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

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.