Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 114

The strategies for mitigation of As contamination related to agricultural activity along with scope
of its future application are also highlighted.

6.2 INFLUENCE OF MICROBES ON THE SPECIATION

AND BIOAVAILABILITY OF ARSENIC

6.2.1 Arsenic speciation
As is normally not associated with life, however it has been observed that various microorganisms
gain energy for their growth from this element (Oremland and Stolz, 2003). These organisms
play an important role in As speciation ( Tables 6.2 and 6.3 ). For example, aqueous As in the

3
oxidation state, As(III), can be oxidized to As(V) by chemoautotrophic arsenite-oxidizing bacteria
(Oremland and Stolz, 2005). There are also heterotrophic arsenite oxidizers that need organic
carbon as energy source. Microbes use As(V) as an electron acceptor in anaerobic respiration,
producing of As(III). i-As species can also be methylated to monomethyl As (MMA), dimethyl As
(DMA) and trimethyl arsine (TMA) oxide (Cullen and Reimer, 1989). DMA can be transformed
by microorganisms via two pathways: (i) reductive conversion to volatile organo-arsine species
(e.g., dimethyl- or trimethyl arsine) and emissions from the soil system; and (ii) demethylation
to produce the end products CO 2 and As(V); the first pathway predominates under anaerobic
conditions, whereas both pathways occur in aerobic soil (Woolson and Kearney, 1973; Yoshinaga
et al ., 2011). The rate of detoxification and the relative importance of the two pathways vary
among different studies, probably due to impact of different soil properties (pH, water logging,
site hydrology, behavior of soil colloids), microbial communities and environmental conditions
(Sadiq, 1997).

+

Table 6.2. Bacterial classes investigated for environmental remediation of arsenic contamination.

Bacterial class Mechanism Reference

Gammaproteobacteria Oxidizes arsenite to arsenate Butt and Rehman (2011), Nagvenkar

and Ramaiah (2010), Srivastava et al .
(2010), Chitpirom et al . (2009),
Aksornchu et al . (2008), Saltikov and
Olson (2002)

Actinobacteria Removal and transformation Nagvenkar and Ramaiah (2010)

Proteobacteria Oxidizes arsenite to arsenate,

Arsenic resistant

Chitpirom et al . (2009), Macur et al .
(2004), Caudill (2003)

Bacilli Biosorption, Reduces arsenate

to arsenite, Biomethylation

Aksornchu et al . (2008), Yamamura
et al . (2007), Jenkins et al . (2003)

Betaproteobacteria Biosorption, reduces arsenate to

arsenite

Aksornchu et al . (2008), Santini and
Vanden Hoven (2004)

Aeromonasbacteria Arsenic resistant Pepi et al . (2007)

Corynebacteria Arsenic tolerant Chang et al . (2008)

Flavobacteria Arsenic resistant Macur et al . (2004)

Alphaproteobacteria Reduces arsenate to arsenite Santini and Vanden Hoven (2004),

Macur et al . (2001)

Deltaproteobacteria Reduces arsenate to arsenite Lloyd and Oremland (2006), Michalke

et al . (2000), Macy et al . (2000)

Methanobacteria, Clostridia As methylation and demethylation Michalke et al . (2000)

Thermos Reduces arsenate to arsenite,

Oxidizes arsenite to arsenate

Gihring and Banfield (2001), Gihring
et al . (2001)

Desulfitobacterium Reduces arsenate to arsenite Niggemeyer et al . (2001)