Use of coral skeleton as environmental archives: The biological basis

Full text: 'There has never been any doubt that corals write valuable information into their skeletons it is their language that has remained blurry and ambiguous'. Paleoceanographers derive information about past environmental conditions from stable isotopes and other tracer records held with geological structures. The skeletons of hermatypic corals are particularly useful for high-resolution studies of tropical paleoceanic environments as they provide an unaltered record of the chemical and physical conditions that existed in the seawater when they were formed. However, these structures do not result from pure chemical CaCO3 precipitation but from highlyregulated biological activities of living organisms. Indeed, trace elements and isotopes were shown to vary widely between and within species or to correlate well with coral growth or extension rates. It was therefore suggested that environmental factors are not completely controlling isotope and trace element signatures in coral skeletons but that biological factors were also important. Furthermore, most of reef-building corals harbor photosynthetic symbionts which stimulate by an unknown mechanism coral calcification, a process called light-enhanced calcification. Consequently, one must consider the effects of these biological activities on the distribution and fractionation of tracers to make correct inferences on climate at the time of skeleton formation. It is, therefore, necessary to understand the physiological mechanisms which control both biomineral formation and carbon supply to the photosynthetic symbiont, called 'vital effects'. This paper will present an up-todate review of the biological control of the biomineralization process in corals which will allow an optimization for the use of coral skeletons as environmental archives. By using the branched scleractinian coral, Stylophora pistillata as a model organism, we have shown that coral skeleton formation results from two biological processes: regulation of the ion concentrations at the site of biomineralization and secretion of macromolecules, called organic matrix, regulating crystallization. Both processes are very rapid and need carrier proteins and various enzymes mainly located within a thin cell layer surrounding the skeleton, called the calicoblastic epithelium. Two major enzymes achieving transepithelial calcium transport have already been characterized as well as some aspects of the sources for carbon supply. Organo-mineral interactions and role of organic matrix will be also presented. (author)