Extracellular electron transfer
Introduction
Extracellular electron transfer (EET) is a process by which certain microorganisms, primarily bacteria, transfer electrons to or from their cells across the cell membrane to external electron acceptors or donors. This phenomenon is significant in various environmental and industrial processes, including biogeochemical cycling, bioenergy production, and bioremediation. EET mechanisms are crucial for understanding microbial ecology and the development of bioelectrochemical systems.
Mechanisms of Extracellular Electron Transfer
EET mechanisms can be broadly classified into two categories: direct electron transfer and mediated electron transfer.
Direct Electron Transfer
Direct electron transfer involves the physical contact between the microbial cell and the electron acceptor or donor. This process is facilitated by specialized proteins, such as cytochromes, located on the outer membrane of the cell. These proteins contain heme groups that can shuttle electrons between the cell and external surfaces. Some bacteria, such as Geobacter and Shewanella, are well-known for their ability to perform direct electron transfer.
Mediated Electron Transfer
Mediated electron transfer involves the use of small, diffusible molecules known as electron shuttles. These molecules can transfer electrons between the microbial cell and the electron acceptor or donor without direct contact. Common electron shuttles include flavins, quinones, and phenazines. The use of electron shuttles allows for electron transfer over longer distances and can enhance the efficiency of microbial processes in environments where direct contact is limited.
Microbial Species Involved in EET
Several microbial species are known for their ability to perform extracellular electron transfer. These include:
- **Geobacter sulfurreducens**: Known for its ability to reduce iron and other metals, Geobacter sulfurreducens is a model organism for studying EET. It utilizes both direct and mediated electron transfer mechanisms.
- **Shewanella oneidensis**: This bacterium is capable of reducing a wide range of electron acceptors, including metals and organic compounds. It employs both direct contact and electron shuttles for EET.
- **Pseudomonas aeruginosa**: Although primarily known as a pathogen, Pseudomonas aeruginosa can perform EET using phenazine derivatives as electron shuttles.
Applications of Extracellular Electron Transfer
EET has numerous applications in environmental and industrial processes:
Biogeochemical Cycling
Microorganisms that perform EET play a crucial role in biogeochemical cycles, such as the carbon, nitrogen, and sulfur cycles. By transferring electrons to or from minerals and organic compounds, these microorganisms facilitate the transformation and mobilization of nutrients in the environment.
Bioenergy Production
Microbial fuel cells (MFCs) harness the ability of EET-capable microorganisms to generate electricity from organic substrates. In MFCs, bacteria oxidize organic matter and transfer electrons to an anode, creating an electric current. This technology has the potential to provide sustainable energy from waste materials.
Bioremediation
EET-capable bacteria can be used in bioremediation to detoxify contaminated environments. By transferring electrons to pollutants, such as heavy metals and organic contaminants, these microorganisms can transform them into less harmful forms. This process is particularly useful in the treatment of industrial wastewater and polluted soils.
Challenges and Future Directions
Despite the potential applications of EET, several challenges remain:
- **Understanding Mechanisms**: The molecular mechanisms of EET are not fully understood, particularly in complex environments. Further research is needed to elucidate the pathways and proteins involved in electron transfer.
- **Enhancing Efficiency**: Improving the efficiency of EET processes is crucial for their practical application. This includes optimizing microbial communities and engineering microorganisms with enhanced EET capabilities.
- **Scaling Up**: Scaling up bioelectrochemical systems, such as microbial fuel cells, for industrial applications presents technical and economic challenges. Addressing these challenges requires advances in materials science and engineering.