Rhizobacteria: Restoration of heavy metal-contaminated soils

Abstract

Heavy metal pollution has become one of the major culprits of environmental disasters. Their toxicity and damaging effects on soils and agricultural lands and direct impact on human health through soils and agricultural lands are considered a major world challenge. Many removal processes, such as mechanical and physicochemical ones have been introduced. However, because of their high cost, attention has been focused on biological solutions, which are less expensive and more efficient. Interestingly, some microorganisms have shown high potential to metal tolerance and removal. Cupriavidus metallidurans is certainly the most known example. The rhizosphere, an important interface between soil and plant, holds a diverse prokaryotic microflora population known as rhizobacteria. An important fraction of these microorganisms have been found to be involved in the removal of heavy metals through a panoply of mechanisms including release of chelating substances, microenvironment acidification, and promotion of redox potentials. The different mechanisms of resistance and interactions with metals have also been discussed. It has been also shown that rhizospheric bacteria may play beneficial roles in phytoremediation processes via facilitating bioavailability of heavy metals to plants such as maize and tomato, as well as for metal stress alleviation along rhizosphere of sensitive crops. In this chapter, we focused on the potentials of rhizobacteria for restoration of metal-affected soils and their role in improving metal uptake for phytoremediation processes. Because of the advantages of being less costly and environmental friendly, microbes are still the best tool for metal removal. But as living organisms, subject to death and decomposition through the geochemical cycles in the soil, their metal cleaning processes are never perfect and the removal may be tentative or not definitive. One of the foci now is on microbial bioengineering and genetic improvement of rhizobacteria and other soil microbial abilities for metal handling. Among the various microbial phyla in the rhizosphere, actinobacteria have drawn great interest due to their high biological compound production, which confer adaptation to a wide spectrum of ecological conditions, including metal contamination in soils. Their extremophilic traits and biological competitiveness in soil present them as possible efficient candidate for bioremediation. We included here their plant growth promotion capacity as well as their potentials for metal bioremediation.