Potential of arbuscular mycorrhizal fungi for bioremediation

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Bioremediation is the use of organisms for the treatment of soil pollution. Root colonizing symbiotic microorganisms such as arbuscular mycorrhizal fungi (AMF) are mainly involved in phytoremediation, that uses plants for soil reme­diation. Phytoremediation comprises a set of technologies that use various plants as a containment, destruction or extraction technique (EPA 2000). These techniques have received considerable interest in recent years because of potential cost savings compared to conventional non biological techniques. Different strategies of phytoremediation can be applied depending on the kind of pollutants. In all cases, vegetation reduces infiltration of water and erosion. Heavy metals cannot be degraded and can only be extracted (phytoextraction) from the soil or immobilized in a non toxic form (phytostabilization). AMF can help alleviate metal toxicity to plants by reducing metal translocation from root to shoot (Leyval et al. 1997). Therefore they may contribute to plant estab­lishment and survival in heavy metal polluted sites and could be used as a com­plement to immobilization strategies. Phytoextraction mainly uses plants accu­mulating high concentrations of heavy metals, which can be harvested, dis­carded and even extracted to recover metals. For this purpose plants with var­ious capacities for metal accumulation are used, like members of the Brassicaceae which are generally considered non mycorrhizal, but also other accumulators producing higher biomass, which can be mycotrophic. Organic pollutants such as polycylic aromatic hydrocarbons (PAH) can be transformed or degraded through microbial activity, which is commonly enhanced in the root zone (rhizodegradation). It is not known whether this enhanced degrada­tion in the rhizosphere is due to plant exudates including enzymes, surfactants, and other physical/chemical effects, and/or to increased microbial activity. Another possible mechanism for degradation of organic pollutants could be directly dependent on plant metabolism. However, this is not a quantitatively important route for PAH (Binet et al. 2000a), and will not be discussed here. AMF may be beneficial for PAH rhizodegradation because they affect root exudation and root associated microbial populations and because, in some ways, they act as an extension of the roots outside the rhizosphere. They may potentially also have a direct effect on PAH degradation. Finally, AMF can be used in bioassays of soil quality or soil toxicity, due to their sensitivity towards a range of soil pollutants. In this way, they could be used to show that adequate soil quality has been re-established after remediation.