Environmental Engineering Reference
In-Depth Information
1.3 Endogenous Electron Shuttling Pathway
Although the exact identity of an endogenous electron shuttle has yet to be
determined, preliminary studies indicate that
Shewanella
may synthesize and
release quinone-like compounds that act as shuttles for electron transfer to solid
Fe(III) oxides [68].
S. algae
BrY produces melanin as a soluble electron shut-
tle for reduction of insoluble Fe(III) oxides [86].
S. algae
-produced melanin
oxidizes cytochromes at the cell surface and reduces solid Fe(III) oxides ex-
tracellularly [86].
S. oneidensis
MR-1 mutants defective in
menC
(encoding
o
-succinylbenzoic acid synthase) are deficient in menaquinone production and
are unable to reduce AQDS, fumarate, thiosulfate, sulfite, DMSO or solid Fe(III)
and Mn(IV) [68]. Menaquinone is detected in the spent media of the wild-type
strain, but not the
menC
mutants. Spent media from the wild-type comple-
mented the
menC
mutant, while spent media from
menC
mutant did not.
S.
oneidensis
MR-1 mutants defective in either
menD
or
menB
(encoding compo-
nents of the menaquinone biosynthetic pathway) are also unable to reduce solid
Fe(III) oxides [76]. Vitamin K
2
(a menaquinone analog) restores the ability of
the
menD
or
menB
mutants (and corresponding membrane fractions) to reduce
either Fe(III) or Mn(IV) [76]. It should be noted that the endogenous electron
shuttle pathway may be the consequence of cell lysis and inadvertent spillage
of menaquinol into the culture medium. Lipid-soluble menaquinol or vitamin
K2 then diffuse into bacterial membranes and functionally complement the
menB-D
mutants. Definitive evidence on the identity of the endogenous elec-
tron shuttle will therefore require further research. A working model of the
endogenous electron shuttling pathway for reduction of solid Fe(III) oxides is
displayed in Fig. 3.
1.4 Fe(III) Solubilization by Bacterially Produced
Organic Ligands
An electrochemical signal attributed to soluble organic-Fe(III) is detected in
a variety of marine and freshwater environments with Au/Hg voltammetric mi-
croelectrodes [83]. Soluble organic-Fe(III) may therefore represent a dominant,
yet underappreciated electron acceptor in anaerobic aquatic systems. Microbial
Fe(III) reduction rates are higher with soluble organic-Fe(III) in pure cultures
of
S. putrefaciens
[2] and in freshwater sediments amended with nitrilotriacetic
acid (NTA).
S. putrefaciens
reduces soluble organic-Fe(III) at rates three orders
of magnitude faster than amorphous or crystalline Fe(III) forms. The mecha-
nism of formation of soluble organic-Fe(III) generally involves non-reductive
dissolution of amorphous Fe(III) oxides by multidentate organic ligands (form-
ing mononuclear complexes with the Fe(III) oxides) at circumneutral pH. The
strength of binding between Fe(III) and the complexing organic ligands influ-
ences soluble organic-Fe(III) reduction activity: organic ligands with strong
