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Shewanella is a genus of marine bacteria capable of dissimilatory iron reduction (DIR). In the context of deep-sea mining activities or submarine mine tailings disposal, dissimilatory iron reducing bacteria may play an important role in biogeochemical reactions concerning iron oxides placed on the sea bed. In this study, batch experiments were performed to evaluate the capacity of Shewanella loihica PV-4 to bioreduce different iron oxides (ferrihydrite, magnetite, goethite and hematite) under conditions similar to those in anaerobic sea sediments. Results showed that bioreduction of structural Fe(III) via oxidation of labile organic matter occurred in all these iron oxides. Based on the aqueous Fe (II) released, derived Fe(II)/acetate ratios and bioreduction coefficients seem to be only up to about 4% of the theoretical ones, considering the ideal stoichiometry of the reaction. A loss of aqueous Fe (II) was caused by adsorption and mineral transformation processes. Scanning electron microscope images showed that Shewanella lohica was attached to the Fe(III)-oxide surfaces during bioreduction. Our findings suggest that DIR of Fe(III) oxides from mine waste placed in marine environments could result in adverse ecological impacts such as liberation of trace metals in the environment.

Anaerobic batch and flow-through experiments were performed to confirm the role of pyrite as electron donor in bacterial denitrification and to look into the feasibility of pyrite-driven denitrification of nitrate-contaminated groundwater. Nitrate reduction was satisfactorily accomplished in experiments with pyrite as the sole electron donor, in presence of the autotrophic denitrifying bacterium Thiobacillus denitrificans and at nitrate concentrations comparable to those observed in contaminated groundwater. The experimental results corroborated field studies in which the reaction occurred in aquifers. Nitrate reduction rates and nitrate removal efficiencies were dependent on pyrite grain size, initial nitrate concentration, nitrate-loading rate and pH. The N and O isotopic enrichment factors (eN and eO) obtained experimentally for pyrite-driven nitrate reduction by T. denitrificans ranged from − 13.5‰ to − 15.0‰ and from − 19.0‰ to − 22.9‰, respectively. These values indicated the magnitude of the isotope fractionation that occurs in nitrate-contaminated aquifers dominated by autotrophic denitrification.