Potential of Coupling Heavy Metal (HM) Phytoremediation by Bioenergy Plants and Their Associated HM-Adapted Rhizosphere Microbiota (Arbuscular Mycorrhizal Fungi and Plant Growth Promoting Microbes) for Bioenergy Production

Abstract

There is growing concern for the contamination of our soils and waters worldwide with heavy metals (HMs), as a result of indiscriminate use of agrochemicals for feeding growing population which require optimal use of resources and sustainable agricultural strategies. This can be simultaneously achieved by using microbes as bio-fertilizers, bio-protectants, and bio-stimulants, and suitable phytoremediation- plant capable of removing heavy metals contaminants from contaminated sites. There is a growing need to adopt such environmentally safe, attractive, and economical techniques that can remove most HMs contaminants as well as yield high biomass for bioenergy production. Phytoremediation and the microbes associated with the roots and inhabiting rhizospheres of the plants used for this purpose, has emerged as an alternative strategy. This article reviews the principles and application of this strategy, and provides an overview of the use of fast growing, non-food bioenergy plants, like Vetiver grass and industrial hemp, and their root-associated microbiota such as Arbuscular Mycorrhizal Fungi (AMF), Mycorrhiza Helping Bacteria (MHB), and Plant–Growth–Promoting-Rhizobia (PGPR) that can both tolerate and immobilize HMs in the roots, i.e. sequestrate contaminant HMs thereby protecting plants from metal toxicity. This mini-review also focuses on other phytoextraction strategies involving rhizosphere microbes, such as (1) inoculating plants used for phytoremediation of HMs contaminated soil and water with rhizobial microflora, and (2) managing their population in the rhizospheres by using a consortium of site specific AMF, PGPR, and MHB, and N-fixing rhizobia as biofertilizers to Phyto-remediate derelict contaminated sites. Various crop management strategies such as Crop Sequencing and Intercropping or Co-cropping of, for example, mycorrhizal and non-mycorrhizal crops, or leguminous and non-leguminous crops, etc., can be employed for improved plant growth. Another possible strategy to exploit soil microbes is to employ pre-cropping with mycotrophic crops to exploit AMF for mycorrhizo-remediation strategy.

Paraburkholderia atlantica is the main rhizobial symbiont of Mimosa spp. in ultramafic soils in the Brazilian Cerrado biome

“…On the other hand, plants growing in heavy metal rich environments produce exudates like enzymes, phytochelators, organic acids, flavonoids, etc. which can also decrease their toxicity in the soil (Khan 2021).…”