Biologically controlled precipitation of calcium phosphate by Ramlibacter tataouinensis

Ramlibacter tataouinensis, a β-proteobacterium strain isolated from an arid environment, was cultured on a solid culture medium supplemented with calcium. Optical and transmission electron microscopies (TEM) showed that the precipitation of nanometer-sized calcium phosphate particles was mainly restricted to the cysts at the center of the colonies and occurred first in the periplasm of the bacteria then inside the cells. Poorly crystallized calcium phosphates, with low Ca/P ratios and located in the periplasm, were nanometer-sized phases elongated tangentially to the cell surface, whereas precipitates inside the cells were crystallized nanocrystalline hydroxyapatites (HAP) with a preferential orientation of their c axes perpendicular to the cell surface. These observations suggest a biologically controlled matrix-mediated calcification. As noticed by previous authors, well-defined fossilized bacteria can thus be preserved in natural phosphate deposits. Moreover, this study shows that, at least for some species, well-defined orientations of phosphates in cell interiors and cell walls could be used, in conjunction with others, as supplementary biogenicity criteria in fossilized materials.