With microsensors, we measured the steady-state microprofiles of O2, pH and Ca2+ on the topside of young segments of Halimeda discoidea, as well as the surface dynamics upon light–dark shifts. The effect of several inhibitors was studied. The steady-state measurements showed that under high light intensity, calcium and protons were taken up, while O2 was produced. In the dark, O2 was consumed, the pH decreased to below seawater level and Ca2+ uptake was reduced to 50%. At low light intensity (12 mmol photons m-2 s-1), Ca2+ efflux was observed. Upon light–dark shifts, a complicated pattern of both the pH and calcium surface dynamics was observed. Illumination caused an initial pH decrease, followed by a gradual pH increase: this indicated that the surface pH of H. discoidea is determined by more than one light-induced process. When photosynthesis was inhibited by dichlorophenyl dimethyl urea (DCMU), a strong acidification was observed upon illumination. The nature and physiological function of this putative pump is not known. The calcium dynamics followed all pH dynamics closely, both in the presence and absence of DCMU. The Ca-channel blockers verapamil and nifedipine had no effect on the Ca2+ dynamics and steady-state profiles. Thus, in H. discoidea, calcification is not regulated by the alga, but is a consequence of pH increase during photosynthesis. Acetazolamide had no effect on photosynthesis, whereas ethoxyzolamide inhibited photosynthesis at higher light intensities. Therefore, all carbonic anhydrase activity is intracellular. Carbonic anhydrase is required to alleviate the CO2 limitation. Calcification cannot supply sufficient protons and CO2 to sustain photosynthesis.