Role of rhizospheric microbial communities

T. Macek et al. Biotechnology Advances 18 2000 23–34 25

4. Uptake and biotransformation of pollutants in plants

Phytoremediation is currently divided into following areas as summarized by Salt et al. [7]. • Phytoextraction: the use of pollutant-accumulating plants to remove metals or organics from soil by concentrating them in the harvestable parts. • Phytodegradation: the use of plants and associated microorganisms to degrade organic pollutants. • Rhizofiltration: the use of plant roots to absorb and adsorb pollutants, mainly metals, from water and aqueous waste streams. • Phytostabilization: the use of plants to reduce the bioavailability of pollutants in the en- vironment. • Phytovolatilization: the use of plants to volatilize pollutants. • The use of plants to remove pollutants from air. Various mechanisms are involved in each of the processes listed above. Plants remediate organic compounds by direct uptake of contaminants, as summarized by Schnoor et al. [5], followed by subsequent transformation, transport, and their accumulation in a nonphytotoxic form which does not necessarily mean nontoxic for humans. In addition, plants support bioremediation by release of exudates and enzymes that stimulate both microbial and bio- chemical activity in the surrounding soil and mineralization in the rhizosphere. The use of plants as a final water treatment step and for the disposal of sludge resulting from waste wa- ter treatment is centuries old [8]. These processes either ‘decontaminate’ the soil, or ‘stabi- lize’ the pollutant within it i.e. preventing its migration to a site of actual danger to human health. Specifically, two subsets of phytoremediation are nearing commercialization. First is phytoextraction, in which high biomass metal-accumulating plants and appropriate soil amendments are used to transport and concentrate metals from the soil into the harvestable part of roots and aboveground shoots, which are harvested with conventional agricultural methods [2,7]. The other is rhizofiltration, in which plant roots grown in water absorb, con- centrate, and precipitate toxic metals and organics from polluted effluents [2,7].

5. Role of rhizospheric microbial communities

Plants can accelerate bioremediation in surface soils by stimulating the growth and metab- olism of soil microorganisms through the release of nutrients and the transport of oxygen to their roots [1,5]. The zone of soil closely associated with the plant root, the rhizosphere, has much higher numbers of metabolically active microorganisms than the surrounding bulk soil. Studies have demonstrated greater than 100-fold increase in microbial counts. Plant roots release compounds including simple sugars, amino acids, enzymes, aliphatics, and aro- matics that encourage growth of specific microbial communities. The interactions between plants and microbes in the rhizosphere are complex and in some cases have evolved to the mutual benefit of both organisms [9,10]. This mutualistic relationship is responsible for the accelerated degradation of soil contaminants in the presence of plants [11]. In addition to this rhizosphere effect, plants themselves are able to passively take up a wide range of organic 26 T. Macek et al. Biotechnology Advances 18 2000 23–34 wastes from soil or water through their roots. Although it is not discussed in this review, plants have a great capacity to bind organic compounds from the air [7]. Classes of organic compounds that are more rapidly degraded in the rhizosphere than in bulk soil include poly- cyclic aromatic hydrocarbons [12,13], total petroleum hydrocarbons [14], chlorinated pesti- cides PCP, 2,4-D, other chlorinated compounds like polychlorinated biphenyls PCBs [15,16], TCE [17], explosives [2,4,6,-trinitrotoluene TNT, hexahydro-1,3,5-trinitro-1,3,5- triazine RDX, dinitrotoluene DNT], organophosphate insecticides diazinon, parathion, and surfactants detergents. Burken and Schnoor [18] discussed beneficial impacts of root exudates and biomass on mineralization of atrazine by poplar trees. Betts [19] reported a field trial in which a substantial decrease in total petroleum hydrocar- bon TPH content using bermuda grass, rye grass, white clover, and tall fescue was ob- served in plots at a large fuel facility in the United States. Recently, it was shown that some compounds present in root exudates can serve not only as nutrients for microorganisms [11,20], but they can also induce the bacterial degradation of PCBs [21–23]. These com- pounds phenolics, flavonoids, terpenes served as growth substrates and they stimulated PCB degradation in the same manner as biphenyl.

6. Metabolism