Structural and Microbial Constraints of FeIII Oxides Submitted to Dissimilatory Iron Reduction

Dissimilatory iron reducing bacteria (IRB) are strongly involved in iron cycling in surface environment associated with the cycle of trace element contaminants like As, Cr, Cd. The fate of iron and these contaminants are strongly linked to composition and solubility of iron minerals. A natural environment subjected to redox oscillations was selected to illustrate the role of IRB activity and non-metabolic reactions in the kinetic control of mineralogical Fe oxides was investigated with synthetic ferrihydrite, goethite, hematite and lepidocrocite minerals.

The sampling site is located in Decize (Bourgogne, France, Dhivert et al, 2015) near a Loire river channel. Three bulk samples were taken: soil from the river bank, flood ground and sediment under water. IRB cultures were obtained by inoculating the site samples in medium containing FeIII-NTA 10 mM as electron acceptor and acetate, lactate, formate and glucose as electron donors. Then synthetic ferrihydrite, lepidocrocite, goethite and hematite minerals were provided as electron acceptors for the IRB enrichments in batch experiments and dissolved FeII and FeIII species were monitored.

Three enrichments of iron reducing bacteria were obtained that totally reduced FeIII-NTA in 1-2 days. With ferrihydrite, lepidocrocite, goethite, and hematite, FeIII reduction rate was nearly 500 times less rapid compared with FeIII-NTA, with both IRB enrichments, because of structural constraints specific to iron oxides. FeIII-reduction rates decreased from one oxide to another according to lepidocrocite > ferrihydrite > goethite > hematite. The next step will be quantification of specific IRB, especially Shewanella and Geobacter, using qPCR techniques, and the characterization of the final solid phases.

This research work has been performed in the frame of PhD project, scholarship co-funded BRGM - Région Centre Val de Loire 2017-2020, contract N° SIRET 582 056 149 00120

Dhivert et al. (2015). Science of the total Environment, 505, 121-136.