Coral calcification, mucus, and the origin of skeletal organic molecules

Abstract

Biocalcification encompasses the kinetic and structural, abiotic and biologically mediated processes involved in the formation of calcium carbonate skeletons by marine organisms and represents a key process in the global carbon cycle. Throughout the geological record, this process has evolved repetitively and has altered global biogeochemical cycles. Besides the structural variability of calcium carbonate polymorphs laid down by different organisms, biogenic carbonate skeletons are characterized by the presence of organic molecules that are incorporated into the growing skeleton. Major advances have identified the macromolecules associated to the organic matrix within marine calcifiers, however, it has yet to be established the actual role these organic molecules play in the calcification process. In this study, we isolated the effect of skeletal organic molecules (SOM) on the precipitation of calcium carbonate on coral skeleton fragments by adding extracted SOM or coral mucus (CM) to oversaturated calcium carbonate solutions. We found that the precipitation rate did not change regardless if organic molecules were present or not. However, the primary polymorph did change between the treatments, suggesting that organic molecules influence the surface processes that lead to the formation of the crystal lattice but not the kinetic processes that transport ions to the crystal surface. Since SOM and CM both altered the crystal polymorph but not the crystallization rate, we argue that SOM may not represent a specialized biomineralization toolkit, but that SOM originate from CM and the requirement of the polyp to adhere to the substratum.