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 TRIS and AZ. Inhibition by TRIS varied from almost 100% to 25% while AZ produced similar effects. Inhibition by TRIS was completely reverted by increasing the dissolved inorganic carbon concentration (DIC). This species group had half-saturation constants for photosynthesis (Km(DIC)) ranging from 0.5 to 1.1 mM of DIC. TRIS produced a significant increase of Km(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 TRIS 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 TRIS 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 Km(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.

► We studied 24 red macroalgae on their mechanisms uptake carbon from different habitats.
► 60% of them have the “proton pump” mechanism inhibited by TRIS buffer.
► These algae are surface-living and characterized by high-light tolerance.
► One mechanism is induced by TRIS for the case of other algae.
► These algae are deep-living and low light-tolerance.