Electron shuttling via humic acids in microbial iron(III) reduction in a freshwater sediment

Abstract

The biological and chemical potential for electron shuttling via humic acids was evaluated by analyzing the depth distribution of humic-acid-reducing and iron-reducing bacteria in a freshwater sediment, and correlating it to the redox characteristics of humic acids and iron. Physicochemical analysis of profundal sediments of Lake Constance revealed a distinct stratification, with oxygen respiration, microbial iron and sulfate reduction, and methanogenesis allocatable to defined layers. Among the acid-extractable iron in the surface layer, ferric iron (Fe(III)) was dominant, whereas ferrous iron (Fe(II)) prevailed below 2 cm depth. Humic acids showed a higher electron-accepting (oxidizing) capacity in the surface layer and a higher reducing capacity in deeper layers. The more reduced redox state of humic acids in deeper layers was probably due to reduction by humic-acid-reducing microorganisms. Most-probable-number analysis revealed that the sediments contained populations of humic-acid-reducing bacteria that (i) were substantially larger than those of the iron-reducing bacteria in the respective sediment layers and (ii) were in the same range as those of the fermenting bacteria. Our results suggest that microbial reduction of humic acids and subsequent chemical reduction of poorly soluble iron(III) minerals by the reduced humic acids represents an important path of electron flow in anoxic natural environments such as freshwater sediments.

Electron Shuttles Enhance Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction

“…Moreover, the 29 N 2 production rate was significantly correlated with the iron(III) reduction rate (P < 0.0001), which further demonstrated the existence of Feammox in DIRB enrichments, leading to more NH 4 + consumption (Figure 3). The previous studies have shown that substrate molecules containing the lighter 14 N atom are typically consumed at higher rates than the heavier 15 N atom due to the isotopic fraction effect. 34−36 The higher concentration of 29 N 2 production than 30 N 2 suggested that preferential conversion of 14 N-NH 4 Cl and 14 N-NO x to N 2 in both the ferrihydrite enrichments.…”

Ferrihydrite-associated organic matter (OM) stimulates reduction by &lt;i&gt;Shewanella oneidensis&lt;/i&gt; MR-1 and a complex microbial consortia

“…However, most experiments utilizing OM as a terminal electron acceptor have been performed with alkaline extracts: reduced humic acids have been shown to transfer electrons abiotically to terminal electron acceptors, such as metals, thereby functioning as electron shuttles between microbes and electron acceptors (Kappler et al, 2004). Humic acids therefore possess a redox-mediating ability which positively affects the cycling of redox-active minerals, including Fe(III) and Mn(IV) / Mn(III), in conjunction with transformation of organic and inorganic compounds.…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…NOM was found to greatly enhance the reduction of a key element in soil and sediment such as Fe(III) metals or Fe(III) oxides by a variety of microorganisms (Lovley 1996;Lovley et al 1998;Nevin & Lovley 2000;Royer et al 2002;Chen et al 2003;Kappler et al 2004). Lovley (1996) postulated that humic substances were likely acting as electron mediators or shuttles between microorganisms and Fe(III) or Fe(III)-oxide minerals.…”