An experimental model approach of biologically-assisted silicate dissolution with olivine and Escherichia coli - Impact on chemical weathering of mafic rocks and atmospheric CO_2 drawdown

摘要

Chemical weathering of Mg, Ca-silicates and alumino-silicates contributes significantly to the drawdown of atmospheric CO_2 over long time scales. The present work focuses on how this mode of weathering may change in the presence of free-living bacteria in oligotrophic waters, which compose most of the surface freshwaters of the Earth. Forsterite (Fo90) was reacted for 1week with a stable Escherichia coli population in water maintained at 37°C and neutral pH in a batch reactor. Control samples with suspensions of pure olivine powders and E. coli cells in pure water were also used for reference. Olivine controls reproduce the Mg, Si and Fe release in solutions predicted from rates published in the literature with pH shifts of less than 0.5 unit. After 1week, under abiotic conditions, weathered surfaces are enriched in Fe and Fe~(3+) relative to the initial composition of the mineral. Bacterial controls (without minerals) show decreasing Eh with increasing cell concentrations (-50mV with 7×10~7cells/mL and -160mV with 8×10~8cells/mL). Magnesium concentrations in bacterial control solutions are in the μg/L range and can be accounted for by the release of Mg from dead cells. More than 80% of the cells were still alive after 1week. The solutions obtained in the experiments in which olivine reacts in the presence of cells show Mg and Si concentrations a few tens of percent lower than in the mineral control samples, with a prominent depletion of Fe(III) content of the mineral surfaces. Magnesium mass balance discounts both significant bacterial uptake and inhibition of the Mg dissolution rates as a consequence of changing pH and Eh. Coating by bacterial cell layers is also negligible. E. coli reduces the chemical weathering of olivine. This study infers that the presence of free-living Proteobacteria, a prevalent group of subsurface bacteria, should decrease the amount of riverine Mg released by chemical weathering of mafic rocks.