Bioaugmentation-Assisted Phytoextraction Applied to Metal-Contaminated Soils: State of the Art and Future Prospects

Bioaugmentation-assisted phytoextraction is a promising method for accelerating the cleanup rate of soils contaminated by metals. On average, bioaugmentation increases metal accumulated by plant shoots by factors of about two (metal concentration) and five, as a result of higher bioaccessibility of metals in soils, with few obvious differences between effects by bacteria or fungi (e.g., plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi). Metal bioaccessibility is always controlled by microbial siderophores as well as organic acids and surfactants. In cases of excess concentrations, fungi immobilize metals, in contrast to bacteria. Unfortunately, the typically low inoculant survival rate may impair bioaugmentation efficiency. In this chapter, microbial inoculant formulations and management are addressed, as well as strategies for selecting the most relevant plant–microorganism couples for optimum phytoextraction of soil metals. In environments subject to variable conditions, ecological engineering approaches may help in attaining maximal efficiency. Experiments at field-scale are reported, and environmental effects of the technique are discussed. Finally, future prospects are addressed with the main question being how maximal concentrations and amounts of metals in plants can be attained.