Utilization of HCO3[minus] at high pH by the brown macroalga Laminaria saccharina

The brown macroalga Laminaria saccharina exhibits a type of HCO3[minus sign] utilization that could be almost completely inhibited either by proton buffers or by acetazolamide, an inhibitor of extracellularly operating carbonic anhydrase. This means of HCO3[minus sign] utilization featured properties similar to direct HCO3[minus sign] uptake in that photosynthetic rates were proportional to the HCO3[minus sign] concentration of the seawater over a wide pH range (pH 7·0-9·5). Despite this, it must be characterized as carbonic anhydrase-catalysed external HCO3[minus sign] dehydration and not as direct HCO3[minus sign] uptake. A mechanism is suggested involving a CO2-concentrating capability located at the cell membrane. This mechanism, which might be common in brown algae, is suggested to have an adaptational advantage in colder regions of the sea (as compared with the direct HCO3[minus sign] uptake of green macroalgae). This means of HCO3[minus sign] utilization is inhibited even by fairly low concentrations of buffer, with consequences for the interpretation of earlier experimental studies on L. saccharina (and possibly other brown algae). These consequences relate both to ecology (e.g. determination of inorganic C affinity) and physiology (e.g. assessing mechanisms for inorganic C uptake).