Environmental Engineering Reference
In-Depth Information
Shewanella
cells to adhere to solid Fe(III) oxides may be controlled by the
composition and length of the LPS molecules [35].
Shewanella
cells respond to
anaerobic conditions by synthesizing cell surface components with affinity for
Fe(III) minerals: biological forces measured at the interface between cantilever-
attached
Shewanella
cells and goethite indicate that attractive forces between
the cell surface and Fe(III) mineral increase approximately five-fold under
anaerobic conditions [49]. A working model of the Type II protein secretion-
linked pathway for direct (enzymatic) reduction of solid Fe(III) oxides at the
Shewanella
OM is displayed in Fig. 1.
1.2 Exogenous Electron Shuttling Pathway
A variety of Fe(III)-reducing bacteria, including
Shewanella
, employ redox-
active compounds (e.g., humic acids, melanin, phenazines, antibiotics, AQDS)
as electron shuttles to reduce extracellular Fe(III) oxides [46]. The Fe(III)
and Mn(IV) reduction-deficiencies of
Shewanella
Type II protein secretion
mutants are rescued by addition of AQDS [18].
S. oneidensis gspD
insertional
mutants are unable to respire anaerobically on solid Fe(III) or Mn(IV), yet
retain the ability to respire all other electron acceptors, including AQDS. The
ability to respire 50 mM solid Fe(III) or Mn(IV) is rescued in the
S. oneidensis
gspD
insertional mutants by addition of 50 µM AQDS, an indication that the
AQDS electron shuttling pathway functionally replaces the Type II protein
secretion-linked pathway for respiration on solid Fe(III) and Mn(IV). AQDS
is toxic to
Shewanella
cells above a critical concentration threshold and the
efflux pump protein TolC protects
Shewanella
cells from AQDS toxicity by
mediating AQDS efflux [79]. Electron transfer to AQDS also requires the OM
protein MtrB, although its role in AQDS reduction remains unknown [79].
Solid Fe(III) reduction by
Shewanella
is also stimulated by redox-active
antibiotics and phenazines [30]. Phenazines display remarkable similarity in
structure to AQDS, and most likely act in a similar manner to shuttle electrons
between
Shewanella
cells and solid Fe(III) oxides. Redox-active antibiotics
(e.g. bleomycin) also function as shuttles for extracellular electron transfer to
solid electron acceptors. Bacterially-produced phenazines (e.g., synthesized by
Pseudomonas chlororaphis
PCL1391) stimulate Fe(III) reduction by bacteria
unable to produce them (e.g.
S. oneidensis
MR-1) [30]. In addition, melanin (a
humic acid analog synthesized by
S. algae
BrY in the presence of high con-
centrations of tyrosine) will enhance rates of Fe(III) oxide reduction. Melanin
may have a dual function by acting as both an electron shuttle and an Fe(II)-
complexing agent that prevents Fe(II) from blocking Fe(III) oxide surface sites
[86]. A working model of the exogenous electron shuttling pathway for reduc-
tion of solid Fe(III) oxides is displayed in Fig. 2.
