Utilization of HCO₃⁻ at high pH by the brown macroalga Laminaria saccharina

Abstract

The brown macroalga Laminaria saccharina exhibits a type of HCO₃⁻ 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⁻ utilization featured properties similar to direct HCO₃⁻ uptake in that photosynthetic rates were proportional to the HCO₃⁻ 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⁻ dehydration and not as direct HCO₃⁻ 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⁻ uptake of green macroalgae). This means of HCO₃⁻ 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).