Rates of bacteria-promoted solubilization of Fe from minerals: a review of problems and approaches

Understanding the effects of bacteria on mineral weathering will require careful, controlled experiments in chemical reactors. Unfortunately, many of the details of protocol for such experiments have not yet been addressed. In particular, experimenters need to find ways to normalize experiments for the mass of bacteria involved in reaction and need to measure the rate of change of this mass. Furthermore, experimenters will need to define whether mineral dissolution occurs during the log or stationary phase of growth. Bacteria cell mass should be quantified both before and after dissolution. In some cases, flow bioreactors will be useful to understand dissolution under steady state conditions. Finally, even the choice of medium will involve trade-offs between encouraging growth of bacteria while mimicking natural solutions. In the case study for mineral dissolution presented here, two bacterial species of the genera Streptomyces and Arthrobacter, each involved in the natural weathering of hornblende, were investigated in growth experiments in medium with hornblende. Experiments with and without desferrioxamine B (its mesylate salt deferoxamine mesylate, DFAM) were also completed. In the presence of bacteria or DFAM, Fe release from hornblende is accelerated by up to a factor of ∼20. Both bacteria produce catecholamide siderophores that are presumed responsible for the enhanced Fe release when bacteria are present. These results represent the first quantification of the rate of release of Fe from a mineral in the presence of the siderophore-producing bacteria, with subsequent identification of the siderophore. The rate of Fe release from hornblende increases non-linearly in bacteria-free experiments with increasing concentrations of DFAM. Such a rate–concentration function is consistent with formation of surface complexes on the hornblende surface. Surface complexation may also explain enhancement of Fe release in the presence of the catechol siderophores produced by the arthrobacter and streptomycete. The effects of siderophores on Fe transport could be significant in many soils.