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

Understanding the effects of bacteria on mineral weathering will require ca reful, controlled experiments in chemical reactors. Unfortunately, many of the details of protocol for such experiments have not yet been addressed. I n particular, experimenters need to find ways to normalize experiments for the mass of bacteria involved in reaction and need to measure the rate of c hange of this mass. Furthermore, experimenters will need to define whether mineral dissolution occurs during the log or stationary phase of growth. Ba cteria cell mass should be quantified both before and after dissolution. In some cases, flow bioreactors will be useful to understand dissolution unde r steady state conditions. Finally, even the choice of medium will involve trade-offs between encouraging growth of bacteria while mimicking natural s olutions. In the case study for mineral dissolution presented here, two bacterial spe cies of the genera Streptomyces and Arthrobacter, each involved in the natu ral weathering of hornblende, were investigated in growth experiments in me dium with hornblende. Experiments with and without desferrioxamine B (its m esylate salt deferoxamine mesylate, DFAM) were also completed. In the prese nce of bacteria or DFAM, Fe release from hornblende is accelerated by up to a factor of similar to 20. Both bacteria produce catecholamide siderophore s that are presumed responsible for the enhanced Fe release when bacteria a re 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 experiment s with increasing concentrations of DFAM. Such a rate-concentration functio n is consistent with formation of surface complexes on the hornblende surfa ce. Surface complexation may also explain enhancement of fe release in the presence of the catechol siderophores produced by the arthrobacter and stre ptomycete. The effects of siderophores on Fe transport could be significant in many soils. (C) 2000 Elsevier Science B.V. All rights reserved.