Mineral-based Niche Differentiation in Metal-Reducing Populations
Iron-based (hydr)oxides and phyllosilicate clays, which vary dramatically in their electrical and physicochemical properties, are ubiquitous in subsurface environments controlling the mineralization of carbon, sequestration of contaminants, and diversity of microbial populations. The extent and rate of Fe(III) reduction, adhesion onto surfaces by both whole cells and purified putative Fe reductase enzymes, and microbial communities all vary as a function of the Fe(III) (hydr)oxide phase. Furthermore, the amount of reduction is always a minor fraction of the available Fe(III). More recently, we have observed that the extent of Fe(III) reduction of goethite and hematite correlates with the initial degree of disorder within the initial phase, suggesting that reduction ceases following the consumption of high energy sites. The absence of cultivated microbes capable of efficiently utilizing more crystalline (lower Eh) Fe(III) (hydr)oxides may simply be an artifact of past cultivation approaches. All DIRBs presently in culture have been isolated using labile Fe(III) sources (e.g, ferrihydrite, Fe(III)-EDTA) and thus may selectively enrich for organisms poised to metals with high reduction potentials (Eh). Thus, the overall goal of this research is to show that Eh is a defining variable for microbial community composition and activity in contaminated subsurface environments.
More specifically, we hypothesize that different groups of metal-reducing bacteria are poised to transfer electrons within specific Eh ranges, corresponding to different metal phases and complexes. This is based on preliminary findings in our laboratory illustrating that enriched iron-reducing microorganisms vary as a function of Fe(III) (hydr)oxide. For instance, while common iron-reducing organisms (e.g., Geobacter spp.) were observed growing in the presence of ferrihydrite and lepidocrocite (high Eh), novel metal-reducing populations (e.g., Aeromonas, Sulfurospirillum, Dechloromonas spp.) were enriched when supplied with Fe(III) in the form of goethite and hematite (low Eh). These findings suggest that metal-reducing microbes occupy mineral-based ecological niches, which we suspect is a consequence of the large range in Eh for the various Fe (hydr)oxides. Thus we are currently trying to (1) identify novel metal-reducing microbes that respond to metals of varying reduction potential, (2) determine the mineralogical and enzymatic constraints on metal-reducing microbes for metals with varying reduction potential, (3) ascertain the role of reduction potential in constraining electron transfer from metal-reducing microorganisms to various metal species, and (4) define the active metal-reducing microbes within sediments containing low Eh metals. If our predictions prove correct, viewing all Fe phases equally may have lead us to grossly underestimate the diversity of metal-reducing microbes and variables responsible for regulation of electron flow within subsurface sediments.