Ferrihydrite-Dependent Growth of Sulfurospirillum deleyianum through Electron Transfer via Sulfur Cycling

Abstract

Observations in enrichment cultures of ferric iron-reducing bacteria indicated that ferrihydrite was reduced to ferrous iron minerals via sulfur cycling with sulfide as the reductant. Ferric iron reduction via sulfur cycling was investigated in more detail with Sulfurospirillum deleyianum, which can utilize sulfur or thiosulfate as an electron acceptor. In the presence of cysteine (0.5 or 2 mM) as the sole sulfur source, no (microbial) reduction of ferrihydrite or ferric citrate was observed, indicating that S. deleyianum is unable to use ferric iron as an immediate electron acceptor. However, with thiosulfate at a low concentration (0.05 mM), growth with ferrihydrite (6 mM) was possible and sulfur was cycled up to 60 times. Also, spatially distant ferrihydrite in agar cultures was reduced via diffusible sulfur species. Due to the low concentrations of thiosulfate, S. deleyianum produced only small amounts of sulfide. Obviously, sulfide delivered electrons to ferrihydrite with no or only little precipitation of black iron sulfides. Ferrous iron and oxidized sulfur species were produced instead, and the latter served again as the electron acceptor. These oxidized sulfur species have not yet been identified. However, sulfate and sulfite cannot be major products of ferrihydrite-dependent sulfide oxidation, since neither compound can serve as an electron acceptor for S. deleyianum. Instead, sulfur (elemental S or polysulfides) and/or thiosulfate as oxidized products could complete a sulfur cycle-mediated reduction of ferrihydrite.Microorganisms have been recognized as important catalysts for many geochemical transformation reactions. In complex natural systems, transformation reactions of one element often drive transformations of other elements. One important example of such complex geochemical interactions are the reactions between iron and sulfur species and the numerous compounds they can form; in addition, their transformations have further impact on the transformation or mobilization of other elements such as manganese, phosphorus, trace elements, or heavy metals (12).In particular, in marine habitats which are rich in sulfate (28 mM in seawater), the formation of iron mono-and disulfides is taken as an indicator of the activity of sulfate-reducing bacteria, since it is known that they can reduce iron indirectly by producing sulfide, which reduces ferric iron oxides chemically. The sulfidation of ferric iron oxides initiates a complex reaction sequence that results in the formation of various iron monosulfides and iron disulfides including pyrite, which is the most abundant stable iron sulfide (10,12,15). In contrast, the production of siderite (ferrous iron carbonate) or vivianite (ferrous iron phosphate) from ferric iron oxides is attributed exclusively to the direct enzymatic reduction by ferric ironreducing bacteria (6).Since the complex process of sulfidation of ferric iron oxides is surface controlled, it is not surprising that the nature of the products depends on the type of ferric iron oxide used, its...

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