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

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.

Keywords:

• iron cycle
• abiotic iron oxidation
• abiotic iron reduction
• banded iron formations

Acknowledgement

We are grateful to the SKB staff for technical, logistic and analytical support at the Äspö HRL. We thank Bernhard Schnetger for his help with the ICMS operation. We would like to thank Marcel Kuypers for his guidance in iron measurements with the nanoSIMS.

Funding

Our study received financial support from the German Research Foundation (DFG) in the framework of the DFG Research Unit FOR 571 ‘Geobiology of Organo- and Biofilms’. This is publication no. 72 of the DFG Research Unit FOR 571 “Geobiology of Organo- and Biofilms.”

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