Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 120

In microbes, a chromosomally encoded efflux system transports As using either a single-
polypeptide system ( ArsB ) or a two-component system ( ArsA and ArsB ), which functions as a
chemiosmotic transporter (Lee et al ., 2001; Silver et al ., 1982). The ArsB protein is a membrane
protein that functions alone as a chemiosmotic As(III) transport protein (Silver, 1996), whereas
the ArsA gene encodes a unique ATPase that binds to ArsB (Wu et al ., 1992). The ArsAB pump is
composed of six transmembrane segments and a catalytic subunit that functions in the reduction
mechanism of the ArsA ATPase (Bruhn et al ., 1996). Efflux mechanism has been reported in
Pseudomonas putida and Escherichia coli (Chang et al ., 2007).
Mycorrhizal fungus Hymenoscyphus ericae demonstrated an enhanced As efflux mechanism
in comparison with nonresistant Hymenoscyphus ericae and lost approximately 90% of preloaded
cellular As(III) (Sharpels et al ., 2000). The archetypical yeast Saccharomyces cerevisiae lacks
functional phytochelatins, and its As tolerance depends upon the efflux of As across the plasma
membrane and on the vacuolar sequestration of As(GS) 3 (Ghosh et al ., 1999).

6.3.2.4 Biomethylation and biovolatilization
As undergoes biomethylation to form non-volatile monomethylarsonic acid (MMAA), dimethy-
larsenic acid (DMAA), volatile trimethylarsine oxide (TMAO) and trimethylarsine [TMA(III)]
(Bentley and Chasteen, 2002; Zeng et al ., 2010). The expression of As methyl transferase
( ArsM) in bacteria Rhodopseudomonas palustris has been found to increase As tolerance in
E. coli strain AW 3110, in which the Ars RBC operon was detected (Qin et al ., 2006). This
ArsM was the first arsenite- S -adenosylmethyltransferase identified in bacteria. It mediates
bacterial As resistance by catalyzing the formation of dimethylarsenate [DMAs(V)], trimethyl
arsine oxide [TMAs(V)O] and trimethylarsine [TMAs(III)] gas that can leave the cell due
to its volatility (Cullen and Bentley, 2005; Qin et al ., 2006; Yuan et al ., 2008). Meng
et al . (2011) constructed transgenic rice with the ArsM gene from R. palustris and demon-
strated that the resulting transgenic rice plant acquired the capability of volatilization of As.
This is the first report of in planta methylation and volatilization. Two other As methyl
transferase gene (termed CmarsM7 and CmarsM8 ) were cloned and characterized from the uni-
cellular eukaryotic red alga Cyanidroschyzon from the Yellowstone National Park, USA (Qin
et al ., 2009).
The conversion of As(V) to volatile methylarsines was described in a pure culture of a
methanogen Methanobacterium bryantii (McBride and Wolfe, 1971). Recently, several pure
cultures of anaerobes, including a methanogen ( M . formicicum ), a fermentative bacterium
( Clostridium collagenovorans ) and sulfate-reducing bacteria ( Desulfovibrio vulgaris and D .
gigas ), were also capable to form methylarsines (Michalke et al ., 2000). As(V) can be con-
verted to monomethylarsine and dimmethylarsine by Achromobacter sp. and Enterobacter sp.,
and to monomethylarsine, dimethylarsine and trimethylarsine by Aeromonas sp. and Nocardia
sp. (Cullen and Reimer, 1989).
Fungi are also able to transform inorganic and org-As compounds into volatile methylarsines
(Tamaki and Franekanberger, 1992). Some yeast and other fungi such as Candida humicola , Gli-
cladium roseum and Penicillium sp., are capable of converting MMA and DMA to TMAO (Cox
and Alexander, 1973). Effective biovolatalization ( 23% of As was volatilized from all culture

media) of As was observed in the heat-resistant Neosartorya fischeri strain, while transforma-
tion of As to volatile derivates was approximately two times lower than the non-heat-resistant
Aspergillus niger strain (Cernansky et al ., 2007). The order of ability of As biovolatalization was
observed in the same study as Neosartorya fischeri >A . clavatus >A . niger (Cernansky et al .,
2009). As resistant fungi Pennicillum janthinellum , Trichoderma asperellum and Fusarium oxys-
porum also accumulate and volatilize As (ranging from 100 to 304.06

∼

g) from culture medium
(Su et al ., 2010). As biovolatilization has also been reported in Trichoderma sp., Rhizopus sp.,
Penicillum sp., and Aspergillus sp. within a range of 3-29% (Srivastava et al ., 2011). The fungi-
mediated biovolatilization process in agriculture soils may lead to reduction of As load in those
contaminated soils.

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