As thiols abiotically reduce iron oxides and various metal-reducing bacteria (MRB) are able to reduce disulfide species, thiols have been proposed to represent electron shuttles in the microbial reduction of iron oxides. Indeed, addition of low disulfide concentrations to pure cultures of MRB has resulted in elevated rates and extent of iron reduction, indicative of a microbial catalytic cycle involving thiol compounds. The prevalence of thiols in the iron reduction zone of natural sediments, however, has not been investigated thoroughly. In this study five distinct sedimentary environments were investigated for the co-existence of thiols and iron reduction by-products, including an alpine freshwater bog, a coastal marsh, proximal deltaic sediments, and the continental slope of two river-fed marine environments. In each of these sediments four main thiols were detected. Three of the thiols reached maximum concentrations in zones of Fe(II) production, while the fourth apparently resulted from sulfidic degradation of plant material. The good correlation between pore water thiols and Fe(II) as well as organic Fe(III) complexes, proposed to be produced as intermediates during microbial iron reduction, provides evidence for the importance of thiol electron shuttling in dissimilatory iron reduction in natural sediments.

Evidence for low molecular weight thiols as electron shuttles in dissimilatory iron reduction in natural sediments

253rd National Meeting (2017)

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