Environmental Engineering Reference
In-Depth Information
mixtures present and the type of microorganisms
present as well as environmental conditions and
nutrient availability (Abdel-Sabour
2007
). For in-
stance, the microbes resistant to high Cr(VI) are
promising source for the Cr(VI) bioremediation.
Further, the use of plant growth-promoting rhi-
zobacteria is one of the inexpensive and environ-
ment-friendly ways to alleviate the Cr toxicity in
plants (Khan et al.
2012
,
2013
; Kang et al.
2012
).
Rhizobacteria are the root-colonizing bacteria
that exert beneficial effects on plant development
via direct or indirect mechanisms (Nelson
2004
)
and have potential to decrease the toxic effects of
heavy metals (Bertrand et al.
2000
). Rhizobac-
teria having 1-aminocyclopropane-1-carboxylate
deaminase (ACC deaminase) enzyme could im-
prove the plant growth under stress conditions
(Nadeem et al.
2006
). Harms of Cr on plants
could be minimized by rhizobacteria via differ-
ent mechanisms like biosorption and bioaccumu-
lation, bioreduction to a less toxic state, and chro-
mate efflux (Nazir et al.
2011
; Khan et al.
2012
).
Cr(VI)-resistant rhizobacterial isolates might
cause changes in plant growth and development
due to involvement of single or multiple pos-
sible mechanisms of action, i.e., solubilization
of insoluble phosphate (Yasmin and Bano
2011
);
production of siderophore (Meyer
2000
); produc-
tion of phytohormones (Humphry et al.
2007
);
indirect mechanisms of action, i.e., reduction of
Cr(VI) to Cr(III) by which it decreases the harm-
ful effects of Cr(VI) to the plants (Salunkhe et al.
1998
); biocontrol (Chandra et al.
2007
); or induc-
tion of systemic resistance in plants against phy-
totoxicity of Cr(VI) (Mishra et al.
2006
). Turick
et al. (
1996
) investigated several bacteria from
various soils for Cr(VI) resistance and reducing
potential. Microbes selected from both Cr(VI)-
contaminated and Cr(VI)-noncontaminated soils
and sediments were capable of catalyzing the
reduction of Cr(VI) to Cr(III) a less toxic, less
water-soluble form of Cr. Cr reduction capac-
ity of these isolates was compared with that of
Pseudomonas aeruginosa
and
Bacillus circulans
.
Bacillus coagulans
, isolated and identified from
Cr-polluted soil, gave maximum reduction poten-
tial among all organisms studied. Morales et al.
(
2007
) isolated
Streptomyces
sp. CG252, which
was highly tolerant to Cr(VI) and has the ability
to reduce Cr(VI) into Cr(III). Similarly, Mistry
et al. (
2009
) reported that Cr-resistant bacterial
strain
Pseudomonas olevorans
had the ability to
reduce the Cr(VI) into Cr(III) and to bioreme-
diate Cr(VI)-containing waste. Recently, Datta
et al. (
2011
) reported Cr(VI) tolerance capability
of different varieties of wheat and several other
studies showed that rhizosphere bacteria stimu-
late plant growth and development under stress
conditions. Kumar et al. (
2009
) suggested that
plant growth-promoting bacteria (
Enterobacter
aerogenes
and
Rahnella aquatilis
) reduce the
toxicity of Ni and Cr in
B. juncea
(Indian mus-
tard) and promoted plant growth.
6.6
Phytoremediation of Cr Using
Potential Plants
A new research area of using plants for the bio-
remediation (phytoremediation) of contaminated
soil and water was reviewed by Brown (
1995
).
Reduction of heavy metals in situ by plants may
be a useful detoxification mechanism for phy-
toremediation. The key role is played by plant
roots, and they are a significant metal sink. Metal
uptake by plants can be passive, facilitated, or
active. The regulation of metal uptake by both
soil-root and root-shoot interfaces varies within
plant species and cultivars. Plants are effective
at removing metals because they require certain
trace elements to survive. Some plant species,
known as hyperaccumulate toxic metal and they
accumulate upto 5 % of their dry weight and as
on to date about 400 plants that hyperaccumulate
metals are reported. One of the earliest examples
of a hyperaccumulator was the Italian serpentine
plant
Alyssum bertolonii
and another more re-
cently identified is the Alpine pennycress
Thlaspi
caerulescens
. Abdel-Sabour et al. (
2002
) studied
the use of hyperaccumulator plant species to ex-
tract Cr from contaminated soils. They investi-
gated three soils (A, B, and C) and four plant spe-
cies, i.e., sorghum (
Sorghum vulgare
L.), clover
(
Trifolium pratense
L.), panikum (
Panicum an-
tidotal
), and canola (
Brassica napus
), and con-
cluded that canola accumulated the highest Cr
Search WWH ::
Custom Search