Anaerobic and aerobic oxidation of ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing bacteria

Abstract

Nine out of ten anaerobic enrichment cultures inoculated with sediment samples from various freshwater, brackish-water, and marine sediments exhibited ferrous iron oxidation in mineral media with nitrate and an organic cosubstrate at pH 7.2 and 30°C. Anaerobic nitrate-dependent ferrous iron oxidation was a biological process. One strain isolated from brackish-water sediment (strain HidR2, a motile, nonsporeforming, gram-negative rod) was chosen for further investigation of ferrous iron oxidation in the presence of acetate as cosubstrate. Strain HidR2 oxidized between 0.7 and 4.9 mM ferrous iron aerobically and anaerobically at pH 7.2 and 30°C in the presence of small amounts of acetate (between 0.2 and 1.1 mM). The strain gained energy for growth from anaerobic ferrous iron oxidation with nitrate, and the ratio of iron oxidized to acetate provided was constant at limiting acetate supply. The ability to oxidize ferrous iron anaerobically with nitrate at approximately pH 7 appears to be a widespread capacity among mesophilic denitrifying bacteria. Since nitrate-dependent iron oxidation closes the iron cycle within the anoxic zone of sediments and aerobic iron oxidation enhances the reoxidation of ferrous to ferric iron in the oxic zone, both processes increase the importance of iron as a transient electron carrier in the turnover of organic matter in natural sediments.

Fe(II) Oxidation Is an Innate Capability of Nitrate-Reducing Bacteria That Involves Abiotic and Biotic Reactions

“…It was previously noted that acetate is consumed before Fe(II) in NDFO growth cultures (5,60), and this is also true for A. ebreus (see Fig. S2b in the supplemental material).…”

Potential for Microscale Bacterial Fe Redox Cycling at the Aerobic-Anaerobic Interface

“…A review by Emerson (2000) provides an excellent overview of the history of research on circumneutral bacterial Fe(II) oxidation, as well as the physiology and systematics of currently known FeOB. It is relevant to note that the potential for both anaerobic phototrophic (Widdel et al 1993;Ehrenreich and Widdel 1994) and chemolithotrophic nitrate-reducing bacteria (Straub et al 1996;Benz, Brune, and Schink 1998;Straub and Buchholz-Cleven 1998) to catalyze circumneutral Fe(II) oxidation is now recognized (Straub et al 2001). However, the overview and experimental studies presented in this paper focus on chemolithotrophic organisms capable of oxidizing Fe(II) with O 2 as an electron acceptor.…”

“…Together these results suggest that the unique role of FeOB in Fe redox cycling in nature is likely to stem from their ability to alter the spatial separation between zones of Fe(II) oxidation and Fe(III) oxide precipitation in a way that promotes Fe redox cycling. It is important to note in this context that FeOB activity generally leads to the development of a narrow band of cells and Fe(III) oxides in the vicinity of aerobic/anaerobic interface in Fe(II)-O 2 opposing gradient systems (Emerson and Moyer 1997;Benz et al 1998); see also Figure 3), in contrast to a much more diffuse zone of Fe(III) oxide deposition in abiotic systems. These observations suggest that FeOB have the capability to focus the zone of Fe(III) oxide deposition very close to (i.e.…”

Cathodic oxygen reduction catalyzed by bacteria in microbial fuel cells

“…Third, the reuse of anodic effluent in the mixed population reactor inevitably caused a small transfer of organic carbon (acetate and metabolites) to the cathode. Perhaps the organisms at the cathode are lithoheterotrophic, as previously described for denitrifying iron oxidizing organisms (Benz et al, 1998). If so, the organisms used the available organics as carbon source for growth, whereas the cathode was used as source of reducing power for the energy metabolism.…”