Calcitic Biominerals: Candidate Model Organisms and Insights from Older Technologies

Biomineral science may benefit from the use of model organisms. It may be helpful to focus on organisms that secrete only one mineral phase and do so in such a way that biological control of crystal orientations is obvious. Three candidate model organisms are the foraminiferan Patellina corrugata, the prymnesiophyte alga Braarudosphaera bigelowii, and the bivalved mollusc Placuna placenta. All three organisms construct their skeletons from calcite. The snail-like test of Patellina corrugata is formed from a single calcite crystal. The axis of coiling is perpendicular to the base of the test, and it coincides with one of the three a axes of calcite. In contrast, the dodecahedral nannosphere of Braarudosphaera bigelowii is formed from twelve pentagonal plates known as pentaliths, each formed from five calcite crystal segments set at ~72° to each other and with their c axes inclined at ~45° in a counterclockwise fashion. In Placuna placenta, the valves are formed from thin (~300 nm) laths of foliated calcite, which are nearly parallel to the positive rhombohedron {108}, a form rarely expressed naturally. It is the use of this extraordinary lattice direction that makes the foliated calcite of P. placenta one of the most interesting of all known biomaterials. It has the potential to serve as a model for the biological production of tough laminates and is a material that can potentially reveal some fundamental aspects of matrix-mediated biomineralization.