Low-temperature feldspar and illite formation through bioreduction of Fe(III)-bearing smectite by an alkaliphilic bacterium

Abstract

Biogenic mineral assemblages that form from circumneutral microbial reduction of iron in smectite have been suggested as biosignatures in the geological record. However, mineralogical transformation of smectite mediated by microbes under extreme pH condition is still poorly known. The objective of this study was to understand the reduction capacity of structural Fe(III) in iron-rich smectite (nontronite, NAu-2) by a novel anaerobic alkaliphile (strain CCSD-1) isolated from the deep subsurface, and associated mineralogical changes. The experiments with CCSD-1 were conducted in a growth medium containing the electron shuttle anthraquinone-2,6-disulfonate (AQDS) at pH 9.4. The Fe(II) concentration was monitored over the course of the experiment via wet chemistry, and unreduced and reduced nontronites were characterized with X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM). The results indicate that strain CCSD-1 utilizes proteinaceous substrates (yeast extract and tryptone) to reduce structural Fe(III) in smectite with the maximum reduction extent of 26.2%. Mineralogical analysis confirmed that biogenic plagioclase (Na-Ca feldspar) and illite were formed after bioreduction. Our work shows that the interaction between alkaliphile and iron-bearing smectite could account for low-temperature feldspar and illite formation and these minerals may be used as biosignatures in sedimentary rocks.

Coupling of Fe(II) oxidation in illite with nitrate reduction and its role in clay mineral transformation

“…It is also poorly understood if these microbially catalyzed biomineralization can readily be recognized from abiotic pathways of clay mineral transformation. In this context, characteristic biogenic minerals (Dong et al, 2012;Zhao et al, 2015;Liu et al, 2015) are important because they may serve as biosignatures.…”

“…In its first report, Kim et al (2004) suggested that microbial transformation of structural Fe(III) in smectite resulted in partial reductive dissolution of smectite and precipitation of newly formed illite under ambient condition (25 o C, 1 atmospheric pressure) within two weeks. Since then extensive research has been conducted on the important role of dissimilatory iron-reducing bacteria and other microorganisms in driving the smectite to illite conversion (Kim et al, 2004;Zhang et al, 2007a,b;Vorhies and Gaines, 2009;Dong et al, 2009;Jaisi et al, 2011;Koo et al, 2014;Liu et al, 2014Liu et al, , 2015. In contrast, the opposite process, the oxidation of structural Fe(II) in illite has never been studied and it remains unclear whether or not this process would result in the back reaction, e.g., from illite to smectite.…”