The biological reduction
of soluble U(VI) complexes to
form immobile
U(IV) species has been proposed to remediate contaminated sites. It
is well established that multihemec-type cytochromes
(MHCs) are key mediators of electron transfer to aqueous phase U(VI)
complexes for bacteria such asShewanella oneidensisMR-1. Recent studies have confirmed that the reduction proceeds
via a first electron transfer forming pentavalent U(V) species that
readily disproportionate. However, in the presence of the stabilizing
aminocarboxylate ligand, dpaea2–(dpaeaH2=bis(pyridyl-6-methyl-2-carboxylate)-ethylamine), biologically
produced U(V) persisted in aqueous solution at pH 7. We aim to pinpoint
the role of MHC in the reduction of U(V)-dpaea and to establish the
mechanism of solid-phase U(VI)-dpaea reduction. To that end, we investigated
U-dpaea reduction by two deletion mutants ofS. oneidensisMR-1–one lacking outer membrane MHCs and the other lacking
all outer membrane MHCs and a transmembrane MHC–and by the
purified outer membrane MHC, MtrC. Our results suggest that solid-phase
U(VI)-dpaea is reduced primarily by outer membrane MHCs. Additionally,
MtrC can directly transfer electrons to U(V)-dpaea to form U(IV) species
but is not strictly necessary, underscoring the primary involvement
of outer membrane MHCs in the reduction of this pentavalent U species
but not excluding that of periplasmic MHCs.