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

294j 15 The Use of Rhizospheric
humans [87]. However, numerous purification steps are needed to prepare water
hyacinth for human consumption. Water hyacinth is also a good source for the
production of biogas. One kilogram of dried water hyacinth can yield 374 l of biogas
containing 60–80% methane with a fuel value of 21 000 BTU m 3 , as shown by
experiments at the National Space Technology Laboratory, USA [87]. The sludge
remaining after the production of biogas can also be used as a fertilizer and soil
conditioner [55,87]. The fiber composition of water hyacinth is chemically and
physically similar to sugarcane bagasse [87]. The plant can be utilized for
manufacturing paper pulp and board [55,87].
15.5
Microbial Enhancement of Metal Ion Removal Capacity of Water Hyacinth
15.5.1
Biology of the Rhizosphere
The rhizosphere is defined as the environment influenced biologically and biochemically
by the living root [23]. It is an ideal environment for many organisms and
communities because it provides water, oxygen, organic substrates and physical
protection. The surface of the plant root is coated with a layer of mucigel composed
of plant mucilage, bacterial cells, metabolic products, organic colloids and mineral
materials. The plant mucilage is released from the roots as exudates and secretions
[114]. The exudates contain amino acids, sugars, organic acids, proteins, polysaccharides
and growth-promoting and growth-inhibiting substances [114]. The
organic carbon source and the binding surface provide a dynamic force for the
colonization of microbes in the rhizosphere and create the so-called rhizospheric
effect [19,26,22,27].
The microbe–plant interaction in the rhizosphere is dynamic and complicated.
Some microbes contribute to plant health by mobilizing nutrients, some are detrimental
to plant health as they compete with the plant for nutrients or cause disease
and some stimulate plant growth by producing hormones or suppressing pathogens
[19,21,23,25].
The rhizosphere of floating aquatic vegetation, which constitutes a different
biotope of bacteria from those in waters without vegetation and the sediment of
the same aquatic system, is densely populated by many specialized organisms [115].
These organisms can control the biochemical environments in the rhizosphere and
protect plants against pests through the release of bioactive chemicals. Polprasert
and Khatiwada [93] reported that the film of bacteria attached to the roots of water
hyacinth is involved in the reduction of BOD5. Bioactive chemicals such as anaerobic
metabolites, alkaloids, phenolics, terpenoids and steroids are found in abundance in
roots and rhizospheres in wetlands. Bioactivities include allelopathy, growth regulation,
extraorganismal enzymatic activities, metal manipulation by phytosiderophores
and phytochelatins, various pest-control effects and poisoning [116].
Alteration of the biological and chemical composition of the rhizosphere can,