## 3 Heavy Metal Removal Approaches
### Some Other Advanced Microbial-Based Approaches for Heavy Metal Removal
#### 3.6.2 Genetically Engineered Organisms  
The utilization of microorganisms for bioremediation has proven to be an efficacious technique for alleviating soil and water contamination caused by heavy metals. Microorganisms possess the capacity to decompose intricate substances into simpler chemicals, thereby offering a long-lasting solution for the mitigation of heavy metal contamination [63,66]. Recent advances in the field of genetic engineering have facilitated the creation of genetically engineered microorganisms or biocatalysts, which have been demonstrated to outperform natural microbes in terms of their aptitude for eliminating persistent compounds from natural surroundings [135]. Various genetic and metabolic engineering strategies have been utilized to produce highly efficient engineered microorganisms. These strategies consist of single-gene editing and pathway construction, as well as modifications of both coding and controlling sequences of pre-existing genes [136]. The central emphasis of the aforementioned adjustments is primarily on the stages that restrict the rate of metabolic processes. It is noteworthy that the employment of genetically modified bacteria has facilitated the  
Figure 2: Processes involved in rhizoremediation for heavy metals. efficient elimination of heavy metals, including but not limited to Cd, As, Cu, Hg, and Ni [137]. However, it is crucial to acknowledge that the speed at which degradation takes place is contingent upon the catalytic efficacy of enzymes that are either present within the cells or have been stimulated to act on a specific substrate [6]. The utilization of recombinant DNA technology, a genetic engineering tool, has been implemented to amplify the genome of genetically engineered microorganisms (GEMs) by integrating foreign genes from other species [138]. The development of genetically engineered metabolic pathways in microbial cells through bioremediation processes has been enhanced as a result. The advanced technological nature of GEMs has attracted public attention [139]. The incorporation of metal regulatory genes in bacteria has facilitated the conversion of heavy metals from toxic forms to less toxic ones [140]. Additionally, heavy metal accumulation has been observed to increase in GEMs that express metallothioneeins (MT) [141]. [PERSON] et al. [142] conducted a comprehensive evaluation of the application of genetically modified bacteria and plants in the bioremediation of heavy metals and other organic pollutant-contaminated environments [142]. Furthermore, [PERSON] and [PERSON] [143] have demonstrated that the integration of the mer operon from bacterial resistance to mercury, which encodes for the reduction of Hg\({}^{2+}\), into the genetically modified bacterium affords the mer genes the capacity to mitigate Hg pollution at high temperatures [143]. The genes that confer resistance to heavy metals, namely alkB, alkB1, alkB2, alkM, xylE, nida, and ndoB, have been observed to be harbored on plasmids within n-alkane-degrading microorganisms. These genes, which have been frequently employed as markers for identifying microbial biodegradation, are known to undergo horizontal transfer [144]. Within the realm of environmental bioremediation, the genetic modification of microbial membrane transporters serves as a significant means of enhancing the removal of hazardous metals [145]. The crucial roles of transporters and binding mechanisms in the remediation of such metals cannot be overstated. Typically, transporter systems are categorized into three principal categories, namely channels, secondary carriers, and primary active transporters. Channel transporters, which are a subset of the aforementioned categories, are defined by their ability to facilitate the passage of molecules across the inner lipid membrane [62,146]. Examples of channel transporters include Fps, Mer T/P, and GlpF. Additionally, secondary carriers, such as Hxt7, NixA, and Pho84, can be found in the inner lipid membrane.