Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 116

such as As(V) reduction and As(III) oxidation can impact As abundance and speciation as well.
Arsenite oxidation by microorganisms is potentially significant process, which depends upon bio-
catalytic activities of microorganisms (Gihring and Banfield, 2001; Gihring et al ., 2001). Some
microbial strains possess genetic determinants that confer resistance to As toxicity (Turpeinen
et al ., 2000; Weeger et al ., 1999). Many bacteria are known for their ability to transform
i-As species by redox reaction (Simeonova et al ., 2004). The bacterial oxidation of As(III) to
As(V) represents a detoxification mechanism with its potential scope in bioremediation because
it generates less toxic and less mobile forms of As (Oremland, 2002).
Speciation of As has been studied using various microbes, ( Staphylococcus aureus , Bacil-
lus subtilis and Escherichia coli ) (Tauriainen et al ., 1997; Turpeinen et al ., 1999; 2002). It is
known that As(V) is subjected to microbial reduction and methylation leading to volatilization
as arsines (Alexander, 1977; Gao and Barau, 1997). However, the reduction and/or methylation
rates of As, which are necessary pre-requisites for production of arsine, vary greatly depend-
ing on the properties of the matrix, such as temperature, different species of As, and microbial
populations. Macy et al . (2000) reported Desulfomicrobium sp. BenR-B showing reduction of
As(V) to As(III) via enzyme arsenate reductase. The mechanisms involved in the microbial trans-
formation and removal of As from the contaminated matrix included adsorption via reduction
reaction ( Desulfomicrobium sp. BenR-B), oxidation/reduction reaction ( Trichoderma harzianum
AS11 and Trichosporon mucoides SBUG801), and methylation reaction ( Paenibacillus sp. and
Pseudomonas sp.) (Macy et al ., 2000). Fungi methylate As for detoxification and are producing
MMA or DMA. Granchinho et al . (2002) reported that the fungus Fusarium oxysporum melonis ,
isolated from the alga Fucus gardneri , is capable of accumulating As(V) from the surrounding
medium and transforming it into As(III) and further methylated species. The predominant As
species found in the MeOH-H 2 O extracts of the fungus after As(V) exposure were As(III), DMA
species at concentrations about three times less than found in the exposed fungus, i.e., 500 ppb.
Both the Archaeal and Bacterial Domains can also produce volatile methylated arsines, which
remove As from contaminated matrix by forming a gaseous compound. An alternative strat-
egy used by bacteria and yeast ( Saccharomyces cerevisiae ) is based upon arsenate reductase
“arsC operon/genes”. Earlier, Challenger (1945) revealed that trimethylarsine was produced by
Scopulariopsis brevicaulus , from As(III). Subsequent work confirmed that other fungi such as
Penicillium sp., Gliocladium roseum and the yeast Candida humicola were also capable of As
biomethylation (Andreae, 1986; Challenger, 1978; Thayer, 1984). Bacteria, including Aeromonas
sp., Flavobacterium sp. and Escherichia coli can produce a variety of methylated As derivatives
including dimethyl- and trimethylarsine. Turpeinen et al . (2004) observed a positive relationship
between the bioavailability of As and the proportion of As(III) in As resistant species of Acineto-
bacter , Edwardsiella , Enterobacter , Pseudomonas , Salmonella and Serratia , which are classified
into Betaproteobacteria or Gammaproteobacteria.
Speciation of As in soil has become an important issue due to the different toxicity levels
of i-As(V) and As(III) species and gaseous species such as arsine, MMA, DMA and TMA.
Because microbial processes play a major role in controlling the speciation of As in soil, it is
important to study how microbial activities affect As biogeochemistry. It is important to under-
stand microbially-mediated mechanisms involved in bioremediation of As-contaminated soils and
associated risks.

6.3 MITIGATION OF AS CONTAMINATION IN SOIL:

MICROBIAL APPROACHES AND MECHANISMS

Bioremediation of As contaminated soils and water shows a great potential for cost effective
environmental improvement. Several microbes-mediated processes like reduction, mobilization,
immobilization through sorption, biomethylation, complexation, and oxidation-reduction can
be used for bioremediation of different As species in soils. These processes basically involve
organic carbon, Fe, Mn and S, affecting As mobility. As stated earlier, microorganisms have