# Strategies for Enhancing Extracellular Electron Transfer in Environmental Biotechnology: A Review
## 3 Strategies for Enhancing EET in Environmental Biotechnology
### Electron Shuttles and Mediators  
(a) Use of natural or synthetic compounds as electron shuttles  
One class of electron shuttles commonly used in environmental biotechnology is quinones. Quinones are naturally occurring compounds that possess redox-active properties. They can undergo reversible two-electron transfers, wherein they can accept electrons from microbial cells and subsequently donate them to electrodes or other acceptors. Quinones can be found in various environmental matrices, including soils, sediments, and wastewater. Examples of natural quinones are humic substances [30]. In addition to natural quinones, synthetic quinones have also been developed for use as electron shuttles. These synthetic compounds are often designed to have specific redox potentials and electron transfer kinetics, allowing for tailored electron transfer processes. For example, anthraquinone derivatives such as 2,6-anthraquinone disulfonate and 9,10-anthraquinone-2,7-disulfonate have been widely used as synthetic electron shuttles. These compounds have shown significant promise in enhancing the efficiency of microbial fuel cells by facilitating electron transfer and decreasing the overpotential required for electron transfer. The possibility of humic substances functioning as a terminal electron acceptor in the respiration by microbes has been explored by [PERSON] et al. [62]. Additionally, their research also investigated the ability of humic substances to act as electron carriers between microorganisms that reduce Fe (III) and Fe (III) oxides that are insoluble. The findings indicated that the utilization of Fe (III) oxide with low crystallinity as the electron acceptor for the proliferation of _G. metallietucens_ was significantly enhanced by the introduction of a minimal concentration of 100 uM of the humic analogue, specifically anthraquinone-2,6-disulfonate. Therefore, the incorporation of humic substances can be employed to augment the facilitation of electron transfer between microorganisms that reduce Fe (III) and Fe (III) oxides. This approach holds potential as a viable technique to promote the restoration of soils and sediments that have been contaminated with organic or metallic pollutants [63]. Similarly, the impact of 14 ESs on EET in _Shewanella oneidensis_ MR-1 was investigated by [PERSON] et al. [64]. The results revealed that anthraquinone-2-sulfonate (AQS) exhibited the most significant effects, as it resulted in the highest cathodic current density, total charge production, and formation of reduction products. Thus, it can be inferred that the presence of AQS, functioning as electron shuttles, significantly contributed to enhancing the efficiency of EET between the electrode and microorganisms. The phenazines, which serve as the major mediators in _Pseudomonas aeruginosa_, have a vital function in promoting effective EET. The gene known as phzM, which encodes for methyltransferase, has been found to have the ability to augment the synthesis of PYO. Overexpression of this gene in _Pseudomonas aeruginosa_ has been observed to lead to a 1.6-times boost in the production of PYO, and a four-time increment in the optimal energy production [65]. [PERSON] et al. [66] incorporated synthesis-related genes associated with phenazine-1-carboxylic acid (PCA) from _Pseudomonas aeruginosa_ into _Escherichia coli_. The endogenous PCA led to an improvement in EET efficiency. This enhancement resulted in an optimal power generation of 249 mA/m\({}^{2}\) for MFCs, which was found to be twice as high as that observed in the original strain. Consequently, the incorporation of exogenous electron shuttles into non-electroactive microorganisms has been shown to significantly enhance electron exchange efficiency and enhance the performance of MFCs. Furthermore, the interactions between electron shuttles and microorganisms or other components of the system must be thoroughly studied to understand their impact on overall performance.