Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

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The Use of Rhizospheric Bacteria to Enhance Metal Ion Uptake
by Water Hyacinth, Eichhornia crassipe (Mart)
Lai M. So, Alex T. Chow, Kin H. Wong, and Po K. Wong
15.1
Introduction
Contamination of the aquatic environment by toxic metal ions is a serious problem
worldwide [1–7]. Unlike organic pollutants, toxic metal ions cannot be degraded by
chemical or biological process. To remediate the aquatic environment, toxic metal
ions should therefore be concentrated in a form that can be extracted conveniently,
possibly for reuse or at least for proper disposal. To achieve cost-effectiveness, toxic
metal ions should be concentrated into a small mass. Conventional treatment
methods, such as chemical precipitation, chemical oxidation or reduction, ion exchange,
filtration, membrane technologies or evaporation process, are generally not
effective at low metal concentration and are expensive [6,8]. Biological methods,
such as using microbial biomass to remove metal ions, are well documented [9–18];
however, the performance of the microbe-based system is very sensitive to external
physical and chemical factors and the operation costs incurred in cell culture and
separation from effluent, cell immobilization and technical support [13] is quite
high. Phytoremediation provides a viable alternative for metal ion removal with the
merits of hyperaccumulating metal ions using low energy and at low cost.
In the plant rhizosphere, bacteria interact with roots to form mucigel, composed
of plant mucilage and bacterial cells, at the root surface [19–25]. The interaction
creates a rhizospheric effect [19,22,25–27]. The microorganisms and their products
interact with plant roots influencing plant growth and development, as well as
change nutrient dynamics and susceptibility to disease and abiotic stress [25,28].
However, the role of rhizospheric bacteria in metal ion accumulation and tolerance
of plants is not well known. Screening for metal-ion-resistant and/or accumulating
bacteria from the rhizosphere and utilizing the natural colonizing ability of bacteria
may increase the efficiency of plants to remove metal ion using the root surface for
adhesion of metal ion removing bacteria. This principle is similar to application of
biotechnology of immobilized cells on solid surface for metal ion removal [29,30].
This chapter attempts to combine these two technologies on water hyacinth, Eichhornia
crassipe (Mart), a common aquatic plant used in phytoremediation.
Plant-Bacteria Interactions. Strategies and Techniques to Promote Plant Growth
Edited by Iqbal Ahmad, John Pichtel, and Shamsul Hayat
Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31901-5
j283