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

Abstract

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.