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7 Use of Wetland Plants in Bioaccumulation of Heavy Metals 127

concentrations of Se and Hg when they were treated with antibiotics than their
normal counterparts (de Souza et al. 1999). Similarly, mycorrhizae (symbiotic fungi
associated with roots), by increasing the absorptive surface area of root hairs, assist
plant either assimilating metals (Meharg and Cairney 2000) or protect plants by
restricting the uptake of metals by immobilizing them (Khan et al. 2000). Thus
periphyton sometimes associated with freshwater wetland plants (as for example,
Phragmites australis) help in enhancement and the ability to accumulate and retain
metals (Lakatos et al. 1999).
Microbial community plays a major role in phytoremediation of wetland plants.
Community diversity and structure of microorganisms, their enzymatic activity,
and microbial-mediated edaphic processes (C and N mineralization, decomposi-
tion) mostly depend upon metal(s) concentration(s) of the root zone of wetland
plants (Baath 1989; Roane and Kellogg 1996; Bruins et al. 2000) that also help
plants to develop mechanisms to ameliorate toxicity of metals and to tolerate and/or
resist multiple metal sequestration in a complex polluted environment (Nies 1995,
1999; Giller et al. 1998; Bruins et al. 2000; Pal et al. 2004). However, metal
concentration plays a critical role in alteration in species composition, density,
and biomass reduction of microorganisms (Baath 1989; Chander and Brookes 1993;
Chander et al. 2001; Baath et al. 2005). It is reported that metals like Cd, Cr, Mo,
Ni, Pb, and Zn shift the bacterial community with increase in the diversity of Gram
positive bacteria with members from Proteobacteria, Acidobacteria,
Verrucomicrobia, and Chlorobi groups in serpentine soils (Mengoni et al. 2004;
Akerblom et al. 2007). However, few bacterial groups remain unchanged to certain
metals with higher concentrations. As for example, actinobacterial community
diversity remained unaffected with additional inputs of Pb and Zn in a Pb/Zn-
contaminated grassland soil, though community diversity became reduced
(Bamborough and Cummings 2009).
Interestingly, many hyperaccumulators used to follow definite strategy to amass
specific bacteria resistant to particular metal(s) around their roots. Plants like
Alyssum bertolonii, A. serpyllifolium subsp. lusitanicum, Sebertia acuminata,or
Thlaspi caerulescens subsp. calaminaria have been shown to host higher
proportions of Cd-, Ni-, or Zn-resistant bacteria in the rhizosphere compared to
non-hyperaccumulating plants or non-vegetated soil (Schlegel et al. 1994;
Delorme et al. 2001; Mengoni et al. 2001; Lodewyckx et al. 2002; Becerra-castro
et al. 2009). These plants gradually develop resistance to a set of metals. Likewise,
higher proportions of different Ni-tolerant bacteria were found in the rhizosphere
of Alyssum serpyllifolium subsp. lusitanicum when the plants are exposed to high
Ni concentrations (Becerra-castro et al. 2009). A synergistic effect between plant
roots and their associated bacteria is thus evident. Production of metabolites by
bacteria is augmented by the indirect supply of necessary substrates in the root
exudates provided by plants. On the other hand, bacteria at the root zone (plant
growth promoting rhizobacteria, PGPR) may help in the production of
phytohormones (such as indoleacetic acid (IAA), cytokinins, and ethylene) (Kidd
et al. 2009). Further, development, physiology, and exudation of root are also
stimulated by the weathering agents that improves nutrient uptake by plants