The mechanism of calcification and its relation to photosynthesis and respiration were studied with Ca2+, pH and O2 microsensors using the scleractinian coral Galaxea fascicularis. Gross photosynthesis (Pg), net photosynthesis (Pn) and dark respiration (DR) were measured on the surface of the coral. Light respiration (LR) was calculated from the difference between Pg and Pn. Pg was about seven times higher than Pn; thus, respiration consumes most of the O2 produced by the algal symbiont's photosynthesis. The respiration rate in light was ca. 12 times higher than in the dark. The coupled Pg and LR caused an intense internal carbon and O2 cycling. The resultant product of this cycle is metabolic energy (ATP). The measured ATP content was about 35% higher in light-incubated colonies than in dark-incubated ones. Direct measurements of Ca2+ and pH were made on the outer surface of the polyp, inside its coelenteron and under the calicoblastic layer. The effects on Ca2+ and pH dynamics of switching on and off the light were followed in these three compartments. Ca2+ concentrations decreased in light on the surface of the polyp and in the coelenteron. They increased when the light was switched off. The opposite effect was observed under the calicoblastic layer. In light, the level of Ca2+ was lower on the polyp surface than in the surrounding seawater, and even lower inside the coelenteron. The concentration of calcium under the calicoblastic layer was about 0.6 mM higher than in the surrounding seawater. Thus Ca2+ can diffuse from seawater to the coelenteron, but metabolic energy is needed for its transport across the calicoblastic layer to the skeleton. The pH under the calicoblastic layer was more alkaline compared with the polyp surface and inside the coelenteron. This rise in pH increased the supersaturation of aragonite from 3.2 in the dark to 25 in the light, and brought about more rapid precipitation of CaCO3. When ruthenium red was added, Ca2+ and pH dynamics were inhibited under the calicoblastic layer. Ruthenium red is a specific inhibitor of Ca-ATPase. The results indicated that Ca-ATPase transports Ca2+ across the calicoblastic layer to the skeleton in exchange for H+. Addition of dichlorophenyldimethylurea completely inhibited photosynthesis. The calcium dynamics under the calicoblastic layer continued; however, the process was less regular. Initial rates were maintained. We conclude that light and not energy generation triggers calcium uptake; however, energy is also needed.