Bicarbonate Transport and Alkalization of the Medium by Four Species of Rhodophyta

Abstract

Photosynthesis by four species of Rhodophyta: Ceramium rubrum, Ceramium tenuissimum, Gelidium crinale and Pterocladia capillacea was optimal at 25 to 30°C and at pH 8.0 to 8.5. The measured rates of photosynthesis exceeded the rate which could be supported by the carbon dioxide arising from the dehydration of bicarbonate in the medium, from 2.7- to 10.9-fold at pH 8.0 and from 9- to 33.4-fold at pH 9.0, in the absence of measurable extracellular carbonic anhydrase activity. Assays for malic enzyme and pyruvate, orthophosphate dikinase were negative, indicating that bicarbonate ions are taken up by a transport system rather than by an accessory carbon fixation pathway. When the algae were allowed to photosynthesize in an unbuffered system a DCMU (3-[3′, 4′-dichlorophenyl]-1, 1-dimethylurea)-sensitive alkalization of the medium was observed. This alkalization was shown to have a 1:1 stoichiometry with photosynthetic carbon fixation. Alkalization of the medium occurred from 3.4 to 5.8 times the theoretical rate of supply of hydroxyl ions arising from the dehydration of bicarbonate. The data are consistent with the operation of a bicarbonate transport system in these macroalgae, in which electrical neutrality and pH balance are maintained by hydroxyl ion efflux. A difference observed between the time-course of alkalization and oxygen evolution suggests that the transport of the two ions is not compulsorily coupled.