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Using stable isotope fractionation factors to identify Cr(VI) reduction pathways: Metal-mineral-microbe interactions
Qiong Zhang1, Ken Amor1, Stephen J G Galer2
1Department of Earth Sciences, University of Oxford, Oxford, United Kingdom.
Water Research
|December 30, 2018
Summary
Microbial chromium reduction and isotope fractionation are influenced by iron minerals. Different bacteria, Pseudomonas and Shewanella, show varied responses, impacting environmental chromium redox behavior assessments.
Area of Science:
- Environmental microbiology
- Geochemistry
- Biogeochemistry
Background:
- Microbial interactions with metals and minerals are crucial in environmental processes.
- Chromium (Cr) contamination poses environmental risks, and understanding its redox transformations is vital.
- Iron (Fe) minerals are abundant in the environment and can influence microbial activity.
Purpose of the Study:
- To investigate the interactions between bacteria and dissolved chromium in the presence of iron minerals.
- To determine the impact of these interactions on chromium isotope variations.
- To elucidate the mechanisms of chromium reduction by different bacterial species in the presence of iron minerals.
Main Methods:
- Cr(VI) reduction experiments using Pseudomonas fluorescens LB 300 and Shewanella oneidensis MR-1.
- Experiments conducted in the presence of iron oxide minerals (goethite and hematite).
- Analysis of Cr isotopic fractionation factors (ε) and microbial cell attachment using SEM imaging.
Main Results:
- Iron minerals differentially affected Cr(VI) reduction rates and isotopic fractionation by the two bacterial species.
- Shewanella showed mineral-independent Cr isotopic fractionation (ε = -2.3‰), while Pseudomonas's fractionation varied with mineral type (ε = -3.3‰ to -4.3‰).
- Bacterial attachment patterns and proposed reduction pathways (membrane-bound vs. extracellular electron transfer) explained the observed differences.
Conclusions:
- Bacterial Cr(VI) reduction and associated isotope fractionation are significantly influenced by the type of iron minerals present.
- The findings highlight the importance of considering mineralogy and bacterial community composition when using Cr isotopes to assess environmental redox processes.
- Different microbial reduction mechanisms lead to distinct isotopic signatures, complicating direct environmental application without detailed context.
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