Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 124

Table 6.4. Continued.

Augmented microbes Plants Reference

Acaulospora morrowiae Zea mays L. Wang et al . (2007)

Gigaspora margarita Pteris vittata L. Trotta et al . (2006)

Glomus caledonium ,
Glomus intraradices

Pteris vittata L. Chen et al . (2006)

Glomus sp. Holcus lanatus L. Gonzalez-Chavez et al . (2002)

Hymenoscyphus ericae Calluna vulgaris L. Hull. Sharples et al . (2000)

FUNGI
Aspergillus niger ,
Aspergillus nidulans

Soil Mukherjee et al . (2010)

Maheswari and Murugesan (2009)

Trichoderma harzianum ,
Trametes versicolor

Eucalyptus globulus Labill. Arriagada et al . (2009)

Ulocladium sp.,
Penicillium sp.

Soil Edvantoro et al . (2004)

phase, and this could potentially solubilize and mobilize As held within or sorbed on the sur-
face of the iron oxides (Benner et al ., 2002; Cummings et al ., 1999; Rowland et al ., 2007).
However, some authors (Kocar et al ., 2006; Tufano et al ., 2008b) have observed that the Fe(III)
reduction is also likely to form secondary iron phases at root surfaces of plants which are hav-
ing a potential to absorb As. Wang and Zhou (2009) indicated that both microbial and chemical
reductions of iron plaque caused slow As release from iron plaque to aqueous phases. However,
microbial iron reduction induced the formation of more crystalline iron minerals, leading to As
sequestration. The concentration of As in iron plaque was 170 mg kg − 1 as compared to the soil As
(42 mg kg − 1 ) demonstrating that the iron plaque formed naturally on the rice root surface could
accumulate As.

6.4.8 Microbes-As interaction
Tolerance and adaptation of microorganisms to As are common phenomena. Presence of tolerant
fungi and bacteria in As rich environment have frequently been observed (Butt and Rehmann,
2011; Cavalca et al ., 2010; Srivastava et al ., 2011; Srivastava et al ., 2010; Su et al ., 2010b;
Xu et al ., 2008). The increased abundance of tolerant microbes can be due to genetic changes,
physiological adaptations involving no alterations in the genotype or replacement of As sensitive
species with microbial species that already tolerant to As.
Metal resistance in bacteria is often encoded by genes located on plasmids. Bacteria develop
As resistance mechanisms through the Ars operon genes (Kaur et al ., 2011; Muckopadhyay et al .,
2002; Oremland et al ., 2005; Silver and Phung, 2005). As stated earlier, typical Ars operon
contains either ( Ars RBC) or five ( Ars RDABC) genes that generally transcribe as a single unit
(Rosen, 1999). Ars R is a repressor that binds the promoter region and regulates the Ars operon.
Ars B is a membrane-located transport protein that can pump As(III) out of cells using proton
motive force. Ars C was shown to be a cytoplasmic As(V) reductase, whereas Ars A is an As(III)
activated ATPase (Zhou et al ., 2000).
As(III) is generally considered more toxic that As(V) for most of the plants. However, plants
vary in their sensitivity or resistance to As (Meharg and Hartley-Whitaker, 2002). Rhizobacteria
encounter As(V) and As(III) in soil solutions before they enter the root and oxidizing As(III)
from soil solutions might help the plant to grow on As-contaminated soils and thus lower the
As(III) toxicity. The presence of numerous As-resistant bacteria with plant growth promoting