# Strategies for Enhancing Extracellular Electron Transfer in Environmental Biotechnology: A Review
## 4 Challenges and Future Directions  
Challenges associated with the diminished effectiveness of EET in practical contexts include the following:  
1. Diversity of microorganisms: The effectiveness of electrochemically active microbial communities in BESs can be impacted by their diverse features [136]. The coexistence of diverse species can lead to competition for resources and electron donors/acceptors, impacting the efficiency of EET. Syntrophic interactions between different species can also affect EET efficacy. Changes in microbial diversity resulting from variations in environmental circumstances or disturbances, such as pH, temperature, and oxygen levels, can induce modifications in EET. The structure and composition of biofilms on electrode surfaces can also be influenced by microbial diversity. Different microbial species use different methods for electron transfer, and the effectiveness and kinetics of EET can be influenced by the presence of different microbial species that possess unique electron transfer mechanisms. A diverse microbial community can also display a broad spectrum of redox potentials, potentially causing inefficiencies if there is a lack of alignment between the electron donor and acceptor potentials. 2. Scaling up: While laboratory-scale studies have demonstrated the potential of BESs for various applications, scaling up these systems for practical implementation poses significant challenges [137], including the design and arrangement of electrodes, achieving uniform current distribution, and integrating nanostructured materials like carbon nanotubes or graphene into larger systems. Ensuring biofilm stability and uniformity across larger electrode surfaces is crucial for efficient performance. Additionally, expanding EET operations requires supplementary energy inputs, ensuring equilibrium between input and output, and considering the financial implications of implementing the technology. Adherence to environmental and safety regulations is also essential for real-world implementation, as the size of systems increases. 3. Substrate availability and selectivity: The efficacy of EET processes is contingent upon the presence of appropriate electron acceptors or donors within the surrounding environment [138]. Furthermore, the selection and accessibility of acceptable substrates for bioelectrochemical systems may be constrained. Numerous systems are dependent on intricate organic substrates or wastewater, the accessibility and economic viability of which may not be generally prevalent. In the context of bioremediation applications, the utilization of EET by microorganisms to break down pollutants relies heavily on the presence of electron acceptors, specifically metal oxides or electrodes, which play a vital role in supporting and maintaining microbial activity. The presence of a limited amount of substrate can result in partial degradation of pollutants, hence diminishing the overall efficiency of remediation processes. Likewise, within MFCs, the process of microbial oxidation of organic materials to produce electricity via EET can be hindered by inadequate substrate availability, resulting in diminished power generation and compromised overall MFC efficiency. Additionally, there is the possibility of substrate competition, when microorganisms exhibit a preference for other pathways instead of EET, resulting in a decrease in the efficiency of electron transfer. 4. Biofouling and system stability: The occurrence of biofouling results in the development of insulating layers on the surfaces of electrodes [139], and this can impede electron transmission between microbes and electrodes. Consequently, this results in instability in bioelectrochemical systems due to variations in biofilm thickness, microbial composition, and detachment-reattachment dynamics. Biofouling can also deteriorate electrode materials, necessitating frequent replacements.