Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 126

P . janthinellum SM-12F4 according to the quantitative analysis of As speciation in cultures. This
shows the widespread ability of diverse microbes to act on As and further its detoxification.
6.5.2 Identification and manipulation of a functionally active microbial population
The study of metal resistance in microbes has led to the discovery of many metal specific genetic
models. In addition, molecular approaches are more frequently being used to study microbial
communities in metal contaminated environments. Advantages of using molecular approaches
include enhanced assessment of specific microbial populations in contaminated environments
(Rochelle et al ., 1991) and increased sensitivity in monitoring specific microbial populations dur-
ing remediation efforts (Mateos et al ., 2006). There are different mechanisms involved amongst
the microbes for As tolerance and removal. These conversion mechanisms are mediated by the
genetic determinants mostly associated with plasmids (Mobley and Rosen, 1982). Having estab-
lished the plasmid mediated nature of As resistance and detoxification, an analysis of the genetic
diversity of the ars determinants would provide a better understanding of the adaptive responses
of the particular microbe to As. Several approaches to investigate the distribution of divergence of
ars determinants in natural environments have been used. Baker and Brooks (1989) used a probe
to detect homologous determinants by DNA-DNA hybridization, followed by more extensive
hybridization studies using a series of ars probes against DNA isolated from bacteria.
6.5.3 Specific functions of microbes
Microbial transformation of As has great implication on the chemical cycling in the environment,
because different forms of As vary in solubility, mobility, bioavailability and toxicity (Smedley and
Kinniburgh, 2002). Microbes affect As reduction and oxidation, methylation and demethylation
and sorption and desorption in soils. As a result, microbes have developed different detoxification
strategies to withstand the growth restriction under As stress. Microbial As(V) reducing mecha-
nisms can be classified into two types: detoxification and dissimilation. A variety of bacteria are
reported to reduce As(V) to As(III). Occasionally, bacteria take a large quantity of As(V) during
the process of phosphate uptake and in order to remove As(V) toxicity, they convert As(V) to
As(III), which has a high mobility and is expelled out of the cells (Rosen, 2002). Such microbial
detoxification of As occurs under both aerobic and anaerobic conditions, but is not coupled to the
energy generation. Although, some anaerobes can gain energy to support growth and cell function
by coupling As(V) reduction to oxidation of soil organic matter. Oremland and Stolz (2003) had
reported 16 As(V)-respiring prokaryotes. Later, Stolz et al . (2010) isolated more than 50 phylo-
genetically diverse As(III)-oxidizing strains distributed among more than 13 genera from various
environments. These include chemolithoautotrophic As(III) oxidizers, which use the energy and
reducing power from As(III) oxidation during CO 2 fixation and cell growth under both aerobic
(Santini et al ., 2000) and nitrate reducing conditions (Oremland et al ., 2002; Rhine et al ., 2006).
Bacteria have also shown to increase the As mobility in nature through reductive dissolution
of Fe(III)-oxides (Grantham et al ., 1997; Lee et al ., 2009; Lovley, 1993). Lovley and Coates
(1997) suggested that Fe-reducing bacteria dissolve Fe(III) to aqueous Fe(II) and subsequently
As associated with Fe minerals was leached out in anaerobic environments.
6.5.4 Genetically engineered (GE) bacteria
Application of GE bacteria is now widely suggested for remediation of various sites contaminated
with metal pollutants. A combination of microbiological and ecological knowledge, biochemical
mechanisms and field engineering designs are essential elements for successful bioremediation
of metal contaminated sites using engineered bacteria. The GE bacteria have higher detoxification
capacity and have been demonstrated successfully for the degradation of various pollutants under
defined conditions (Barac et al ., 2004). Various bacterial strains of Ralstonia eutropha (its mtb
gene for remediation of Cd 2 + ), Mycobacterium marimum (its MerH gene for remediation of