Smectite Reduction by Shewanella Species as Facilitated by Cystine and Cysteine

Abstract

Exogenous electron transfer mediators employed by Fe(III)-reducing bacteria are believed to govern the kinetics and equilibrium of bioreduction of Fe(III) in solid phase. In contrast to a large number of studies on humic substances and analog anthraquinone-2,6-disulfonate (AQDS), our knowledge of other potential electron shuttles involved in Fe(III) reduction is limited. The purpose of the present study was to understand the role of cystine and cysteine in reduction of iron-rich smectite (nontronite, NAu-2) by Shewanella species. A series of abiotic and biotic experiments were conducted in nongrowth media (bicarbonate buffered, pH = 7.0). Fe(II) and cysteine concentrations were monitored over the course of the bioreduction experiments with wet chemistry, and the unreduced and reduced nontronites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that all Shewanella species tested here were capable of reducing cystine to cysteine. Either cystine or cysteine amendments significantly stimulated the Fe(III) bioreduction rate and extent. The initial reduction rate was linearly correlated with cystine or cysteine concentration. The reduction extent (18.7–22.3%) calculated from bioreactor with cystine or cysteine was slightly lower than those with AQDS (26.3%). Mineralogical analysis demonstrated that cystine or cysteine facilitated the reaction of smectite to illite as a result of Fe(III) bioreduction. Thus, we concluded that, in our experiments, cystine and cysteine functioned as electron carrier in the smectite reduction systems, and were favorable factors influencing smectite illitization.

Keywords:

• electron shuttle
• microbial iron reduction
• nontronite
• smectite illitization

Acknowledgments

This research is financially supported by grants from the National Basic Research Program of China (No. 2011CB808800 and 2012CB822004), Natural Science Foundation of China (41030211 & 41072253), and the Subsurface Biogeochemical Research (SBR) Program, Office of Science (BER), U.S. Department of Energy (DOE) (No. DESC0005333), and a China Postdoctoral Science Foundation funded project (No. 2013M531761). The JEOL 2100 TEM used in this study was supported by NSF grant EAR-0722807. The authors are grateful to the Editor-in-Chief, Dr. William C. Ghiorse, and an anonymous reviewer whose comments improved the quality of this manuscript.