Microbe-Assisted Phytoremediation of Petroleum Hydrocarbons

Introduction

The word ‟phytoremediation” was first described by Ilga Raskin (“phyto” Gr. Plant; and “remediation” L. able to cure) in the early 1990s. It is a general term that describes the process of using plants to decrease the quantity, mobility, or toxicity of contaminants in contaminated media like soil or groundwater (Van Epps, 2006). It is also defined as a form of bioremediation that involves a plants-microbial synergism to detoxify contaminants. Phytoremediation is a green process in which vegetative plants remove or degrade contaminants from an environment (Cameselle et al., 2013).

As an in-situ bioremediation technique, phytoremediation employ the inherent abilities of living plants. Over the years, interactions among plants, microorganisms water, and soil have been demonstrated to play a significant role in manipulating environmental components and it is on this principle the concepts of phytoremediation are established (Ahalya and Ramachandra, 2006). General information on phytoremediation has been developed from a number of laboratory and field studies such as in constructed wetlands, oil spills, and accumulation of heavy metals by agricultural plants (Sumiahadi and Acar, 2018). The technology is solar-energy driven and operates based on the principles of using nature to cleanse nature which showcase its eco-friendliness (Nwaaichi et al., 2015). Currently, phytoremediation as a promising technology solving the problem of different pollutions faced by mankind. Phytoremediation in addition to addressing environmental pollution problems, it also provides several ecosystem services (Chakravarty et al., 2017).

Phytoremediation is applied in terrestrial and aquatic environments as a beginning or finishing treatment option after initial clean-up processes (Ahalya and Ramachandra, 2006). Presently, phytoremediation is the only known most-passive cleanup technology in which growing, and in some cases harvesting the plants on a contaminated site renders it safe; especially where the levels of the contaminants are low or moderate. It is used to clean up heavy metals, organic pollutants (e.g. pesticides, petroleum hydrocarbons, and solvents), explosives, radioactive contaminants, and landfill leachates. In essence, phytoremediation is the most economical cleanup technology for different organic and inorganic pollutants (Pilon-Smits, 2005). One of the successful areas of phytoremediation application is in the treatment of hydrocarbon polluted media. A number of investigations have reported encouraging findings in the decontamination of soil and water using phytoremediation (Frick et al., 1999). Although hydrocarbon degradation can spontaneously proceed due to microbial activity, a number of studies have shown that the presence of plant species increases the rate of disappearance of the hydrocarbon contaminants (McIntosh et al., 2017; Rodriguez- Campos et al., 2018; Riskuwa-Shehu and Ismail, 2018). In addition, researchers have established that plants’ role is majorly indirectly because the fundamental mechanism for the cleanup of hydrocarbon contaminants is rhizodegradation (Frick et al., 1999, Germinda et al., 2002; Hall et al., 2011; Lu et al., 2019).

In rhizodegradation, microorganisms and plants are involved both individually and in synergy for the degradation or transformation of petroleum hydrocarbons into products that are environmentally less harmful and less persistent than the parent compounds (Germinda et al., 2002; Kotoky et al., 2018). The interaction between plants and microorganisms in the rhizosphere is the primary mechanism by which petroleum hydrocarbons are degraded in soils. In the rhizosphere, plants increase significantly the microbial activity by nutrient supplementation through root exudation (Rohrbacher and St-Arnaud, 2016). The plants make oxygen available either by excreting oxygen or by creating void spaces in the subsurface that allows for greater oxygen diffusion from the atmosphere (Tsao, 2003; Van Epps, 2006). Microbial populations benefit plants through recycling and solubilization of mineral nutrients as well as by supplying vitamins, amino acids, auxins, cytokinins, and gibberellins, which stimulate plant growth (Vaziri et al., 2013).