title: Dominance of sulfur-fueled iron oxide reduction in low-sulfate freshwater sediments.
abstract: A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant pathway in iron reduction. This process was mediated by (re)cycling of sulfur upon reaction of sulfide and iron oxides to support continued sulfur-based respiration--a cryptic sulfur cycle involving generation and consumption of sulfur intermediates. Although canonical iron respiration was not observed in the sediments amended with the more crystalline iron oxides, iron respiration did become dominant in the presence of ferrihydrite once sulfate was consumed. Thus, despite more favorable energetics, ferrihydrite reduction did not precede sulfate reduction and instead an inverse redox zonation was observed. These findings indicate that sulfur (re)cycling is a dominant force in iron cycling even in low-sulfate systems and in a manner difficult to predict using the classical thermodynamic ladder.
authors: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
authors: Colleen M
authors: CM
authors: Hansel
authors: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
authors: Chris J
authors: CJ
authors: Lentini
authors: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA.
authors: Yuanzhi
authors: Y
authors: Tang
authors: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
authors: David T
authors: DT
authors: Johnston
authors: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
authors: Scott D
authors: SD
authors: Wankel
authors: Philip M
authors: PM
authors: Jardine
mesh_terms: D003901:Desulfovibrio / Q000235:genetics / Q000378:metabolism*; D005290:Ferric Compounds / Q000378:metabolism*; D005618:Fresh Water; D005740:Gases; D019015:Geologic Sediments / Q000737:chemistry* / Q000382:microbiology; D007501:Iron / Q000378:metabolism; D058085:Iron Compounds; D008903:Minerals; D010084:Oxidation-Reduction; D010087:Oxides; D010101:Oxygen Consumption; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D013431:Sulfates / Q000378:metabolism*; D013440:Sulfides; D013455:Sulfur / Q000378:metabolism; D013457:Sulfur Compounds; D013816:Thermodynamics
chemical_list: D005290:Ferric Compounds; D005740:Gases; D058085:Iron Compounds; D008903:Minerals; D010087:Oxides; D012336:RNA, Ribosomal, 16S; D013431:Sulfates; D013440:Sulfides; D013457:Sulfur Compounds; C094886:goethite; C405951:sulfur monoxide; C000499:ferric oxide; D013455:Sulfur; C092844:ferric oxyhydroxide; D007501:Iron