Figure 5. Time course experiment of lindane disappearance in solution with chilli and coriander. Controls (white columns), chilli (black columns), coriander (grey columns). Data points represent the means of 5 replicates determinations. Bar = SD.
the effect of the rhizosphere on these compounds. Five plant species were harvested and separated into roots, stems and leaves: Chenopodium vulgare, Solanum nigrum, Cytisus striatus, Vicia sativa and Avena sativa. Concentrations of total HCH in plant organs ranged between 1.7 and 62.5 mg kg-1, depending on plant species and organs. Leaves systematically contained the highest amount of HCH, probably due to the volatilization of HCH isomers from the soil surface and subsequent sorption by leaves. In one species, C. striatus, the metabolites pentachlorocyclohexene, cyclopentiltrichloroethene and 6 trichloro, 4-en-hexanoic derivates were also detected in the all plant tissues analysed . Data obtained from the bulk and rhizosphere soils from C. striatus and A. sativa suggest that both plant species tend to reduce the levels of all HCH isomers in the rhizosphere. This could be due to the enhanced biodegradation in the rhizosphere, root exudation of enzymes able to dechlorinate HCH isomers and/or sequestration by partitioning into the lipophilic plant tissues or uptake by the roots.
In the case of HCH, phytoremediation has thus shown its own limitations, even in the favourable case of hydroponic systems, where the pollutant is made highly bioavailable. Since rhizosphere microbial activity is known to aid the release of bound pesticide residues in soil, which can in turn enhance uptake and transformation, by plants, a combination of bioremediation and phytoremediation, or the phytostimulation of rhizosphere microorganisms, is likely to be more successful.
Phytostimulation of bacteria present or added in soil seems the most promising approach to remove lindane from contaminated sites. It has long been known that plants release a vast range of organic materials through roots into the rhizosphere. These exudates contain water soluble, insoluble and volatile compounds including sugars, amino acids, organic acids, flavonones, phenolic compounds and even enzymes [12, 7377]. The root exudates can enhance the acquisition of nutrients by plants; stimulate microbial growth in the rhizosphere; and change pH, water flux and availability of oxygen. Microorganisms able to use phenolic compounds as a carbon source often have enzymes that can co-metabolise pollutants with similar structures. Plants have also a great capacity to release secondary metabolites having a surfactant activity, which is favourable for phytoremediation purposes. Thus the degradation of several chlorinated pesticides has been reported to be higher in a vegetated soil than a non-vegetated soil. For example, the biodegradation of HCH isomers is enhanced in rhizosphere soils of Kochia sp., as compared to bulk soil, even if the mechanism by which this occurs is not yet known .
A dynamic synergy does exist between plant roots and soil microorganisms. The microbial activity in the immediate vicinity of the root (rhizosphere) seems to offer a favourable environment for co-metabolism of soil-bound and recalcitrant chemicals. The microbial transformations of organic compounds are not always driven by energy needs, but also by the necessity to reduce toxicity for which microbes may have to suffer an energy deficit. Thus, the processes may be helped and driven by the abundant energy provided by root exudates. Certain soil microorganisms can also produce biosurfactant compounds that may facilitate the removal and degradation of organic chemicals by increasing their availability to plants. Plants can thus take advantage of an increased bioavailability of nutrients and degradation of phytotoxic soil contaminants.
A successful example of exploring rhizosphere microorganisms for the decontamination of pesticide-contaminated soils has been highlighted by the recent findings of a coordinated Indo-Swiss project . The project was aimed at investigating possibilities to remediate agricultural soils contaminated with lindane, using a plant-rhizosphere system. Mineralisation of 14C lindane by rhizospheric soils of plants growing in lindane-contaminated fields indicated that microorganisms capable of degrading the insecticide were present. Enrichment culture techniques resulted in the isolation of bacteria growing in the presence of lindane. Klebsiella sp., Pseudomonas sp., and Pseudoarthrobacter sp. degraded up to 50% of lindane with the formation of 2,3,4-trichlorobenzene. Root exudates of some plants stimulated the growth of lindane degrading bacterium Pseudomonas sp., indicating the need for an approach, which includes both plants and interacting microorganisms for an efficient degradation of pesticides. To treat lindane-contaminated soils, it thus appears that phytostimulation would be the most appropriate technique.
Soil composition influences sorption, soil pH, bulk density and water retention, all of which affecting aeration, nutrient availability and thus bioavailability and biodegradability of contaminants . A high density of indigenous S. paucimobilis was found in the plant debris fraction of soil and it was postulated that plant organic matter integrated into the soil aggregates served as a microhabitat rich in growth substrates. This can be the basis for the application of plant-derived organic amendments to soil as a phytostimulation strategy, like rice straw or other cellulosic material, because of their efficiency, availability in large quantities and low cost . For example, the Daramend® technology for bioremediation of HCH-contaminated soils is based on the application of solid plant-derived organic matter providing nutrients and a non-toxic habitat for indigenous microorganisms; it creates a concentration gradient that facilitates diffusion of organic contaminants from pockets of higher to lower concentrations on the amendment surface, where they are more bioavailable . Since the texture, nutrient requirements and microbial populations of each soil are specific, the usefulness and composition of amendments should be assessed on a case-by-case basis.
Detailed strategies for optimising treatments on sites contaminated with HCHs remain to be established and could involve enhanced natural attenuation and optimisation of environmental conditions, to stimulate growth and biodegradation by indigenous microorganisms. Supplemental nutrients and/or organic amendments could be added to enrich the soil and stimulate the bacteria degrading HCH. Bioaugmentation with phytostimulation or a vegetative cover should also be tested, either by increasing the population of microorganisms able to degrade HCH isomers, or by increasing the bioavailability of the insecticide.
Both examples developed here show that plants and soil microorganisms have certain limitations with respect to their individual abilities to remove and degrade organic pollutants like pesticides, and other molecules containing chlorines and/or aromatic ring structures. However, plants and bacteria have very specific and complementary metabolic pathways, and their combined appropriate use can breakdown many man-made chemicals. Therefore, a synergy between rhizospheric microorganisms leading to increased availability of hydrophobic compounds and plants leading to their removal and/or degradation, may overcome many of the limitations, thus providing a sound basis for enhancing biological remediation of contaminated environments.
Phytostimulation or rhizoremediation is of particular importance because it refers to an important contribution that microorganisms in the root-zone (rhizosphere) make to the overall breakdown and removal of organic pollutants by plants. Plant-microbial interactions in the rhizosphere are thus of utmost importance for the degradation of recalcitrant chemicals in the environment [12, 30, 80].
However, further research into the mechanisms by which plants can stimulate biodegradation and the complexity of the soil-plant-microbe system due to its interwoven nature is thus required to better explore and exploit their huge potential. Such studies must be done not only at laboratory scale, but also under real conditions, as demonstration projects, to optimise the phytoremediation process and convince regulators and the general public of the technique's feasibility [81, 82]. To increase its acceptance as a remediation concept, phytoremediation must also become an economically interesting approach and biomass disposal or use after the treatment is thus an important issue to consider. For example, the biomass of fibres, oil or fragrance producing plants like vetiver, could be used to recover these added-value products, if however their level of contamination is nil or low enough. Alternatively, contaminated biomass could be used for renewable energy generation, either by direct combustion, gasification or pyrolysis, or indirectly via biogas or biofuel production [83, 84].
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