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

The brown macroalga Laminaria saccharina exhibits a type of HCO3- utilizati on that could be almost completely inhibited either by proton buffers or by acetazolamide, an inhibitor of extracellularly operating carbonic anhydras e. This means of HCO3- utilization featured properties similar to direct HC O3- uptake in that photosynthetic rates were proportional to the HCO3- conc entration 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- deh ydration and not as direct HCO3- uptake. A mechanism is suggested involving a CO2-concentrating capability located at the cell membrane. This mechanis m, which might be common in brown algae, is suggested to have an adaptation al advantage in colder regions of the sea las 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 int erpretation of earlier experimental studies on L. saccharina land possibly other brown algae). These consequences relate both to ecology (e.g. determi nation of inorganic C affinity) and physiology (e.g. assessing mechanisms f or inorganic C uptake).