Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 119

communis was capable of removing 2290 mg kg − 1 As from culture medium amended with 5 mg
As(V) L − 1 by accumulation in their cells. As(III) in water is an inorganic equivalent of non-ionized
glycerols and can be transported across cell membranes by glyceroporin channel proteins (Rosen,
2002; Ma et al ., 2008).
The intracellular uptake of metal ions from a substrate into living cells may lead to the biological
removal of metals by fungi. Su et al . (2010) reported that fungi ( Penicillium janthinellum , Fusar-
ium oxysporum and Trichoderma asperellum ) bioaccumulate As(V) under laboratory conditions.
Trichoderma asperellum and Fusarium oxysporum showed superior abilities for the absorption of
extracellular As and accumulation of intracellular As, which accounted for 82.2 and 63.4% of the
total accumulated arsenic, respectively. In contrast, Penicillium janthinellum presented an equal
distribution of intracellular and extracellular As. According to Adeyemi (2009), three filamen-
tous fungi ( Aspergillus niger , Serpula himantioides and Tremetes versicolor ) were investigated
for their potential abilities to bioaccumulate As. Accumulation of As in the fungal biomass was
observed in the order of Tremetes versicolor > Serpula himantioides >Aspergillus niger .
Biosorption is the passive sequestration of metals and metalloids by live or dead biological
mass, which is at present the most practical and widely used approach for the bioremediation of
metals, and metalloids. Biosorption is effective in treating water and wastewater (Schiewer and
Volesky, 2000), but its potential in soil is less attempted (Ledin, 2000). Ion exchange, adsorption,
microprecipitation, and electrostatic and hydrophobic interactions facilitate biosorption. Whereas
mechanisms of metal binding by individual cellular organelles and chemical moieties are known
(Schiewer and Volesky, 2000), sorption of metals to intact cells or microbial biomass is governed
by a multiplicity of mechanisms and interactions and thus not always fully understood. Langley and
Beveridge (1999) attempted to understand the role of carboxyls in the binding of metal cations
to O -side chains of lipopolysaccharide (LPS) and concluded that metals bound most likely to
phosphoryl groups in the LPS. This negatively charged side chains influence binding of metals
to Gram-negative bacteria by affecting cell hydrophobicity. The kinetics of metal binding onto
fungal biomass depends upon heterogeneous non-equivalent interactions, multiplicity of binding
sites, charge types, accessibilities and the properties of bound metals (Gadd, 2000).
According to Volesky and Holan (1995), biosorption includes several mechanisms such as
ion exchange, chelation, adsorption and diffusion through cell walls and membranes. These
mechanisms may differ depending upon the fungal species used, the origin and processing of
the biomass and solution chemistry (Ceribasi and Yetis, 2001). The metal biosorption abilities of
different fungi ( Rhizopus nigricans and Mucor rouxii ) have been previously reported (Bai and
Abraham, 2003; Yan and Viraraghavan, 2003). Biosorption of toxic metals is based upon ionic
species associating with the fungal cell surface such as extracellular polysaccharide, proteins and
chitins of fungi (Zafar et al ., 2007). Metal sorption activity of fungal cells depends on structural
cell wall polysaccharides and chitin:glucan ratio (Tereshina et al ., 1999). Many fungi contain
chitin and chitosan as integral parts of their cell wall structure and these are effective biosorbents
for metals (Gadd, 2004). The deacetylated amino group of glucosamine of chitosan may act as
binding sites for metals (Pillichshammer et al ., 1995). Zhou (1999) also revealed that in fungus
Rhizopus arrhizus metal biosorption occurred due to wall chitin and chitosan. The biomass of
the fungus Penicillium purpurogenum has been found effective in biosorption of As (Say et al .,
2003). The maximum adsorption capacities of heavy metal ions onto the fungal biomass under
noncompetitive conditions were 35.6 mg g − 1 for As(III), 70.4 mg g − 1 for Hg(II), 110.4 mg g − 1 for
Cd(II) and 252.8 mg g − 1 for Pb(II).

6.3.2.3 Efflux
Different biological systems have evolved diverse strategies to tolerate the toxic effect of As.
Presumably, microorganisms bear at least one arsenate reductase and a membrane bound pump
complex for the efflux of As(III) back to the external medium. It is perhaps an indication of
the early evolutionary success of such a mechanism during the rise of an oxidizing atmosphere
(Canovas et al ., 2004).