Acclimation responses of Gracilaria sp (Rhodophyta) and Enteromorpha intestinalis (Chlorophyta) to changes in the external inorganic carbon concentration

The acclimation responses of two intertidal macroalgae, Gracilaria sp. and Enteromorpha intestinalis, to different dissolved inorganic carbon (DIC) levels were investigated under laboratory conditions. The effect of DIC availability on growth rate, biochemical composition (C. N, pigments and Rubisco) and on the degree of inhibition of external and total carbonic anhydrase activities (by acetazolamide and 6-ethoxyzolamide, respectively), and of a putative HCO3- exchanger protein (sensitive to the inhibitor 4.4 ' -diisothiocyanatostilbene-2,2 ' -disulfonate, DIDS) was species-specific. Pigment and Rubisco contents co-varied negatively with DIC availability in Gracilaria sp. However, no such pattern was observed in Enteromorpha intestinalis. The mechanisms of DIC uptake were also modulated by the external DIC concentration. Under limiting DIC conditions, the induction of mechanisms for CO2 acquisition above the diffusive rate was observed in Gracilaria sp., while a repression of the DIDS-sensitive mechanism was obtained for Enteromorpha intestinalis. The results revealed the plasticity of these intertidal macroalgae to acclimate to different ambient DIC levels, and indicate the important role of DIC as a factor controlling biochemical and physiological processes.