Biogeochemical analysis of the calcification patterns of cold-water corals Madrepora oculata and Lophelia pertusa along contact surfaces with calcified tubes of the symbiotic polychaete Eunice norvegica: Evaluation of a 'mucus' calcification hypothesis

The scleractinian cold-water corals (CWCs), including the species Madrepora oculata and especially Lophelia pertusa, have been studied extensively in an attempt to decipher environmental signals recorded during biomineralisation in order to extract environmental chronologies. However, understanding the mechanisms of carbonate precipitation is a prerequisite to interpret variations in geochemical signatures locked into the skeleton during coral growth; to date results are still inconclusive. Here a novel approach, comparing the calcification patterns within the coral microstructure of species L. pertusa and M. oculata and the geochemistry along the contact surfaces with calcified polychaete tubes is undertaken to provide additional information on the mechanisms of biomineralisation in colonial corals. The fact that no significant difference in microstructures, variations in growth rate, or geochemical composition between the corallite theca and the calcified polychaete tube was detectable leads to the conclusion that both have been deposited by the coral tissue in L. pertusa and M. oculata. Based on prior knowledge on the symbiotic relationship between CWCs and the polychaete Eunice norvegica, an involvement of mucus in the calcification of the parchment tubes had been suspected. However, we found only evidence for aragonite precipitated by coral tissue, without evidence for an involvement of mucus in the calcification.