Bioelectrochemical Systems and Shewanella: Power Generation and Remediation

Introduction to Bioelectrochemical Systems. Bioelectrochemical Systems (BES), such as microbial fuel cells (MFCs), are innovative devices that utilize electroactive microbes, known as exoelectrogens, to generate electricity. These microorganisms facilitate extracellular electron transfer (EET) from their cells to an electrode, thereby catalyzing the direct conversion of chemical energy into electrical energy. A typical MFC design incorporates a membrane that physically separates the anodic and cathodic chambers, with the system's charge balance maintained by the movement of ions across this ionic membrane. This technology can utilize a wide range of electron donors, including various waste streams like wastewater and lignocellulosic waste, positioning it as a seminal platform for coupling sustainable water treatment with renewable energy recovery.

Applications and Challenges of MFC Technology. The microbial EET processes that power BES are foundational for developing advanced technologies in contaminant remediation, renewable energy recovery, and biofuel production integrated with water treatment. MFCs are capable of operating on diverse renewable organic substrates, including simple compounds like acetate and glucose, as well as complex mixtures like synthetic wastewater, brewery starch, and landfill leachate. This versatility allows MFCs to serve a dual purpose: treating wastewater by reducing its biological oxygen demand while simultaneously generating electricity to offset operational costs. Despite this significant potential, the current major obstacle preventing the widespread application of MFC technology is its characteristically low power density, which remains a key focus of ongoing research.

Shewanella Species as Model Exoelectrogens. Several species within the Shewanella genus are renowned for their pronounced EET activity and frequently dominate the microbial communities that colonize MFC electrodes exposed to wastewater. When compared under identical BES configurations, different Shewanella strains produce current densities ranging from 0.7 to 3 pA cm⁻², with S. putrefaciens W3-18-1 generating the highest output and Shewanella loihica PV-4 producing the lowest. A remarkable capability of Shewanella is its ability to reductively precipitate highly toxic metals while simultaneously generating electricity, a process known as bioelectrochemical remediation.

Shewanella in Metal and Dye Remediation. A prime example of this remediation capability is demonstrated by S. oneidensis-driven MFCs, which use lactate as an electron donor. These systems achieve a maximum current density of 32.5 mA m⁻² after receiving a 10 mg L⁻¹ Cr(VI) addition to the cathode. The cathodic efficiency for chromium reduction increases steadily over an eight-day operation period with successive Cr(VI) additions, confirming effective and continuous detoxification coupled with current production. Subsequent studies have shown that other Shewanella spp., including strains W3-18-1, MR-4, and ANA-3, can generate current during Cr(VI) reduction at rates up to fivefold higher than the model organism S. oneidensis MR-1. Furthermore, S. oneidensis MR-1 can reduce pollutants like the azo dye Acid Orange 7 (AO7) in a biocathode configuration, achieving a decolorization efficiency of over 96%.

Biosensing Applications of Shewanella BES. Beyond power generation and remediation, Shewanella-based BES can be engineered into highly sensitive biosensors for detecting toxic compounds. For instance, formaldehyde biosensors have been developed that detect current responses over a concentration range of 0.01% to 0.10%. Similarly, fumarate-biosensing systems deliver a symmetric current peak directly proportional to analyte concentration in a linear range between 2 pM and 10 mM. The true power of this platform, however, lies in its compatibility with genetic engineering, enabling the development of sophisticated, customizable biosensors.

Genetically Engineered BES for Advanced Sensing. Researchers have successfully created genetically engineered S. oneidensis strains for specific sensing tasks. By placing the metal reduction (Mtr) pathway under the control of an arsenic-sensitive promoter, an engineered strain produces a quantifiable increase in current output in response to arsenic contamination. This BES-based biosensor displays an exceptionally low detection limit of 40 pM As(III). The modularity of this transcriptional circuit allows for the development of sensors for other analytes by simply exchanging a single genetic component. Another example involves using trimethylamine-N-oxide (TMAO) to control EET rates via the mtrCAB pathway, where specific promoters respond to TMAO to induce iron reduction and current production. These systems demonstrate how Shewanella-based BES can simultaneously treat waste, monitor for toxins, and dynamically modify treatment processes by controlling the expression of key biochemical pathways.