Different mechanisms of inorganic carbon acquisition in red macroalgae (Rhodophyta) revealed by the use of TRIS buffer

The effects on photosynthesis of acetazolamide (AZ, an inhibitor of the external carbonic anhydrase) and TRIS buffer at pH 8.7 were assessed in 24 species of red macroalgae. Only Palmaria palmata was unaffected by both substances. The rest of species were classified into three groups according to their sensitivity to TRIS and AZ. Photosynthesis of fourteen species was significantly inhibited by both IRIS and AZ. Inhibition by TRIS varied from almost 100% to 25% while AZ produced similar effects. Inhibition by IRIS was completely reverted by increasing the dissolved inorganic carbon concentration (DIC). This species group had half-saturation constants for photosynthesis (K-m(DIC)) ranging from 0.5 to 1.1 mM of DIC. TRIS produced a significant increase of K-m(DIC). Altogether, these results indicate that the algae sensitive to TRIS are capable of using HCO3- efficiently at pH 8.7. Furthermore, the buffering capacity of IRIS was responsible for its inhibitory effect on photosynthesis suggesting that HCO3- use was facilitated by excretion of protons outside the plasma membrane, which creates regions of low pH resulting in a higher-than-ambient CO2 concentration. In contrast, photosynthesis by two Porphyra species analysed was slightly stimulated by IRIS and completely inhibited by AZ, suggesting that the mechanism was different. In a third group of seaweeds, photosynthesis was insensitive to TRIS but it was significantly inhibited by AZ. These species had relatively high values of K-m(DIC) indicating that they relied on purely diffusive entry of CO2 generated by external carbonic anhydrase activity. Consequently, the results demonstrate that external carbonic anhydrase is widespread among red macroalgae since only P. palmata was insensitive to AZ. The functional significance of this enzyme was quite variable among the tested species. (C) 2011 Elsevier B.V. All rights reserved.