Eric Tambutté

Coral Calcification, Cells to Reefs

In spite of more than one century and half of studies, mechanisms of coral biomineralization, leading to coral growth and reef formation, still remain poorly known, although major global threats to coral reefs, such as ocean acidification, primarily affect this process. Coral skeletons are used as environmental archives but the vital processes that govern incorporation of trace elements and stable isotope are still unknown. Our knowledge on coral physiology is restricted to the organismal level due to the lack of appropriate cell model, however the advent of new approaches, such as coral genomic, is changing drastically our knowledge on these animals even if only a few data are available concerning the field of biomineralization. This chapter reviews our present knowledge and discusses the different theories on coral calcification, from the molecular to the reef level. Conclusion is presented in a list of key issues to be resolved in order to understand the intimate mechanisms of calcification of corals, essential to determine the origin of the sensitivity of corals to ocean acidification, to improve paleoceanographic reconstructions or coral reef management, or “just” to understand how genes of a soft organism control the formation of an extracellular 3D-skeleton.

Calcein labelling and electrophysiology: insights on coral tissue permeability and calcification

The mechanisms behind the transfer of molecules from the surrounding sea water to the site of coral calcification are not well understood, but are critical for understanding how coral reefs are formed. We conducted experiments with the fluorescent dye calcein, which binds to calcium and is incorporated into growing calcium carbonate crystals, to determine the permeability properties of coral cells and tissues to this molecule, and to determine how it is incorporated into the coral skeleton. We also compared rates of calcein incorporation with rates of calcification measured by the alkalinity anomaly technique. Finally, by an electrophysiological approach, we investigated the electrical resistance of coral tissues in order to better understand the role of tissues in ionic permeability. Our results show that (i) calcein passes through coral tissues by a paracellular pathway, (ii) intercellular junctions control and restrict the diffusion of molecules, (iii) intercellular junctions should have pores of a size higher than 13 Å and lower than 20 nm, and (iv) the resistance of the tissues owing to paracellular junctions has a value of 477 ± 21 Ohm cm2. We discuss the implication of our results for the transport of calcium involved in the calcification process.