BIOCALCIFICATION MECHANISM IN GORGONIANS - CA-45 UPTAKE AND DEPOSITION BY THE MEDITERRANEAN RED CORAL CORALLIUM-RUBRUM

The biocalcification of Mediterranean red coral Corallium rubrum (Anthozoa, Octocorallia) was investigated by using Ca-45 as a tracer to follow calcium transport and deposition. Kinetic experiments were performed by tracing the appearance of radioisotope in the three studied compartments of the apical part of the red coral: tissue fraction, spicules, and axial skeleton. The results demonstrate that calcium is absorbed by tissue before being deposited in the skeletal structures. The interval of time required by calcium to reach the site of calcification is about 1 hour. The spicule fraction represents the most important site of Ca-45 deposition in comparison with the other two compartments. By using different kinds of inhibitors, we demonstrate that calcium enters the cell along its electrochemical gradient via verapamil-sensitive calcium channels and is concentrated in spicule-forming vesicles or exported to the site of axis formation by an energetic process whose activation energy is 12 Kcal/mol for both skeletal structures. Modifications of the CO2/HCO3- buffering system were used to examine the possible origins of inorganic carbon for CaCO3 formation. Our results suggest that metabolic CO2 may serve as a main source. The carbonic anhydrase enzyme appears to be involved in the hydratation of CO2 to HCO3, this latter ion being subsequently transported to the calcification site by a Cl-/HCO3- exchanger.