Dissolution of Al-substituted goethites by an aerobic Pseudomonas mendocina var. bacteria

Abstract

Goethite particles in soil environments often contain Al 3+ substituted for Fe 3+ in octahedrally coordinated sites. Al substitution has been shown to alter mineral stability and abiotic dissolution rates. This study focused on the effects of Al substitution (to 8.8 mol%) on synthetic goethite dissolution by an aerobic Pseudomonas mendocina var. bacteria. In contrast to dissimilatory iron reducing bacteria (DIRB), this bacteria is not capable of using Fe as a terminal electron acceptor for oxidative phosphorylation, and hence only requires μM concentrations of Fe for metabolism. Pure and substituted goethites were reacted with microorganisms in Fe-limited growth media wherein the only source of Fe was the solid phase, so that microbial populations could only grow by obtaining Fe through mineral dissolution. Because at least some Fe was taken up by the bacteria, we could not measure Fe release rates directly from dissolved Fe concentrations. Rather, we relied upon microbial growth measurements as indirect indicators of mineral dissolution. Increasing Al substitution resulted in particles with progressively decreasing mean particle length and aspect ratios, as well as fewer domains, as measured by atomic-force microscopy (AFM); but with increasing structural order as determined by XRD line widths. Experiments conducted in the dark at 22°C, exposed to the atmosphere, showed that maximum microbial population did not correlate with particle specific surface area, which is in contrast with previous studies using DIRB. Maximum microbial population increased a small amount with increasing Al content of the goethites, in contrast with several previous investigations of abiotic dissolution. Because dense biofilms formed, we were unable to use AFM to observe mineral dissolution features. AFM imaging suggested that more highly substituted goethites formed denser aggregates, and previous investigations have shown that aggregate structure is important for microbial attachment, which is prerequisite for dissolution. Hence, effects of Al substitution on aggregate structure is a focus of ongoing research.