**4. Future approach of bioremediation technology**

#### **4.1 Genetically engineered microorganisms (GEMs)**

The potential for microorganisms to remediate water and soil pollutants to increase treated water consumption and soil fertility for agricultural output is gaining attention [11, 38]. Recently, research has been conducted to enhance the application of altered organisms delineated specifically to boost their affectability toward hazardous metals [11, 16, 38]. An organism whose genetics have been transformed by the use of synthetic methods, which are driven by an artificial genetic exchange between bacteria, is referred to as a "genetically engineered microorganism [11, 18]. By developing GEM, genetic engineering has enhanced the application and disposal of hazardous wastes in laboratory settings. In addition, the following protocols must be considered during the GEM process: (a) alteration of enzyme selectivity and affinity, (b) pathway development and modulation, (c) bioprocess advancement, surveillance, and control, and (d) bioaffinity bioreporter sensor utilization for chemical detecting, toxicity reduction, and endpoint evaluation [13, 18].

As there are several possibilities for improving degradation performance through genetic engineering approaches, such as genetically controlling the rate kinetics of known metabolic pathways to increase degradation rate, or completely infusing bacterial strains with new metabolic pathways for the degradation of previously recalcitrant compounds [6, 8]. Despite important genes for microorganisms are carried on a single chromosome, defining the specific genes needed for the catabolism of some of these novel substrates may be carried on plasmids [18, 76, 77]. Plasmids were entangled in the catabolism process. As a result, GEM can be successfully used for biodegradation purposes, necessitating immediate research and large-scale deployment. Genetically engineer microbes offer the benefit of developing microbial strains which can tolerate unfriendly upsetting circumstances and can be utilized as a bioremediation tool under different and complicated natural conditions [18, 37, 76, 77]. Additionally, GEM has encouraged the development of "microbial biosensors" capable of precisely quantifying the degree of pollution in a contaminated site.

#### **4.2 Engineered plants approach**

The current advancement in omics technologies, including genomics, proteomics, transcriptomics, and metabolomics, play a critical role in finding characteristics that optimize remediation solutions [7, 11, 78]. Consequently, phytoremediation was developed, a process for eliminating toxins or their metabolites from plant tissues. This usually shortens the life of the plant and finally volatilizes the toxins into the atmosphere [78]. This disadvantage can be mitigated by managing plants' metal resistance, accumulation, and breakdown capacity in the presence of various inorganic toxins. To improve metal decomposition in plants, bacterial genes responsible for metal reduction can be integrated into plant tissues. As a result, plant-based bioremediation for a variety of significant metal poisons is cutting-edge due to its

eco-friendliness. They are more effective at reducing dangerous substances than Physicochemical approaches, which are less environmentally friendly and potentially detrimental to human health [7, 8].

Notwithstanding, microbial genes can bridle in the transgenic plant for decontamination and collection of inorganic pollutants [7, 11]. The metal-detoxifying chelators, for example, metallothioneins and phytochelatins can give resistance to the plant by upgrading take-up, transport and amassing of different heavy metals [14, 78]. Similarly, transgenic plants with bacterial reductase can augment the volatilization of Hg and Se while absorbing the arsenic in plant shoots [17, 78]. Also, high-biomassproducing plants including poplar, willow and Jatropha can be applied for both phytoremediation and energy generation [7, 14, 26, 78]. Nonetheless, metals can only be removed from soil or water, which is why consuming metal-contaminated plants is advantageous. Thus, metal-accumulating biomasses should be properly preserved or disposed of to avoid posing an environmental hazard [20, 78].

#### **4.3 Engineered** *Rhizosphere* **approach**

Bioremediation methods include the introduction of growth stimulators (electron acceptors/donors) or nutrients to the rhizosphere to promote microbial growth and bioremediation characteristics of microbes or genetically engineered plants [6, 26, 78]. Multiple small organisms were generated with heavy metals by drainage using synthesized catalysts such as chromate and uranyl reductase in a particular rhizosphere [19, 26, 78]. Although genomics has been studied and applied mostly in microbial genetics and agriculture, such as genetic crops, and now serve as a bioremediation instrument [26, 76]. The application of genomics to bioremediation enables the microorganism to be dissected based on biochemical constraints as well as sub-atomic levels associated with the component [26, 76, 77].

#### **4.4 Integrated bioturbation: phytoremediation process**

Bioturbation is a very prolific and appealing technology for remediation, cleaning, management, and recovery of environmental contamination caused by microbial activity [11]. Furthermore, phytoremediation is successful at removing both inorganic and organic pollutants from residues or soils [7, 11, 12]. Nonetheless, investigation of resourceful bioremediation approaches for damaged aquatic environments that are based on these two processes to improve wastewater and soil treatment is necessary [10, 17]. Nonetheless, investigation of resourceful bioremediation technologies based on these two processes is important to improve soil and wastewater treatment [11, 17]. In addition, phytoremediation has been generally illustrated as a bioremediation process for heavy metals, such as lead, cadmium, copper, arsenic removal from contaminated soil or water [76, 77]. In essence, aquatic bioturbation combined with phytoremediation is a more effective and alternative method of removing heavy metals by improving cadmium transfers from overlying water to sediment and then into the root system of plants [15, 38].

Additionally, studies have demonstrated that earthworm movement greatly boosted phytoavailability by increasing soil macroporosity and generating cast around plant roots (**Figure 3**), implying that the physical effect of the earthworm's bioturbation is a viable mechanism [20, 26]. Interaction between plants and soildwelling microorganisms can also enhance phytoremediation known as rhizosphere bioremediation. The study by Leveque et al. [52] to investigate the contribution of

**Figure 3.**

*Proffered approach to illustrate metal phytoavailability in earthworms' activities (adapted from [20, 26]).*

earthworm (as bioremediator or bioturbation agent) to phytoremediation showed that earthworms significantly increased the phyto-availability of metal by generating soil macroporosity and developing cast near plant roots in which the main mechanism appears to be the physical impact of earthworm bioturbation. Moore et al. [21], demonstrated the contribution and the effect of bioturbators in the remediation of organic contaminants using the phytoremediation technique. In the study, *Typha latifolia* plant species recorded rapid growth in high pollutant concentrations in the environs due to its appreciable efficiency in the phytoaccumulation of contaminants from the sediments, which showed the ability to extract atrazine molecules by producing flux between the soil and the plant root. This plant was able to transform contaminants from atrazine to lower metabolites such as hydroxyatrazine, DEA and DIA [79].
