THE EFFECT OF BACTERIAL CELL WALL ADSORPTION ON MINERAL SOLUBILITIES

The transport of mass in geologic systems, such as liquid hydrocarbon migration in sedimentary basins or contaminant transport in groundwater systems, is strongly influenced by rock porosity and permeability. In turn, these rock properties are at least partially controlled by mineral solubilities and dissolution rates. Bacteria can affect the dissolution rates of rock-forming minerals. However, bacterial effects on mineral solubilities have not been studied. In this study, we demonstrate that the adsorption of mineral-forming cations onto a bacterial surface leads to enhanced mineral solubilities, and we present a thermodynamic approach for quantitatively modelling this bacterial effect. We determined the solubility of CuO in both abiotic and bacteria-bearing systems, measuring Cu concentrations, both in solution and adsorbed onto the bacteria. We measured the dissolution of tenorite (CuO) in a fixed pH, fixed ionic strength system, with and without bacteria present in suspension. Tenorite is analogous to environmentally-important metal oxides such as Fe- and Al-oxides, however its solubility under circumneutral pH conditions is much higher, making its solubility easier to measure experimentally. These experiments involved the Gram-positive aerobic bacterial species Bacillus subtilis, a common soil species whose cell wall adsorptive properties, for both protons and a range of aqueous metals, have been thoroughly characterized. After exposure of the tenorite to the bacterial suspension, we measured Cu concentrations in two system reservoirs separately: dissolved Cu in solution and adsorbed Cu on the bacterial surface. We also measured the concentration of dissolved organic carbon (DOC) in each sample. In the bacteria-bearing systems, we observed greatly elevated extents of mineral dissolution, caused by high concentrations of Cu adsorbed onto the cell walls. Our results indicate that bacterial cell wall adsorption can significantly lower mineral saturation states in geologic systems, causing both an increase in the extent of mineral dissolution as well as an inhibition of secondary mineral formation.