Comparison of fine scale growth structures in coral skeletons and abiogenic aragonite

Coral reefs are spectacular examples of biomineralization yet the fundamental processes by which corals precipitate aragonite remain elusive. Numerous studies have investigated `daily' growth bands and other fine scale features in coral skeletons as clues for understanding the biomineralization process. However, a comparison of such structures with abiogenic aragonites has generally been lacking, despite long-standing observations of the similar morphologies of abiogenic spherulites and coral sclerodermites. We have precipitated aragonite from seawater over a range of saturation states, generating crystals with spherulitic morphologies and banding patterns similar to those found in coral skeletons. Crystal morphologies revealed with scanning electron microscopy (SEM) imaging show similar shapes between coral crystals and spherulites precipitated at pH 8.8. We examine fine banding in abiogenic spherulites, acridine orange staining of these bands and variations in Mg/Ca ratios from the center to edge of spherulites. Atomic Force Microscrope (AFM) images of abiogenic aragonites show micron-scale and nanoscale features that are similar to those found in coral skeletons. We propose that a diurnal cycle in the saturation state of the calcifying fluid may be sufficient to explain many of these features, though these findings do not rule out a role for biomolecules which have often been invoked to explain such features.