Coccolith crystals: Pure calcite or organic-mineral composite structures?

The localization of organic material within biominerals is central to developing biomineral formation mechanisms. Coccoliths, morphologically sophisticated calcite platelets of intracellularly calcifying coccolithophores, are not only eco-physiologically important, but also influence biogeochemical cycles through mass production. Despite their importance and over a century of research, the formation mechanism of coccoliths is still poorly understood. Crucial unsolved questions include the localization of organic material within coccoliths. In extracellular calcifiers the discovery of an organics-containing nano-structure within seemingly single crystals has led to the formulation of a two-step crystallization mechanism. Coccoliths are traditionally thought of as being formed by a different mechanism, but it is unclear whether coccolith crystals possess a nano-structure. Here we review the evidence for and against such a nano-structure. Current SXPD analyses suggest a nano-structure of some kind, while imaging methods (SEM, TEM, AFM) provide evidence against it. We suggest directions for future research which should help solve this puzzle. STATEMENT OF SIGNIFICANCE: Coccolithophores, unicellular calcifying algae, are important primary producers and contribute significantly to pelagic calcium carbonate export. Their calcite platelets, the coccoliths, are amongst the most sophisticated biomineral structures. Understanding the crystallization mechanism of coccolith crystals is not only central to coccolithophore cell biology but also lies at the heart of biomineralization research more generally. The crystallization mechanism of coccoliths has remained largely elusive, not least because it is still an open question whether the micron sized coccolith crystals are pure calcite, or contain organic material. Here we review the state of the art and suggest a way to solve this central problem.