Mechanisms of inorganic carbon acquisition in Gracilaria gaditana nom. prov. (Rhodophyta)

Abstract.

The mechanisms for acquisition of dissolved inorganic carbon (DIC) in the red macroalga Gracilaria gaditana nom. prov. have been investigated. The capacity for HCO₃ ⁻ use by an extracellular carbonic anhydrase (CA; EC 4.2.1.1), and by an anion exchanger with similar properties to that of red blood cells (AE1), has been quantified. It was illustrated by comparing O₂ evolution rates with those theoretically supported by CO₂, as well as by photosynthesis-pH curves. Both external and internal CA, and a direct uptake were involved in HCO₃ ⁻ use, since photosynthesis and pH evolution were affected by acetazolamide, 6-ethoxyzolamide (inhibitors of external and total CA, respectively) and 4,4′-diisothiocyanatostilbene-2,2′-disulfonate, (DIDS; an inhibitor of HCO₃ ⁻ exchanger protein). The activity of the external CA was detected by a potentiometric method and by an alternative method based on the study of O₂ evolution after addition of CO₂ and acetazolamide. The latter method showed a residual photosynthetic rate due to direct HCO₃ ⁻ use. Inhibitors caused a reduction in the pH compensation points in pH-drift experiments. The CO₂ compensation points for photosynthesis increased when the inhibitors were applied, indicating a suppresion of the pathways involved in the carbon-concentrating mechanism. The net photosynthesis rates as a function of DIC concentration displayed a biphasic pattern that could be supported by the occurrence of the two mechanisms of HCO₃ ⁻ use. The potential contribution to HCO₃ ⁻ acquisition by the DIDS-sensitive mechanism was higher after culturing at a high pH. Our results suggest that the HCO₃ ⁻ use by Gracilaria gaditana is carried out by the two DIC uptake mechanisms. These operate simultaneously with different affinities for DIC, the indirect HCO₃ ⁻ use by an external CA activity being the main pathway. The presence of a carbon-concentrating mechanism confers eco-physiological advantages in a fluctuating ecosystem subjected daily to high pHs and low DIC concentrations.