Inducing magnetite formation by microbial Fe(II) oxidation in the presence of mineral nucleation sites: Evaluating a novel approach to heavy metal remediation in the environment.
Final Report Abstract
Microbially driven reduction and oxidation (redox) reactions are fundamental to global biogeochemical cycles and play a major role in electron transfer processes as well as in the formation and dissolution of mineral phases in the environment. The biogeochemical iron (Fe) cycle is of particular importance due to the element's abundance, buffering capacity, and sorption capacity to metals and nutrients. Mixed valence redox-active iron and iron-sulphur minerals such as magnetite, green rust, greigite, pyrrhotite and Fe-phyllosilicates are ubiquitous in soils and sediments, and are often found within metabolically diverse microbial communities. To date, however, the potential interactions between these minerals and the microbes within these mixed communities remains poorly understood. Throughout this project however we have been able to show that magnetite which is a mixed-valent iron mineral is able to support redox reactions between Fe(N)-oxidizing and Fe(IM)-reducing bacteria. Typically, microbes will use one compound or mineral as an electron acceptor, whilst using a different compound or mineral as an electron donor. However, in this project we showed that mixed valence minerals such as magnetite can act as either an electron donor or acceptor to different types of bacteria, depending on the redox conditions of the microenvironment. In this way, the magnetite was effectively behaving as a biogeochemical battery which could support both Fe(II)-oxidizing and Fe(MI)-reducing bacteria in a way that helps either to metabolise in conditions where other electron donors or acceptors are limiting, i.e. in littoral sediments or during day and night cycles. Developing upon the idea of the biogeochemical battery, a wide range of open questions and potential research avenues emerged including whether Fe metabolizing bacteria can use magnetite for long range electron transport in soils and sediments? We addressed this question by comparing the redox cycling of Fe by Fe-metabolizing bacteria with different sizes of magnetite. Through this study we showed that whilst Fe(II) oxidation is confined to the surface of the mineral, Fe(III)-reduction takes place throughout the bulk of the magnetite. By being able to transfer and pump electrons through the entire crystal, we suggest that magnetite can therefore be used as a mediator in long range electron transfer processes. The work within this project also extended into finding potential remediation strategies for arsenic and chromium using magnetite. We were able to show that the reactive properties of magnetite can be enhanced through the action of Fe-metabolizing bacteria and that both Fe(II) oxidized and Fe(III) reduced materials show greater reactivity towards chromium than the unreactive material. This opens up new possibilities for using bacteria in remediation applications. Finally, we showed that redox cycling of Fe in magnetite is possible in environmentally relevant settings by investigating the potential for microbial Fe(III) reduction and Fe(II) oxidation to take place in a Chinese soil loaded with magnetite. Such work also has relevance regarding the search for biomarkers in the rock record which could give an indication of past microbial activity on magnetite grains.
Publications
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(2017) Interactions between magnetite and humic substances: redox reactions and dissolution processes. Geochemical transactions 18 (1) 6
Sundman, Anneli; Byrne, James M.; Bauer, Iris; Menguy, Nicolas; Kappler, Andreas
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Redox cycling of Fe(II) and Fe(NI) in magnetite by Fe-metabolizing bacteria, (2015), Science 347, 1473-1476
Byrne J. M., Klueglein, N., Pearce, C., Rosso K. M., Appel, E., Kappler, A.
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Size dependent microbial oxidation and reduction of magnetite nano- and micro-particles, (2016), Scientific Reports 6, 30969
Byrne J. M., van der Laan G., Figueroa A. I., Qafoku O., Wang C., Pearce C. I., Jackson M., Feinberg J., Rosso K. M., Kappler A.
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(2017) Current and future microbiological strategies to remove As and Cd from drinking water. Microbial biotechnology 10 (5) 109
Byrne J. M., Kappler A.
