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).

Inorganic carbon acquisition under emerged conditions was studied in the red macroalga Bostrychia scorpioides by investigating the effects of changes in the pH of the water film surrounding the thalli when emerged and of inhibitors of carbonic anhydrase (CA; EC 4.2.1.1). The CO2 uptake rate increased by approximately 150% when the pH of the water film was changed from 8·1 to 10·5. Acetazolamide (AZ), an inhibitor of the external CA, inhibited CO2 uptake by 50% at pH values higher than 8·7. The effect of ethoxyzolamide (EZ), an inhibitor of both external and internal CA, was more pronounced than that of AZ, inhibiting the CO2 uptake rates by 80% at all the pH values assayed. At pH 10·5, CO2 flux into the cells exceeded by several orders of magnitude the flux supported by the spontaneous dehydration of HCO3− to CO2 in the water film. From these results it was concluded that the main source of inorganic carbon was atmospheric CO2. The driving force for the flux of atmospheric CO2 into the cell must be the CO2 concentration gradient. Therefore, low internal CO2 concentration might be necessary to account for the CO2 fluxes measured. The function of the internal CA could be to speed up the transformation of CO2 into HCO3− inside the cell. The function of the external CA could not be fully described from the experiments although it was demonstrated that the external enzyme enhanced the CO2 flux from air to cell surface.

Enteromorpha intestinalis grows along the Swedish west coast in rockpools which are isolated from the open seawater for long time periods and where, therefore, the inorganic carbon content is low and the pH is high during the day. In order to investigate how E. intestinalis could grow under such apparently CO2-constraining conditions, we measured its photosynthetic responses to inorganic carbon in the presence of an inhibitor of external/surface-bound carbonic anhydrase (acetazolamide) as well as an inhibitor of HCO−3 transport via anion exchange (4,4′-diisothiocyanatostilbene-2,2′-disulfonate). The results show that both HCO−3 dehydration via surface-bound carbonic anhydrase and HCO−3 transport via a 4,4′-diisothiocyanatostilbene-2,2′-disulfonate-sensitive mechanism were present in E. intestinalis, but only HCO−3 uptake via the putative transporter was operative in rockpool water during most of the photic period (pH > 9·4, inorganic carbon < 0·45 mol m−3 and CO2 < 0·05 mmol m−3). The advantage of such a mechanism, rather than extracellular HCO−3 dehydration, is discussed with regard to photosynthesis of marine macroalgae under in situ conditions conducive to high pH values adjacent to the thalli.