Biomedical Engineering Reference
In-Depth Information
7.2.1
Isolation
Polaromonas
sp. strain JS666, the first bacterium capable of coupling growth to the aerobic
oxidation of
cis
-DCE, was isolated from activated carbon used for treating water contaminated
with PCE, TCE and
cis
-DCE in Dortmund, Germany (Coleman et al.,
2002a
). Microcosms
containing
cis
-DCE as the sole carbon source were inoculated with activated carbon from the
pump-and-treat plant. A pure culture was obtained by sequential transfers to minimal salts
medium (MSM) [modified from that used by Hartmans et al. (
1992
)], with
cis
-DCE as a sole
source of carbon and energy (Coleman et al.,
2002a
). Biodegradation of
cis
-DCE in enrich-
ments began after 50 days (Coleman et al.,
2002a
). Similar samples from 18 other solvent
contaminated sites did not yield
cis
-DCE degraders, whereas 23 of 27 field samples yielded VC
degraders (Coleman et al.,
2002b
). Others also have isolated VC degraders from the environ-
ment (Verce et al.,
2000
). The results indicate that VC oxidizing bacteria are more common than
cis
-DCE oxidizers and that VC may be more readily degraded via natural attenuation than
cis
-
DCE. It is also possible that the small fraction of microcosms with
cis
-DCE oxidation reflects
difficulties in cultivating DCE degraders. Therefore, future research is necessary to determine
the distribution and abundance of
cis
-DCE oxidizers. However, the large number of sites where
cis
-DCE accumulates suggests that
cis
-DCE oxidizers are not widespread in the environment.
Phylogenetic analysis of the 16S ribosomal ribonucleic acid (rRNA) gene indicated that
strain JS666 is a
-Proteobacterium closely related to the Antarctic marine isolate,
Polaromo-
nas vacuolata,
(97% sequence identity). Strain JS666 shares a 98% sequence identity to the 16S
rRNA gene of
Polaromonas
sp. GM1, a psychrotolerant arsenite-oxidizing bacterium (Osborne
et al.,
2010
), and a 97% 16S rRNA nucleic acid identity with
Polaromonas naphthalenivorans
CJ2, which has been implicated in naphthalene degradation at sites contaminated with coal-tar
waste (Jeon et al.,
2006
). Neither organism possesses the ability to degrade
cis
-DCE. There is
increasing evidence that bacteria of the
Polaromonas
genus may play an important role in the
biodegradation of contaminants (Mattes et al.,
2008
; Yagi et al.,
2009
).
b
7.2.2 Kinetics, Thresholds and Tolerances to
cis
-DCE and Oxygen
JS666 will grow in a minimal medium supplemented with
cis
-DCE as the sole carbon source
(Figure
7.2
). Successive additions of
cis
-DCE were administered in a manner preventing its
complete depletion because JS666 experiences long lags before
cis
-DCE degradation resumes
if cultures are deprived of
cis
-DCE for even short periods (Jennings,
2005
). The slowing of
cis
-DCE degradation after several additions is something often observed with JS666, and can
result from pH decline or accumulation of chloride. The growth yield was 6.1
0.4 gram (g)
protein/mole
cis
-DCE, and at 20 degrees Celsius (
C), the doubling time was 74
8 h (Coleman
et al.,
2002a
). Complete mineralization of
cis
-DCE was indicated by the release of 1.94 moles
chloride per mole of
cis
-DCE degraded (Coleman et al.,
2002a
).
In batch cultures at 20
C, the half-velocity constant (K
s
) was 1.6
M),
and the maximum specific substrate utilization rate (k) ranged from 12.6 to 16.8 nanomoles/
minute/milligram (nmol/min/mg) of protein (Coleman et al.,
2002a
). The high k and low K
s
indicate that JS666 is capable of degrading
cis
-DCE to extremely low levels without kinetic
limitations, which is important if the organism is to degrade
cis
-DCE completely at contami-
nated sites. In the laboratory, JS666 was routinely able to degrade
cis
-DCE to below detection
limits of 0.03 micrograms per liter (
0.2 micromolar (
m
g/L) (Coleman et al.,
2002a
), which is well below the
drinking water standard for
cis
-DCE of 70
m
m
g/L (
www.epa.gov
).
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