The coupling between CO₂ and Ca²⁺ exchange and photosynthesis by corals (Favia sp.) was studied with microsensors for Ca²⁺, O₂, pH and CO₂. The profiles of these compounds, measured perpendicular on the coral surface, were strongly influenced by light. During illumination, the concentration of O₂ and the pH at the polyp surface was higher than in the surrounding seawater, while the concentrations of Ca²⁺ and CO₂ were lower. In the dark the inverse was observed. Furthermore, simultaneous recording of concentration changes at the coral surface, in response to light and inhibitors, were performed with pairs of the sensors. The concentration changes of CO₂ and pH were slow, while those of Ca²⁺ and O₂ were immediate and fast. The concentration changes of the O₂ and Ca²⁺ concentrations at the coral surface were synchronous in response to changes in light conditions and to inhibition of the photosynthesis. Also, the spatial distribution of photosynthetic activity over a single polyp coincided with the distribution of Ca²⁺ concentration changes. These results show that Ca²⁺ dynamics at the polyp surface is not an indirect effect of increased CaCO₃ precipitation at the skeleton, but indicates the presence of a Ca²⁺ uptake mechanism that is directly correlated to photosynthesis. Inhibition of carbonic anhydrase strongly decreased photosynthesis, especially at higher light intensities. This, combined with the observed increase in CO₂ concentration changes and absolute increase in CO₂ concentration at the tissue surface, demonstrated the importance of carbonic anhydrase for CO₂/DIC uptake and transport to the site of photosynthesis.