Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 118

of mineral phases. Redox cycling of naturally occurring trace elements and their host minerals
often controls the release or sequestration of inorganic contaminants. Redox processes control
the chemical speciation, bioavailability, toxicity, and mobility of many major and trace elements
(Borch et al ., 2010). Anaerobic microorganisms can potentially use As(V) as an electron accep-
tor for the oxidation of organic matter, yielding energy to support their growth (McLaren and
Kim, 1995; Oremland and Stolz, 2003). Microorganisms, including bacteria, archaea and fungi,
display resistance to As(V) toxicity, which involves a common mechanism of resistance by the
reduction of intracellular As(V) to As(III) by As(V) reductase with As(III) being pumped out via
efflux pumps (Oremland and Stolz, 2003).
Microbial reduction of As(V) to As(III) has been proposed to contribute to As mobilization
(Tufano et al ., 2008a), although this may not be universally true (Campbell et al ., 2008). Several
biogeochemical processes can directly or indirectly lead to redox transformations of As. The
reduction of As(V) to As(III) under anaerobic conditions has been reported to be mediated by a
diverse populations of anaerobic microorganisms, including methanogens, fermentive bacteria,
and sulfate- and iron- reducers. This indicates that arsenate can act as terminal electron acceptor
for anaerobic respiration (dissimilatory arsenate reduction).
Known As(III) oxidizing bacterial strains are distributed in more than 20 genera and have been
isolated from various environments. They include both chemolithotrophs such as Acidicaldus sp.
and Acidithiobacillus sp. (Brierley and Brierley, 2001; D'Imperio et al ., 2007) and heterotrophs
such as Agrobacterium sp., Alcaligenes sp., Burkholderia sp., Thiomonas sp., Acinetobacter sp.
and Pseudomonas sp. (Cai et al ., 2009; Krumova et al ., 2008; Quéméneur et al ., 2008; Santini
and Vanden Hoven, 2004). As resistant bacteria are usually associated with its ability to reduce
As(V) to As(III) (Huang et al ., 2010). Study of aox gene transcripts (Quéméneur et al ., 2008) or
the Ars and Acr transporter genes transcripts would help for better understanding of the processes
involved in As(III) oxidation.
Microorganisms may further indirectly induce As(III) oxidation or As(V) reduction. In partic-
ular, they can produce reactive organic or inorganic compounds that subsequently undergo redox
reactions with As(V) or As(III). According to Messens et al . (2002), Staphylococcus aureus
pI258and Bacillus subtilis are expressing a thioredoxin-coupled arsenate reductase (ArsC). The
ArsC from E . coli plasmid R773 and ACR1, ACR2 and ACR3 were identified on chromosome
XVI of Saccharomyces cerevisiae . ACR1 encodes a transcription regulatory protein. Eukary-
otic ACR2p encodes the arsenate reductase. ACR3 encodes the ACR3p membrane transporter
that effluxes arsenite from the cells. ACR2p from Saccharomyces cerevisiae represent two dis-
tinct glutaredoxin-linked ArsC classes. All are small cytoplasmic redox enzymes that reduce
arsenate to arsenite by the sequential involvement of three different thiolate nucleophiles that
function as a redox cascade (Bobrowicz et al ., 1997; Ordonez et al ., 2009). In case of fungi,
Aspergillus sp. strain P37, an arsenate hyper-tolerant strain, isolated from the As polluted Rio
Tinto in South Western Spain showed that arsenate triggered an increase in the accumulation
of GSH. Such an accumulation could contribute to arsenate detoxification as GSH can serve
as the electron donor in enzyme-catalyzed arsenate reduction and GSH binds arsenite to form
As(GS) 3 , which allows sequestration in vacuoles mediated by an ATP binding cassette-type glu-
tathione conjugate transporters (Rosen, 2002). This indicates that Aspergillus sp. strain P37,
which maintained higher GSH levels and has higher arsenite efflux following arsenate exposure
in comparison to sensitive A . nidulans TS1, has enhanced arsenate reductase capacity (Cánovas,
2004).
6.3.2.2 Bioaccumulation and biosorption
Bioaccumulation of As mainly involves the biosorption of As by microbial biomass and its
byproducts; and physiological uptake of As by microorganisms through metabolic processes.
Microorganisms can take up As(V) via phosphate transporters (Zhao et al ., 2009), and then
reduce the As(V) internally to As(III), which is then either extruded from the cells or sequestered
in intracellular components (either as free As(III) or as conjugates with glutathione or other thiols)
(Mateos et al ., 2006). Takeuchi et al . (2007) reported that an As resistant isolate Marinomonas