We investigated the precipitation of carbonate and phosphate minerals by 19 species of moderately halophilic bacteria using media with variable Mg(2+)/Ca(2+) ratios. The precipitated minerals were calcite, magnesium (Mg) calcite, and struvite (MgNH(4)PO(4) x 6H(2)O) in variable proportions depending on the Mg(2+)/Ca(2+) ratio of the medium. The Mg content of the Mg-calcite decreased with increasing Ca(2+) concentration in the medium. According to the saturation indices, other minerals could also have precipitated. We observed important differences between the morphology of carbonate and phosphate, which may help us to recognize these minerals in natural systems. We studied the growth and pH curves of four bacteria in media specific for carbonate and struvite precipitation. We consider the biomineralization processes that produce carbonate and phosphate minerals, and propose a hypothesis for the lack of struvite in natural environments and ancient rocks.