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

Goethite particles in soil environments often contain Al3+ substituted for Fe3+ in octahedrally coordinated sites. Al substitution has been shown to a lter mineral stability and abiotic dissolution rates. This study focused on the effects of Al substitution (to 8.8 mol%) on synthetic goethite dissolu tion by an aerobic Pseudomonas mendocina var. bacteria. In contrast to diss imilatory iron reducing bacteria (DIRB), this bacteria is not capable of us ing Fe as a terminal electron acceptor for oxidative phosphorylation, and h ence only requires mu M concentrations of Fe for metabolism. Pure and substituted goethites were reacted with microorganisms in Fe-limit ed growth media wherein the only source of Fe was the solid phase, so that microbial populations could only grow by obtaining Fe through mineral disso lution. 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 mi neral dissolution. Increasing Al substitution resulted in particles with progressively decreas ing mean particle length and aspect ratios, as well as fewer domains, as me asured by atomic-force microscopy (AFM); but with increasing structural ord er as determined by XRD line widths. Experiments conducted in the dark at 2 2 degrees C, exposed to the atmosphere, showed that maximum microbial popul ation did not correlate with particle specific surface area, which is in co ntrast with previous studies using DIRE. Maximum microbial population incre ased a small amount with increasing Al content of the goethites, in contras t with several previous investigations of abiotic dissolution. Because dens e 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 dissoluti on. Hence, effects of Al substitution on aggregate structure is a focus of ongoing research. Copyright (C) 2000 Elsevier Science Ltd.