A microsensor study of light enhanced Ca2+ uptake and photosynthesis in the reef-building hermatypic coral Favia sp.

The coupling between CO2 and Ca2+ exchange and photosynthesis by corals (Favia sp.) was studied with microsensors for Ca2+, O-2, pH and CO2. The profiles of these compounds, measured perpendicular on the coral surface, were strongly influenced by light. During illumination, the concentration of O-2 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 O-2 were immediate and fast. The concentration changes of the O-2 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.