Microbes and Mg: Investigating Stable Mg Isotope Fractionation in Microbally Mediated Carbonates

Microbially induced precipitation is a phenomenon in which minerals are precipitated as a byproduct of the metabolic processes and organic presence (e.g., biofilms) of microorganisms. Commonly responsible are consortia of eubacteria, archaebacteria, and fungi collaborating to complement each other’s metabolisms, exchange genetic material, and form the biofilms in which they live. The mineralized remains of these communities have been associated with carbonates in marine or lacustrine settings and, in addition, microbes have been either observed or suggested to be involved in the creation of speleothems, travertine deposits, various carbonate grains (e.g., ooids), and cements. Magnesium isotope compositions of carbonate minerals are sensitive to the mechanisms of carbonate mineral formation and diagenesis, Mg cycling, and environmental conditions. Data from laboratory experiments suggest that Mg isotope fractionations are dependent on precipitation rates and/or temperature. However, little work has been done to determine whether the information gleaned from sterile, abiotic precipitation experiments are valid if the magnesium bearing minerals are biotically induced. Calcite was precipitated in free drift experiments, conducted at 25 °C with Mg/Ca molar ratios between 0.2 and 5.6. Data from this study suggests that microbial activity during calcite precipitation influences Mg isotope fractionation in anaerobic environments. The calcite precipitated under these conditions was found to be enriched in 26Mg relative to calcite grown under anaerobic/sterile conditions as well as aerobic experiments with and without microorganisms. Further inferences were hampered by nonsystematic shifts in isotope measurements introduced by unresolvable organic materials associated with the Mg purification process, and possibly the culture media.