Effects of Nickel and Soil Humic Acid During Biological Hematite Reduction byShewanella putrefaciensCN32

Abstract

The effect of nickel on the reductive dissolution of hematite by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens CN32 was studied in batch culture mode. Experiments were conducted under nongrowth conditions (108 cell mL−1) with hematite (2.0 g L−1) in 10 mM PIPES (pH 6.8) and H2 as the electron donor, spiked with nickel (0 to 1.63 mM) and soil humic acid (SHA; 100 mg L−1), and incubated for 5 days. Abiotic reductions using hydroxylamine HCl (HA-HCl) assays to estimate microbially reducible Fe(III), and biotic nitrate (0 to 1.43 mM) assays were also performed. A 50% inhibitory concentration (IC50) was defined as the nickel concentration that decreased the extent of electron acceptor consumption by 50% compared to a no-nickel control. Nickel addition resulted in a minimal effect on hematite bioreduction (IC50 of 132 mM), however SHA addition significantly increased the inhibitory effect of nickel (IC50 of 0.34 mM). SHA had an insignificant effect on nickel inhibition during nitrate reduction, while abiotic HA-HCl Fe(II) results suggest SHA-Ni(II) complexes may increase bioavailable Fe(III). Fe(II) and Ni(II) sorption to cells and hematite increased in the presence of SHA, suggesting that surface passivation or saturation of sorption surface area through the formation of ternary SHA-Ni(II)–hematite complexes appear responsible for the increased inhibitory effects by nickel observed.