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We investigated the precipitation of carbonates by Halobacillus trueperi in both solid and liquid media at different salt concentrations and different magnesium/calcium ratios. H. trueperi precipitated at all assayed salt concentrations. When salt concentration increased, the quantity and the size of bioliths decreased and the time required increased. The precipitated minerals (determined by X-ray diffraction) were calcite, magnesium calcite and monohydrocalcite in variable proportions depending on the salinity and the physical state of the medium; the magnesium content of the magnesium calcites also varied with regard to the culture type. According to the saturation indices other minerals could also precipitate. Scanning electron microscopy showed that dominant morphologies of the bioliths were spherulitic with fibrous radiated interiors. We show that H. trueperi plays an active role in the precipitation of carbonates and we hypothesize about this process of biomineralization.

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