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Hg 2 + ), E. coli, Sphingomonas desiccabilis, Bacillus idriensis (its ArsM gene for volatilization
of As) etc. have genes that would help in bioremediation of specific metals (Valls et al ., 2000;
Schue et al ., 2009; Liu et al ., 2011).
Bioremediation of As from contaminated sites by using GE bacteria could be an option to
transform arsenic to its non-toxic forms (Valls and De Lorenzo, 2002). Expression of phy-
tochelatin (PC) synthase of Arabidopsis thaliana (ATPCS) in E . coli cells (Sauge-Merle et al .,
2003) increased the accumulation of intracellular arsenic by 50-fold by sequestering arsenite
in a non-toxic form and competing with the arsenic efflux transporter. In another scenario,
E . coli expressing arsenite S-adenosylmethionine methyltransferase gene (arsM ) cloned from
( Rhodopseudomonas pulustris ) has been found to methylate toxic i-As to less-toxic volatile
trimethylarsine (Cullen and Bentley, 2005; Qin et al ., 2006; 2009). Recently Liu et al . (2011)
demonstrated that As could be removed through volatilization by GE bacteria, showing arsM
genes. They overexpressed arsM genes in Sphingomonas desiccabilis and Bacillus idreinsis ,
which resulted in 10-fold increase in methylated As release compared to their wild types. The
transcriptional regulator arsR present in the bacterial ars operon can be considered an As-specific
metallothionein like protein endowed with an unambiguous binding site for arsenite (Paez-Espino
et al ., 2009). These results open the possibility of designing As-specific bioadsorbants by merg-
ing dedicated transport systems with naturally occurring or evolved intracellular As-binding
polypeptides.
6.5.5 Enhancement of bioremediation by use of surfactants
Bio-surfactants are microbial compounds that exhibit high surface and emulsifying activity. The
major classes of bio-surfactants include glycolipids, lipopeptides and lipoproteins, phospholipids,
fatty acids, polymeric surfactants, and particulate surfactants. Bio-surfactants production, struc-
ture and their various natural and industrial uses and their role in pollutant removal have been
extensively reviewed (Banat, 1995; Banat et al ., 2000; Cameotra and Bollag, 2003; Eliora and
Rosenberg, 2001; Singh and Cameotra, 2004). The efficiency of bio-surfactants for stimulat-
ing As detoxification/removal is uncertain due to specificity observed between biosurfactant and
microorganisms. A strategy suitable for effective As-remediation would be to stimulate biosurfac-
tants produced by indigenous microbial population or use of commercial bio-surfactants produced
by biological organisms found to be already present at the contaminated site (Singh and Cameotra,
2004).
6.5.6 Priming and encapsulation
The major concern in bioremediation is the survival along with the soil colonization of the
microbial inoculants consisting of pure cultures or consortia. Bioremediation very often fails
because of the low survival rate of the inoculated microorganisms. The capacity of microorganisms
to proliferate in the soil after bioaugmentation is important as their performance, activities and
persistence are related to the microorganisms characteristics and inoculation procedure (Thomson
et al ., 2005). For the success of bioaugmentation, the previous conditioning of the microorganisms
before inoculation needs improvement. One method is 'priming' or 'activated soil' strategy. This
consists of pre-inoculation of the microbial inoculums in sterilized soil and to be incubated for
7-15 days before bioaugmentation (Gentry et al ., 2004). The advantages of this method are:
(i) pool of useful complementary microorganisms, (ii) pool of cultivable and non-cultivable
microorganisms, (iii) no steps of extraction and culture of microbes and (iv) a better microbial
survival rate since soil serves as a carrier for microbes.
Another method is immobilizing microorganisms into carriers or 'encapsulation'. For exam-
ple, alginate, clay, peat etc., protect microbes against biotic (Cassidy et al ., 1996; Da Silva and
Alvarez, 2010; Gentry et al ., 2004; McLoughlin, 1994; Tyagi et al ., 2011) and abiotic environ-
mental stress such as toxicity of metals (Braud et al ., 2007). Alginate encapsulated plant growth
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