Tambutté E., 1996. Calcification process of hermatypic scleractinians, Stylophora pistillata. In situ growth in Mururoa. PhD from Nice Sophia Antipolis University.

ABSTRACT

The aim of this work was to determine the cellular mechanisms involved in the calcification processes of hermatypic scleractinians. The new biological material used is clones of the coral Stylophora pistillata developed into microcolonies whose skeleton is entirely covered by tissues, thus avoiding the non specific adsorption of radioisotopes. In the protocol set up for measuring calcification, we use 45Ca. One characteristics of this protocol is the presence of an efflux phase at the end of the incubation period which allows the emptying of an extracellular compartment, the coelenteron. Thanks to the utilization of both the microcolonies and this new protocol, we evidence four compartments involved in calcification processes. The first compartment is extracellular and corresponds to the coelenteron. It seems that the second compartment (7 nmol Ca2+. mg-1protein) which corresponds to the whole tissues and has a long half-time (20 min) is indirectly involved in calcium transport for calcification processes. The third compartment which is at the origin of the calcium used for the synthesis of the skeleton has a very short half-time (2 min) and we suggest that this compartment corresponds to the calicoblastic epithelium. The cells of this epithelium are sensitive to L-type calcium channels inhibitors. The molecular characterization of a L-type calcium channel and the utilization of specific antibodies have revealed the existence of such a channel on both the oral and the calicoblastic ectoderms. Calcium transport in this compartment is transcellular and coupled to cellular energy dependant mechanisms, probably Ca2+ ATPases. The fourth compartment corresponding to the skeleton is characterized by an unidirectional calcium flux of 975 pmol Ca2+. mg-1 protein. min-1. Thanks to the study of this compartment, we have evidenced that calcification is linear along time.
We have demonstrated that organic matrix incorporation is coupled with calcium incorporation into the skeleton. We suggest that the amount of aspartic acid in the external medium is a limiting factor for calcification. TBT, an antifouling agent, could inhibit the synthesis of the organic matrix in the animal tissues and thus affect calcification.
Scanning and transmission electron microscope studies of the calicoblastic epithelium and the skeleton have revealed the importance of anchoring structures on the skeleton, the desmoidal processes.

Key-words : Scleractinian - Stylophora pistillata - Calcification - Calcium - L-type channel - Desmoidal process - Organic matrix - Aerial exposure