Calcification in the green alga Halimeda: III. The sources of inorganic carbon for photosynthesis and calcification and a model of the mechanism of calcification

Calcification and photosynthetic rates in Halimeda tuna were measured by the 14C method under conditions of differing pH and total inorganic carbon (ΣCO2) concentrations. The effects of pH and ΣCO2 on photosynthesis and respiration were also monitored with a polarographic O2 electrode. The results obtained indicate that the intercellular pH and ΣCO2 differ from those of the external medium. Experiments carried out over a range of pH values show that Halimeda can use HCO−3HCO3- for photosynthesis. Photosynthesis appears to stimulate calcification by removing CO2 from the intercellular spaces. As these spaces are isolated from the external sea water by the layer of cell wall of the adpressed peripheral utricles, the removal of CO2 results in a rise in [CO2−3CO32-] and a rise in pH. This results in an increased rate of CaCO3 precipitation. Respiratory CO2 evolution has an inhibitory effect on calcification by decreasing the pH and [CO2−3CO32-]. A model for calcification in Halimeda is proposed based on the results of this and previous papers. Calcification in Halimeda is seen to be a result of the anatomy of the thallus in which the sites of calcification are within a semi-isolated chamber where removal or addition of CO2 due to photosynthesis or respiration can effectively change [COCO2−3CO32-] thereby resulting in precipitation of CaCO3. In the Appendix to this paper theoretical calculations illustrate the effects of CO2, HCO−3HCO3-⁠, and CO2−3CO32- removal or addition in a closed system on the relative concentrations of the other inorganic carbon species