INTRODUCTION
Microbial corrosion of iron-containing metals is a substantial economic problem, but the mechanisms are poorly understood (
1–3). Fundamentally, iron corrosion is the oxidation of metallic iron to ferrous iron:
This reaction must be coupled with an electron accepting reaction to proceed. Thus, microorganisms can stimulate iron corrosion by either accelerating reaction 1 or by contributing to one or more reactions that consume the electrons from reaction 1.
Sulfate reducers are often implicated in iron corrosion (
1,
4–6).
Desulfovibrio spp. have been the most studied pure culture isolates for investigating iron corrosion under sulfate-reducing conditions, dating back to the some of the earliest studies on microbial corrosion (
1,
3,
7). Three mechanisms for
Desulfovibrio species to consume electrons derived from Fe
0 oxidation have been proposed (
Fig. 1A). The first mechanism proposed (
8) was abiotic oxidation of Fe
0 coupled to proton reduction to generate H
2:
combined with consumption of the H
2 via sulfate reduction:
Several mechanisms that might enhance H
2 production have been proposed (
Fig. 1B). Hydrogenases released from moribund cells may accelerate reaction 1 by catalyzing H
2 production from Fe
0 (
9). H
2S generated from sulfate reduction may promote H
2 production from Fe
0 in two ways. Sulfide can react with Fe
0 to generate H
2 (
1):
and iron sulfide precipitates might facilitate electron transfer from the Fe
0 to H
+, accelerating reaction 2 (
2,
10).
One of the most intriguing proposed mechanisms for
Desulfovibrio species to participate in iron corrosion is direct microbial electron uptake from Fe
0 (
1,
11–14).
D. ferrophilus (previously known as strain IS
5) can grow with H
2 as the sole electron donor, but it was inferred to directly consume electrons from Fe
0 based on the observation that it reduced sulfate faster than several other H
2-oxidizing sulfate reducers (
11). However, this inference relies on the unsubstantiated assumption that direct electron transfer is faster than H
2-mediated electron transfer from Fe
0 to microbes. Furthermore, possible adaptions in
D. ferrophilus for enhanced growth on H
2 derived from Fe
0—such as producing an extracellular hydrogenase to accelerate Fe
0 oxidation, a higher affinity for H
2, or possibly a better capacity for attachment to Fe
0—were not considered (
3).
In subsequent studies,
D. ferrophilus grew with pure Fe
0 as the electron donor, but not with stainless steel (
15). This distinction is important because pure Fe
0 abiotically generates H
2 via reaction 1 (
16,
17), but stainless steel does not (
18). In contrast to
D. ferrophilus, stainless steel is an effective electron donor for
Geobacter and
Methanosarcina strains capable of direct electron uptake from Fe
0 (
15,
18,
19). Notably, protease digestion of
D. ferrophilus extracellular proteins did not affect sulfate reduction rates with Fe
0 as the electron donor (
20), a result inconsistent with a microbe making direct electrical contact with Fe
0 because protease degrades outer-surface electrical contacts (
21). Therefore, the evidence available to date suggests that
D. ferrophilus most likely accepts electrons from Fe
0 via an H
2 intermediate (
15).
D. vulgaris is the most intensively studied sulfate reducer for biochemical and physiological investigations, and has served as a model sulfate reducer for many corrosion studies (
7). Direct Fe
0-to-microbe electron transfer has also been proposed for
D. vulgaris (
13,
14,
22), but as with the
D. ferrophilus studies, the possibility of H
2-mediated metal-to-microbe electron transfer was not rigorously eliminated. Studies with
Geobacter (
17,
18),
Shewanella (
23,
24), and
Methanosarcina (
19) species have provided evidence for direct electron uptake from Fe
0 by (i) eliminating the possibility that H
2 was serving as an electron shuttle between Fe
0 and cells and (ii) demonstrating with gene deletions that outer-surface
c-type cytochromes were required for electron uptake from Fe
0. In contrast, no studies have previously been reported on
D. vulgaris corrosion with strains that were unable to use H
2 (
7).
D. vulgaris lacks outer-surface cytochromes (
25), and no other
D. vulgaris outer surface electrical contacts are known. Unlike the microbes previously shown to directly accept electrons from Fe
0 (
17–19,
23,
24),
D. vulgaris does not directly reduce Fe(III) (
26), an ability common to most microbes that can directly exchange electrons with extracellular electron donors and acceptors (
27).
Higher rates of corrosion following the addition of riboflavin (
14,
28,
29) led to the suggestion that riboflavin can function as an electron shuttle that Fe
0 reduces:
with
D. vulgaris oxidizing the reduced riboflavin with the reduction of sulfate:
However, those studies did not determine whether Fe0 could donate electrons to riboflavin or whether reduced riboflavin can serve as an electron donor for sulfate reduction. The alternative possibility that riboflavin might stimulate other aspects of microbial metabolism was also not evaluated. Furthermore, electron transfer via H2 was still possible in those studies.
A rigorous strategy to evaluate the possibility of H
2 serving as an intermediary electron carrier is to determine whether strains unable to use H
2 as an electron donor can respire with Fe
0 as the sole electron donor (
17–19,
23,
30). In instances in which the wild-type strain of interest can consume H
2, this can be accomplished by deleting genes necessary for H
2 metabolism (
17,
23,
30). A strain of
D. vulgaris in which genes for all of the annotated hydrogenases on the genome were deleted is available as one of a large collection of mutant strains (
31). We report here on studies on Fe
0-dependent sulfate reduction conducted with this hydrogenase-deficient strain.