The Environment Recording Unit in coral skeletons - a synthesis of structural and chemical evidences for a biochemically driven, stepping-growth process in fibres

This paper gathers a series of structural and biochemical in situ characterizations carried out to improve our knowledge of the fine scale growth patterns of fibres in coral skeletons. The resulting data show a clear correspondence between the mineral subunits of fibres and the spatial distribution of organic macromolecules. New observations using atomic force microscope confirm the close relationship between mineral and organic phases at the nanometre scale. Synthesis of these data results in a significant change in our concept of the mineralization process in coral skeletons. In contrast to the usual view of an aggregate of purely mineral units independently growing by simple chemical precipitation, coral fibres appear to be fully controlled structures. Their growth process is based on cyclic secretion of mineralizing compounds by the polyp basal ectoderm. These biochemical components of the coral fibres, in which sulfated acidic proteoglycans probably play a major role, are repeatedly produced (proteoglycans are those glycoproteins whose carbohydrate moieties consist of long unbranched chains of sulfated amino sugars). This results in a stepping growth mode of fibres and a layered global organization of coral skeletons. Therefore, in contrast to the widely accepted geochemical interpretation, we propose a fibre growth model that places coral skeletons among the typical "matrix mediated" structures. The crystal-like fibres are built by superimposition of few micron-thick growth layers. A biomineralization cycle starts by the secretion of a mineralizing matrix and the final step is the crystallization phase, during which mineral material grows onto the organic framework. Thus, each growth layer is the actual Environment Recording Unit. From a practical standpoint, these results may contribute to develop a new high resolution approach of the environment recording by coral skeletons.