Biological Oxidation of Fe(II)-Bearing Smectite by Microaerophilic Iron Oxidizer Sideroxydans lithotrophicus Using Dual Mto and Cyc2 Iron Oxidation Pathways

Abstract

Fe(II) clays are common across many environments, making them a potentially significant microbial substrate, yet clays are not well established as an electron donor. Therefore, we explored whether Fe(II)-smectite supports the growth of Sideroxydans lithotrophicus ES-1, a microaerophilic Fe(II)-oxidizing bacterium (FeOB), using synthesized trioctahedral Fe(II)-smectite and 2% oxygen. S. lithotrophicus grew substantially and can oxidize Fe(II)-smectite to a higher extent than abiotic oxidation, based on X-ray near-edge spectroscopy (XANES). Sequential extraction showed that edge-Fe(II) is oxidized before interior-Fe(II) in both biotic and abiotic experiments. The resulting Fe(III) remains in smectite, as secondary minerals were not detected in biotic and abiotic oxidation products by XANES and Mossbauer spectroscopy. To determine the genes involved, we compared S. lithotrophicus grown on smectite versus Fe(II)-citrate using reverse-transcription quantitative PCR and found that cyc2 genes were highly expressed on both substrates, while mtoA was upregulated on smectite. Proteomics confirmed that Mto proteins were only expressed on smectite, indicating that ES-1 uses the Mto pathway to access solid Fe(II). We integrate our results into a biochemical and mineralogical model of microbial smectite oxidation. This work increases the known substrates for FeOB growth and expands the mechanisms of Fe(II)-smectite alteration in the environment.

Gallionellaceae pangenomic analysis reveals insight into phylogeny, metabolic flexibility, and iron oxidation mechanisms

“…humic substances). The decaheme MtoA has been shown to play roles in the oxidation of mineral-bound Fe(II), specifically Fe(II) smectite clay (37). As a MHC, MtoA may have multiple benefits that help in oxidizing minerals.…”

Oxidation Rates and Redox Stabilization of Ferrous Iron in Trioctahedral Smectites

“…Each kinetic study was conducted in an artificial freshwater media (Emerson and Merrill Floyd, 2005) for microaerophilic lithophiles that was chosen to replicate the conditions utilized in microbial cultures, e.g., (Zhou et al, 2022), and consistent with natural bicarbonate-buffered aquatic environments. The media contained 1 mL per liter each of ATCC Vitamin Supplement (ATCC MD-VS) and ATCC Trace Mineral Supplement (ATCC MD-TMS).…”

“…In order to determine the rate of oxidation by dissolved oxygen, either air or a gas mixture of 78% N2, 20% CO2, and 2% O2 was first humidified and then passed through a 0.22 µm filter membrane to each serum bottle at a rate of 50 mL min -1 . The gas mixture composition was selected to mirror the headspace used in microaerophilic microbial oxidation studies (Zhou et al, 2022). Suspensions were continuously stirred for the duration of the experiment.…”

“…The rate of abiotic oxidation of trioctahedral Fe(II) smectite suggests that these phases represent potential electron donors for microorganisms. Recent work (Zhou et al, 2022) has…”

“…Irrespective of the accessibility of the recalcitrant Fe(II) pool, a series of studies now indicate that structural Fe(II) in smectites and other phyllosilicates is accessible to microbial oxidation under microaerophilic conditions (Benzine et al, 2013;Shelobolina et al, 2012b;Zhou et al, 2022). Such microbially-mediated oxidation may be pervasive in altered oceanic crust where ferrous smectites frequently coat cracks and vesicles (Alt and Teagle, 2003;Andrews, 1980;Colwell and D'Hondt, 2013).…”