Photosynthetic carbon utilization by Enteromorpha intestinalis (Chlorophyta) from a Swedish rockpool

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 HCO3- transport via anion exchange (4,4'-diisothiocyanalostilbene-2,2'-disulfonate). The results show that both HCO3- dehydration via surface-bound carbonic anhydrase and HCO3- transport via a 4,4'-diisothiocyanatostilbene-2,2'-disulfonate-sensitive mechanism were present in E. intestinalis, but only HCO3- 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 HCO3- dehydration, is discussed with regard to photosynthesis of marine macroalgae under in situ conditions conducive to high pH values adjacent to the thalli.