Environmental Engineering Reference
In-Depth Information
Table 1.2
c
-type cytochromes and their role in U(VI) reduction
Bacterial species Role/description/comments
References
Desulfovibrio
vulgaris
Removal of cytochrome
c
3
from crude extracts eliminated
U(VI) reduction activity (in vitro). Reduced cytochrome
c
3
was oxidized during U(VI) reduction.
[133, 134,
135]
Desulfovibrio
desulfuricans
U(VI) reduction by a cytochrome
c
3
mutant was inhibited by
at least 90% with H
2
as the electron donor.
[136, 126]
Geobacter
sulfurreducens
Elimination of two outer membrane cytochromes and two
putative outer membrane cytochromes significantly
decreased (50-60%) the ability of
G. sulfurreducens
to
reduce U(VI).
[137]
Shewanella
oneidensis
Deletions of outer membrane cytochromes MtrC and/or
OmcA significantly affected the in vivo U(VI) reduction
rate. There was a close association of the extracellular UO
2
nanoparticles with MtrC and OmcA.
[138, 71]
Thiobacillus
denitrificans
Membrane-associated cytochromes
c
4
and
c
5
played a major
role in nitrate-dependent U(IV) oxidation. Insertion
mutations resulted in a decrease (<50%) in U(IV) oxidation
activity. Complementation restored activity to the wild-type
level.
[139]
affect the immobilization process by causing local changes in electrochemical
potential (E
h
) and pH, or by producing metal-complexing ligands [44]. A wide
variety of microorganisms, including bacteria, fungi and algae, have been shown
to possess capabilities of U(VI) biosorption [45-49]. Uranium can be biosorbed
either to the cell wall or to extracellular components associated with the cell wall,
such as polysaccharides, glycoprotein or lipopolysaccharides. Various functional
groups, including carboxylate, phosphate, and hydroxyl groups, are usually found
(Table 1.3) on the extracellular components.
Bioprecipitation
Microorganisms are also able to precipitate metals and radionuclides as carbonates
and hydroxides through localized alkalization at the cell surface or to precipi-
tate them with enzymatically generated ligands, such as carbonate, sulfide, and
phosphate [50-53], providing an appreciable way to immobilize U(VI).
Bioaccumulation
Bacterial cells have been shown to have the ability to accumulate U(VI) intracellu-
larly and then immobilize it through several mechanisms. The chelation of uranium
by polyphosphate bodies is a well-studied mechanism [54]. McLean and Beveridge
[39] speculated that the polyphosphate bodies in a bacterial cell might sequester
uranium in the cytoplasm and form strong complexes with it to protect the cell.
The accumulation of uranium in the form of needle-like fibrils in the cytoplasm
is another type of bioaccumulation [55]. It has been suggested [44, 56] that this
