Precipitation of carbonates and phosphates by bacteria in extract solutions from a semi-arid saline soil. Influence of Ca2+ and Mg2+ concentrations and Mg2+/Ca2+ molar ratio in biomineralization

We examine the ability of bacteria from a saline soil (Arenic-Gypsic-Hyposalic Solonchak) from the Santa-Pola marine saltern (Alicante, Spain) to precipitate carbonate and phosphate minerals. Solid culture media were prepared from natural soil extracts, from natural soil extracts amended with organic material in different proportions, and by modifying the Ca2+ and Mg2+ concentrations and the Mg2+/Ca2+ molar ratio. A high percentage of moderately halophilic bacteria were able to form biominerals (calcite, magnesian calcite and/or struvite) in natural soil extracts with small amounts of added organic material and in ionically modified natural soil extracts. Light microscopy revealed three different types of bio-mineralization: colonies that precipitate carbonates, colonies that precipitate struvite, and colonies that precipitate both types of minerals. The precipitation of carbonates and struvite is not simultaneous, but successive. The concentration of Ca2+ and the Mg2+/Ca2+ molar ratio were more influential than the absolute amount of Mg2+ on the precipitation of minerals. The higher the calcium concentrations are, the more carbonate-forming colonies there are and the fewer struvite-forming colonies. The bioliths precipitated were analyzed by Powder X-Ray Diffraction (PXRD) and Scanning Electron Microscope (SEM). In the magnesian calcites precipitated (Ca1-nMgnCO3), n (amount of magnesium that replaces calcium by formula) varied from 0.058 to 0.342. Struvite crystals are polyhedral and around 500 mu m, while carbonate crystals are smaller (< 100 mu m), spherical and usually in aggregates. The precipitation of minerals studied shows an active role of the bacteria, but the geochemical conditions are clearly influential. It may therefore be considered induced biomineralization. The results point out the possibility that in some soils the C, N, P, Ca and Mg cycles are coupled due to bacterial biomineralization.