Biotic and abiotic oxidation and reduction of iron at circumneutral pH are inseparable processes under natural conditions

Danny Ionescu,Christine Heim,Lubos Polerecky,Volker Thiel,Dirk De Beer

doi:10.1080/01490451.2014.887393

Abstract

Oxidation and reduction of iron can occur through abiotic (chemical) and biotic (microbial) processes. Abiotic iron oxidation is a function of pH and O2 concentration. Biotic iron oxidation is carried out by a diverse group of bacteria, using O2 or NO3 as terminal electron acceptors. At circumneutral pH, both processes occur at similar rates and compete with each other. Abiotic iron reduction is catalyzed by iron-sulfur minerals or different types of organic compounds, whereas biotic iron reduction is carried out by a diverse group of microorganisms, often using chemical agents to dissolve solid iron minerals. We used iron oxidizing microbial mats to assess the potential impact of microbial activity on the deposition of banded iron formations (BIF). The mats were collected during several years from experimental tanks connected to groundwater aquifers with different Fe2+ concentrations. To separate between biotic and abiotic iron oxidation, live and killed mats were incubated with 57Fe2+. Separate analyses of the water and solid phase revealed that the iron oxidation and reduction rates per mL of solid matter (biomass and iron precipitates) were 0.4-73 mmol L−1 d−1 and 30–280 mmol L−1 d−1, respectively. No significant differences in iron oxidation rates were observed between the live and killed samples. The iron reduction rates, however, were higher in the live samples in mats from 3 out of 4 environments. We suggest that in natural systems, in the presence of organic matter, biotic and abiotic iron oxidation and reduction are not separable processes. Fe2+ will be biotically and abiotically oxidized as well as bind to exposed charged groups of organic substances. Either way, this iron may serve as a nucleation matrix for further abiotic iron precipitation. The oxidized iron is then susceptible to iron reduction, which can likewise be a direct metabolic or an abiotic process. Nevertheless, it is important to note the significance of organic matter, since both the abiotic oxidation and reduction of iron are often mediated by substrates of biological origin.

Highlights

Banded Iron Formations (BIF) are the major deposits of iron in the Earth crust (Posth et al 2011)
The main question regarding the involvement of microorganisms in the formation of BIF is whether these are capable of precipitating such vast amounts of iron (Konhauser et al 2002)
The bulk measurements of the 57Fe/56Fe ratios showed the expected trend, with the ratio decreasing with time in the liquid phase and increasing with time in the solid phase; individual subsamples collected during the incubation occasionally deviated from this trend (Table S2)

Summary

Introduction

Banded Iron Formations (BIF) are the major deposits of iron in the Earth crust (Posth et al 2011). A major division between the proposed hypotheses involves the question whether they are formed by abiotic (Wang et al 2009) or biotic (Czaja et al 2013; Konhauser et al 2002) processes. Posth et al (2013) have shown that the magnetite and siderite, both key minerals found in current-days BIFs, could be formed by interaction of ferrihydrite and organic matter under high pressure or temperature. Their results eliminate the necessity for a direct link between BIF minerals and a respective microorganism. The main question regarding the involvement of microorganisms in the formation of BIF is whether these are capable of precipitating such vast amounts of iron (Konhauser et al 2002)

Methods

Results

Conclusion