Experimental study of the effects of Bacillus subtilis on gibbsite dissolution rates under near-neutral pH and nutrient-poor conditions

In order to examine the effects of Bacillus subtilis on rates and extents of gibbsite dissolution, we conducted batch-type dissolution experiments with viable, non-viable, and no bacteria. Dissolved Al and organic carbon were monitored at regular time intervals for approximately 10 days under near-neutral pH and nutrient-poor conditions that simulated the conditions under which natural fluid–mineral interactions occur. Viable bacteria not only significantly enhanced the gibbsite dissolution rate, but they also enhanced the extent of dissolution by a factor of 30 relative to the bacteria-free control experiments. The dissolved Al concentrations in the experimental solutions are correlated to the concentration of dissolved organic carbon, suggesting that organic molecules derived from the bacteria are responsible for the observed dissolution behavior. Non-viable bacteria also cause enhancement of the rate and extent of gibbsite dissolution relative to the controls, but the observed Al concentrations are five to seven times lower than those observed for the systems containing viable bacteria. This result implies that abiotic surface complexation between aqueous Al and the bacterial cell wall functional groups is not the cause of the dissolution enhancement. However, the dissolved organic carbon concentrations in the experiments containing non-viable bacteria are virtually identical to those observed for the systems containing viable bacteria. This similarity strongly suggests that the bacterially derived organic molecules from both types of bacteria are lysis products rather than metabolic exudates, and that the lysis products from intact bacteria are more capable of enhancing the rate and extent of gibbsite dissolution than those of the non-viable bacteria. These conclusions are supported by IC and GC-MS analyses of the two different types of dissolved organic carbon, and by results from dissolution experiments that were engineered to prevent bacterial lysis. The experimental results presented here are the first to indicate that bacterial lysis can significantly affect mineral dissolution in weathering environments, and that lysis products may be as important as bacterial exudates in controlling the rate and extent of mineral dissolution.