from the Gibraltar Strait (Southern Spain) have been examined for both their external carbonic anhydrase activity (CA; 4.2.1.1.) and their affinity for inorganic carbon (C i). Information on the ability to use HCO 3 − was obtained by measuring the effect of increasing seawater pH on the photosynthesis and by examining the O 2 evolution curves vs. C i concentration. The species exhibited a range of characteristics of C i utilisation: the green macroalgae were saturated at seawater C i concentration with K m ranging from 250.0 to 493.3 mmol m −3 and the photosynthetic O 2 production, expressed as conductance for CO 2 ranging from 432.8 to 16.1×10 −6 m s −1. Only three out of the eleven tested red and brown macroalgae were saturated at C i concentration of seawater and for these algae HCO 3 − use was clearly indicated by the high initial slope of the O 2 evolution rate vs. C i curve. The most efficient HCO 3 − users were species from the high intertidal habitats and rockpools. Two methods were used to infer the presence of external CA: standard method (direct enzyme assay) and use of acetazolamide (AZ), a non-permeant inhibitor of CA, in assays of photosynthetic rate. Some discrepancies between the two methods were found and we obtained indications that the use of AZ could be a more sensitive method to detect external CA. It so, we suggest that external CA might be present in all the macroalgae examined. There appeared to be no correlation between the ability to use HCO 3 − and the presence of external CA. Therefore, the presence of external CA was not a strong indication for an efficient use of HCO 3 −, although some dependence between the requirement for external CA and habitats was found.