BIOMINERALIZATION: INTEGRATING MECHANISM AND EVOLUTIONARY HISTORY

In this review we combine insights on CaCO₃ skeleton formation pathway with trends and constraints from evolutionary history, genetics, transcriptomics, proteomics, and isotope geochemistry to develop a powerful conceptual unified model for CaCO₃ biomineralization applicable to all phyla. Central to this unified model is the role for both intracellular and extracellular calcifying fluids and the increasing (but still incomplete) control over these spaces that organisms have evolved over geologic time. All biomineralizers regulate the transport of carbon and calcium to, and removal of protons from these calcifying fluids. A meta-analysis of the previous literature on C and O isotopic composition of CaCO₃ skeletons confirms that these isotopes covary in most marine calcifiers, supporting unified transport mechanisms. All biomineralizers control the mesostructure, location, and rate of crystallization. Structurally diverse proteins and polysaccharides illustrate new ways for organic molecules to control biomineral phases, phase transitions, and crystal growth shared by multiple phyla. Despite these commonalities, CaCO₃ biomineralizers are highly diverse organisms, and the fossil record demonstrates very different fates during past mass extinctions. Here too, the unified model can accommodate such diversity as it includes a sequence of fundamental steps, the relative contribution of which varies across phyla, but the steps are convergent.