Abstract: Uraninite (UO2) is the most desirable end product of in situ bioreduction because of its low solubility under reducing conditions. For effective long-term immobilization of uranium (U), there should be no biotic or abiotic reoxidation of the insoluble biogenic U(IV). It is therefore critical to understand the long-term stability of U(IV) under oxic- and nutrient-limited conditions at U-contaminated subsurface sites. It has now been established that following in situ bioremediation of U(VI) via nutrient addition in the subsurface, a range of physical, chemical, and biological factors control the rate and extent of long-term stability of U(IV). Some of these factors are tied to site specific conditions including existence of oxidants such as Fe(III)(hydr)oxides, Mn(IV) oxides, oxygen, and nitrate; the presence of organic carbon and the reduced forms of U (e.g., mononuclear U(IV) or nanometer-sized uraninite particles); as well as the carbonate concentration and pH of groundwater. This review analyzes the contribution of these factors in controlling U(IV)-reoxidation, and highlights the competition among U(IV) and other electron acceptors and possible mechanisms of reoxidation of various forms of U(IV).
Abstract: The present status of contamination by heavy
metals, and the impact of cage culture on sediments at the
Uranouchi Inlet, Kochi Prefecture, Japan, were investigated
from two stations influenced by intensive aquaculture and a
control station, during May–July 2006. The moisture
content of the sediments at the aquaculture stations was
over 65%, and the organic matter was always over
100 mg/g dry wt. In contrast, the highest moisture content
and organic matter at the control station was 45.5% and
62.9 mg/g dry wt, respectively. Concentrations of zinc
(Zn) (178 ± 4.8 mg/kg dry wt) and copper (Cu) (125 ±
1.2 mg/kg dry wt) were highest at the aquaculture stations.
Lead (Pb) was highest (50.7 ± 0.77 mg/kg dry wt) at
the aquaculture station though it was as high as 33.2 ±
0.77 mg/kg dry wt at the control station. One-way ANOVA
showed that the differences in concentrations of Zn and Cu
in sediments from the aquaculture and control stations were
highly significant (P = 0.01), whereas Pb showed no such
trend. Occurrence of a large fraction of labile Zn (56.1%)
and Cu (40.3%) in these sediments warrants attention.
Although factors other than metals may explain the distribution
observed, the information presented here may be
useful in predicting long-term effects of heavy metal contamination
from aquaculture in the marine environment.
Abstract: Pollution in industrial areas is a serious environmental
concern, and interest in bacterial resistance to
heavy metals is of practical significance. Mercury (Hg),
Cadmium (Cd), and lead (Pb) are known to cause damage
to living organisms, including human beings. Several
marine bacteria highly resistant to mercury (BHRM)
capable of growing at 25 ppm (mg L−1) or higher
concentrations of mercury were tested during this study to
evaluate their potential to detoxify Cd and Pb. Results
indicate their potential of detoxification not only of Hg, but
also Cd and Pb. Through biochemical and 16S rRNA gene
sequence analyses, these bacteria were identified to belong
to Alcaligenes faecalis (seven isolates), Bacillus pumilus
(three isolates), Bacillus sp. (one isolate), Pseudomonas
aeruginosa (one isolate), and Brevibacterium iodinium (one
isolate). The mechanisms of heavy metal detoxification
were through volatilization (for Hg), putative entrapment in
the extracellular polymeric substance (for Hg, Cd and Pb)
as revealed by the scanning electron microscopy and energy
dispersive x-ray spectroscopy, and/or precipitation as
sulfide (for Pb). These bacteria removed more than 70%
of Cd and 98% of Pb within 72 and 96 h, respectively, from
growth medium that had initial metal concentrations of
100 ppm. Their detoxification efficiency for Hg, Cd and Pb
indicates good potential for application in bioremediation of
toxic heavy metals.
Abstract: The hypothesis that mercury-resistant bacteria exposed to polluted environments such as coastal areas can tolerate, detoxify, or biotransform a variety of other toxicants was examined. Several mercury-resistant marine bacteria from the coastal waters of India were evaluated for their ability to biotransform the heavy metals mercury, cadmium and lead as well as xenobiotics like polychlorinated biphenyls and tributyltin. These salt-tolerant bacteria removed mercury by means of volatilization and were successfully used to detoxify mercury-amended growth medium for the culturing of mercury-sensitive Phormidium sp. Over 70% cadmium and 95% of the lead from the growth medium were either cell-bound (cadmium) or precipitated (lead) by some of these bacteria. A pseudomonad strain, CH07, aerobically degraded fourteen toxic polychlorinated biphenyls including congeners with five or more chlorine atoms on the biphenyl ring and was also equally efficient in degrading more than 54% of the tributyltin. These bacteria offer great biotechnological opportunities in the bioremediation of toxic chemicals.
Abstract: Several strains of bacteria unusually highly resistant to mercury were isolated from seawater and marine sediment samples
and identified by 16S rDNA sequencing and were also characterized by a battery of biochemical and morphological tests. The
bacterial isolates were identified to belong to the genera Pseudomonas, Alcaligenes, Brevibacterium and Bacillus. Many of the
chosen isolates were tested for growth in the presence of different heavy metals and a variety of xenobiotics. Growth curves of all
six bacteria highly resistant to mercury examined for growth at different concentrations of Hg exhibited prolonged lag phase,
during which time necessary physiological adaptations to toxic milieu were undergone. All the strains tested for antibiotic
resistance showed little to no effect of antibiotics on their normal growth. Results of this study demonstrate the occurrence of
diverse groups of marine prokaryotes capable of high tolerance to mercury with a potential to degrade a variety of toxic heavy
metals and xenobiotics.
Abstract: Bioremediation of toxic substances includes
microbe-mediated enzymatic transformation of toxicants to
non-toxic, often assimilable, forms. Mercury-resistant
marine bacteria are found to be very promising in dealing
with mercury, and a host of other highly toxic heavy metals
and xenobiotics. In the present studies we have shown that
the Pseudomonas aeruginosa CH07 (NRRL B-30604) has
been able to degrade a variety of PCB congeners including
a complete degradation of CB-126 and CB-181. The culture
was able to remove over 70% Cd from growth medium
when supplemented with 100 ppm Cd. The same bacterium
rapidly biotransformed/removed toxic mercury from
wastewater in a bioreactor system.
Abstract: As insights from tolerance responses of native microflora
are useful in deciphering their involvement in biogeochemical
cycling of heavy metals, we enumerated mercury-
resistant bacteria (MRB) using sea water nutrient
agar medium amended with 10 ppm Hg from oceanic
and coastal waters of the Bay of Bengal (BOB) during
the summer monsoon (July–August 2001) period.
MRB were present in all samples and, intriguingly, the
MRB per cent based on total viable counts (TVC) increased
significantly (r = 0.86; P < 0.001; df = 44) with
depth. On an average, MRB contributed to over 20%
of TVC on the surface, 12% at 100 m, 35% at 500 m
and a staggering 49% at 1000 m. The fact that a major
portion of the natural, culturable bacterial flora was
mercury-resistant from the offshore regions of the
BOB points to the global nature of mercury pollution.
The higher percentages of MRB in the offshore waters
of the BOB might signify the already prevalent adverse
impact of heavy metals on the metabolic performance
of heterotrophic microflora.
Abstract: Hitherto, aerobic degradation of polychlorinated
biphenyls (PCBs) has been reported to be
limited to the less chlorinated biphenyls. We report
here a marine mercury-resistant bacterium, Pseudomonas
CH07 (NRRL B-30604) which was capable of
degrading a variety of highly chlorinated congeners of
PCBs from the technical mixture Clophen A-50. Of the
two most toxic coplanar PCBs present in Clophen
A-50, one coplanar pentachloro congener CB-126 and
one toxic sterically hindered heptachloro congener
CB-181 were found to be degraded completely and the
other coplanar tetrachloro congener CB-77 was
degraded by more than 40% within 40 h by this
microorganism. The apparent absence of bphC in this
bacterium leads to the proposal of a different mechanism
for degradation of PCBs.
Abstract: A sharp rise in mercury-resistant bacteria (MRB) capable of tolerating very high concentration of Hg was observed over the last 3-4 years in the coastal environs of India. While none or negligible colony-forming units (CFU) of bacteria were counted on seawater nutrient agar with 0.5 ppm ( 2.5 microM) Hg (II) as HgCl2 until 1997, from 13 to over 75% of the CFU grew on 20 times higher, 50 microM, Hg concentrations from almost every recently examined marine sample. Although exceptionally high counts of MRB (96% of CFU) were recorded from samples collected from the polluted zones off Mumbai, the MRB capable of growth on seawater nutrient agar with 50 microM Hg were quite abundant in most samples collected from many locations with few or no pollution effects. We noticed for the first time the occurrence of aerobic heterotrophic bacterial isolates capable of growth with 250 microM Hg. Such MRB grew with higher concentrations of many other toxic xenobiotics than the Hg sensitive ones. Based on the unusually high populations of viable MRB and some simple experiments, we propose that many marine bacterial species are selected, possibly through acquisition of plasmids and/or transposable elements and modifying Hg, whose concentration, according to recent studies, is on the rise in marine habitats.
Abstract: A sharp rise in mercury-resistant bacteria (MRB) capable of tolerating very high concentration of
Hg was observed over the last 3–4 years in the coastal environs of India. While none or negligible
colony-forming units (CFU) of bacteria were counted on seawater nutrient agar with 0.5 ppm
(2.5 lM) Hg (II) as HgCl2 until 1997, from 13 to over 75% of the CFU grew on 20 times higher, 50
lM, Hg concentrations from almost every recently examined marine sample. Although exceptionally
high counts of MRB (96% of CFU) were recorded from samples collected from the
polluted zones off Mumbai, the MRB capable of growth on seawater nutrient agar with 50 lMHg
were quite abundant in most samples collected from many locations with few or no pollution
effects. We noticed for the first time the occurrence of aerobic heterotrophic bacterial isolates
capable of growth with 250 lM Hg. Such MRB grew with higher concentrations of many other
toxic xenobiotics than the Hg sensitive ones. Based on the unusually high populations of viable
MRB and some simple experiments, we propose that many marine bacterial species are selected,
possibly through acquisition of plasmids and/or transposable elements and modifying Hg, whose
concentration, according to recent studies, is on the rise in marine habitats.
Abstract: Bacteria highly resistant to mercury isolated from seawater and sediment samples were tested for
growth in the presence of different heavy metals, pesticides, phenol, formaldehyde, formic acid, and trichloroethane
to investigate their potential for growth in the presence of a variety of toxic xenobiotics. We hypothesized
that bacteria resistant to high concentrations of mercury would have potential capacities to tolerate
or possibly degrade a variety of toxic materials and thus would be important in environmental pollution
bioremediation. The mercury-resistant bacteria were found to belong to Pseudomonas, Proteus, Xanthomonas,
Alteromonas, Aeromonas, and Enterobacteriaceae. All these environmental bacterial strains tolerant to mercury
used in this study were capable of growth at a far higher concentration (50 ppm) of mercury than previously
reported. Likewise, their ability to grow in the presence of toxic xenobiotics, either singly or in combination,
was superior to that of bacteria incapable of growth in media containing 5 ppm mercury. Plasmid-curing assays
done in this study ascertained that resistance to mercury antibiotics, and toxic xenobiotics is mediated by
chromosomally borne genes and/or transposable elements rather than by plasmids.
Abstract: General contamination of heavy metals in the environment is a major global concern, which has provoked the emergence of phytoremediation technologies for cleaning aquatic environment. Heavy metals are released into the environment from a wide range of natural and anthropogenic sources. Macrophytes are known as good indicators of heavy metal contamination in aquatic ecosystems and they act as biological filters by accumulating heavy metals from the surrounding environments. Concentrations of heavy metals such as Hg, Cd, Co, Cu, Mo, Ni, Pb, Tl and Zn were measured in macrophytes and water samples from the mouth of five rivers namely; Gavaraget, Argichi, Makenis, Masrik each of them meeting the Lake Sevan, Armenia. The collected plants were Batrachium rionii, Myosotis palustris, Lythrum salicaria, Scrophularia alata, Calamagrostis epigeios, Lepidium latifolium, Glyceria plicata, Veronica anagallis-aquatica, Butomus umbellatus, Sparganium erectum. The highest concentration of Ni (5.5 mg/kg) was observed in Glyceria plicata whereas concentrations (mg/kg) of all other metals were highest (Hg, 0.02; Cd, 0.46; Co, 3; Cu, 18.9; Pb, 6.9; Tl, 0.13 and Zn, 113) in Batrachium rionii. Range and trend in concentrations of Co (<0.5µg/l), Cd (<0.5µg/l), Tl (<0.1µg/l) and Hg (<0.3µg/l) in water samples were similar at all the sites. Occurrence of heavy metals was much higher in macrophytes and water from Gavaraget and Masrik than that of the Argichi and Makenis due to the discharge of sewage into the river Gavaraget and industrial wastewaters into the river Masrik. The fact that the concentrations of different heavy metals in these macrophytes were far higher than in their respective water column indicates to their role in the biogeochemical cycles of heavy metals. This study aimed at understanding the importance of macrophytes in accumulation of heavy metals and suggesting remedial measures for the preservation and restoration of the lake ecosystem.
Abstract: The hypothesis put-forth for this study that mercury resistant bacteria (MRB) can tolerate, detoxify or biotransform a variety of other toxicants was examined. Experiments were carried out to understand the potential of a few MRB to tolerate and/or biotransform polychlorinated biphenyls (PCBs), tri-butyl tin (TBT), mercury, cadmium and lead to validate this hypothesis. Enumeration of MRB was carried out from water and sediments over a period of four years (1999-2002) and comparison was made with previous records (1993-1997). It was found out that there was sudden and unusual increase in MRB in the coastal environs of India. This information is useful not only to ascertain the recent changes the coastal environs in terms of Hg increases. Apparently, most bacterial community in our coasts and offshore is Hg resistant.
Many randomly chosen environmental strains of MRB tolerating  25 ppm mercury were characterized biochemically and through 16S rDNA sequencing. Alcaligenes faecalis, Bacillus sp., Pseudomonas and Brevibacterium iodinium were identified. Their genomic DNA was investigated for presence of mer operon and six of its structural genes and also for plasmids. Nine of them had merA (important in reducing Hg2+ to Hg0), four possessed merB. Whereas, merD was present in three isolates and all examined strains volatilized mercury. MerR was found in four isolates. It is also likely that mer gene arrangement in the marine strains has a different orientation as the universal mer oligomers did not amplify total mer from all 11 isolates examined through PCR during this study. The recognition of non-mer mediated mercury volatilization during this study is of great importance in recognizing detoxification mechanisms offered by several mercury-resistant marine bacteria. Absence of plasmids, as confirmed both by molecular analyses and by two indirect assays prompts to conclude that mercury resistance in marine bacteria is largely chromosome-mediated. All these findings are interesting and prompted research on their detoxification mechanisms.
Many of the chosen MRB strains grew in medium amended with Cd and Pb and a few were capable of removing >70% Cd and >98% Pb in 96 h from growth medium. As revealed by the SEM and EDS analyses, Pb was found entrapped in the exopolymeric substance (EPS) or, precipitated as lead sulfide. Bioremediation of mercury from wastewater in a laboratory scale bioreactor has been demonstrated efficiently by this investigation. Efficiency of these MRB in detoxifying the heavy metals like Hg, Cd, Pb and many other toxic substances helps in hypothesizing that these bacteria possess detoxification mechanisms, which are probably overlapping in their genetic nature sharing some common features.
One MRB strain CH07, a marine pseudomonad, degraded 14 chlorobiphenyls containing  6 chlorines from among different congeners of PCBs in Clophen A-50. Of these, CB-126 (coplanar congener) and CB-181 (sterically hindered) were completely degraded within 40 h. This invention was granted a US Patent (no. 6,544,773). It also degraded nearly 54% of the initial TBT. Efficiency of such MRB in degradation of TBT is quite promising and their potential in bioremediation of some very deleterious environmental toxicants including toxic heavy metals as well as xenobiotics is promising.
Bioremediation of mercury-containing ASN-III medium to promote growth of mercury-sensitive Phormidium sp. was a successful demonstration of detoxification efficiency of MRB. Furthermore, Hg-bioremediation of agricultural soil prior to sowing of a salt tolerant rice variety proves that MRB are useful in field applications. It appears that they release relatively less toxic gaseous mercury into the atmosphere. In principle, if a single strain can perform several metabolic activities, the efficiency and predictability of the process may be significantly enhanced. The marine pseudomonad strain, CH07 (deposited with accession number: NRRL B-30604) and many other strains isolated and identified during this study, prove their potential for bioremediation. Apparently, they are useful in treating contaminated sites in the open environments. It can be surmised that despite the alarming present scenario of chemical pollution, there is hope from these MRB possessing an array of armory for alleviating environmental health hazards.
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