Sources of inorganic carbon (Ci) for photosynthesis and calcification and the mechanisms involved in their uptake in scleractinian corals were investigated in microcolonies of Galaxea fascicularis. Direct measurements of Ca2+, pH and O2 on the surface and inside the polyp's coelenteron were made with microsensors. Gross photosynthesis (Pg) and net photosynthesis (Pn) were measured on the surface. Light respiration (LR) was calculated from Pg and Pn. The effect of light/dark and dark/light switches on Ca2+ and pH dynamics on the surface and inside the coelenteron were followed. To evaluate the different sources of Ci for photosynthesis and calcification, Ci-free seawater and 6-Ethoxyzolamide and Acetazolamide, inhibitors for carbonic anhydrase (CA) were used.In normal seawater, Pg was about seven times higher than Pn, the LR was ca. 80–90% of the Pg. Thus, most of the O2 produced in Pg are immediately consumed in respiration, indicating the presence of a highly active internal C-cycle. As the internal C-cycle is highly active, a large part of the Ci for calcification will have passed through the metabolism of the symbiont. The high LR provides ATP for energy requiring processes in light.Ci for photosynthesis and calcification can come from seawater in the form of free Ci, respiration of photosynthates (internal C-cycle) or respiration of the ingested plankton. These sources form a common carbon pool (C-pool) that is used for the different processes.In Ci-free seawater, Pg decreased by about 12.5%, indicating that most of the photosynthetically fixed Ci can temporarily be supplied from internal sources. The initial decalcification, observed directly upon the switch to Ci-free seawater, showed that the Ca-pools in the coral are exchangeable. Part of the Pg in Ci-free seawater may depend on this decalcification for its Ci supply.Three localities of CA were defined. One on the surface facing seawater and one on endodermal cells facing the coelenteron, while the third is intracellular. The inhibition of CA decreased Pg by about 30%, while it increased the concentration of Ca2+ as a result of a decrease in its precipitation. The reduction of photosynthesis and calcification by CA inhibition demonstrated that both processes need the enzyme for the supply of Ci. The pH on the surface and inside the coelenteron decreased upon 6-Ethoxyzolamide addition indicating a role of CA in pH control.