Importance of c-Type cytochromes for U(VI) reduction by Geobacter sulfurreducens

Evgenya S Shelobolina,Sara A Sullivan,Byoung-Chan Kim,Ching Leang,Huifang Xu,Laurie N Didonato,Derek R Lovley,Jessica E Butler,Anton A Korenevsky,Maddalena V Coppi,Hiromi Konishi

doi:10.1186/1471-2180-7-16

Abstract

BackgroundIn order to study the mechanism of U(VI) reduction, the effect of deleting c-type cytochrome genes on the capacity of Geobacter sulfurreducens to reduce U(VI) with acetate serving as the electron donor was investigated.ResultsThe ability of several c-type cytochrome deficient mutants to reduce U(VI) was lower than that of the wild type strain. Elimination of two confirmed outer membrane cytochromes and two putative outer membrane cytochromes significantly decreased (ca. 50–60%) the ability of G. sulfurreducens to reduce U(VI). Involvement in U(VI) reduction did not appear to be a general property of outer membrane cytochromes, as elimination of two other confirmed outer membrane cytochromes, OmcB and OmcC, had very little impact on U(VI) reduction. Among the periplasmic cytochromes, only MacA, proposed to transfer electrons from the inner membrane to the periplasm, appeared to play a significant role in U(VI) reduction. A subpopulation of both wild type and U(VI) reduction-impaired cells, 24–30%, accumulated amorphous uranium in the periplasm. Comparison of uranium-accumulating cells demonstrated a similar amount of periplasmic uranium accumulation in U(VI) reduction-impaired and wild type G. sulfurreducens. Assessment of the ability of the various suspensions to reduce Fe(III) revealed no correlation between the impact of cytochrome deletion on U(VI) reduction and reduction of Fe(III) hydroxide and chelated Fe(III).ConclusionThis study indicates that c-type cytochromes are involved in U(VI) reduction by Geobacter sulfurreducens. The data provide new evidence for extracellular uranium reduction by G. sulfurreducens but do not rule out the possibility of periplasmic uranium reduction. Occurrence of U(VI) reduction at the cell surface is supported by the significant impact of elimination of outer membrane cytochromes on U(VI) reduction and the lack of correlation between periplasmic uranium accumulation and the capacity for uranium reduction. Periplasmic uranium accumulation may reflect the ability of uranium to penetrate the outer membrane rather than the occurrence of enzymatic U(VI) reduction. Elimination of cytochromes rarely had a similar impact on both Fe(III) and U(VI) reduction, suggesting that there are differences in the routes of electron transfer to U(VI) and Fe(III). Further studies are required to clarify the pathways leading to U(VI) reduction in G. sulfurreducens.

Highlights

In order to study the mechanism of U(VI) reduction, the effect of deleting c-type cytochrome genes on the capacity of Geobacter sulfurreducens to reduce U(VI) with acetate serving as the electron donor was investigated
Total uranium was quantitated by diluting samples withdrawn from the cell suspension in 100 mM bicarbonate and bubbling the samples for 15 min with air to convert any reduced uranium, U(IV) and U(V) [30], to U(VI) such that it could be detected by kinetic phosphorescence analysis (KPA)
Our results demonstrate that uranium accumulated to a similar extent in the periplasm of U(VI) reductionimpaired mutants and wild type G. sulfurreducens

Summary

Introduction

In order to study the mechanism of U(VI) reduction, the effect of deleting c-type cytochrome genes on the capacity of Geobacter sulfurreducens to reduce U(VI) with acetate serving as the electron donor was investigated. Uranium is a long-lived radionuclide that poses an ecological and human health hazard. In order to prevent further contamination of aquifers with uranium and halt the expansion of uranium contaminated ground water plumes, it is necessary to immobilize uranium in a geochemically inert form in situ [1,2,3,4]. Stimulation of dissimilatory metal reduction in laboratory incubations of uranium contaminated sediment [6] and in a uranium contaminated aquifer during in situ uranium bioremediation field trials [7,8,9] resulted in the concomitant removal of soluble, hexavalent U(VI) from the ground water and domination of the microbial community by indigenous Fe(III)-reducing bacteria belonging to the family Geobacteraceae of the delta subdivision of the Proteobacteria

Methods

Results

Conclusion