Archaea-based Microbial Fuel Cell Operating at High Ionic Strength Conditions

In this work two archaea microorganisms (Haloferax volcanii and Natrialba magadii) used as biocatalyst at a microbial fuel cell (MFC) anode were evaluated. Both archaea are able to grow at high salt concentrations. By increasing the media conductivity, the internal resistance was diminished, improving the MFCs performance. Without any added redox mediator, maximum power (Pmax) and current at Pmax were 11.87 / 4.57 / 0.12 {\mu}W cm-2 and 49.67 / 22.03 / 0.59 {\mu}A cm-2 for H. volcanii, N. magadii and E. coli, respectively. When neutral red was used as redox mediator, Pmax was 50.98 and 5.39 {\mu}W cm-2 for H. volcanii and N. magadii respectively. In this paper an archaea MFC is described and compared with other MFC systems; the high salt concentration assayed here, comparable with that used in Pt-catalyzed alkaline hydrogen fuel cells will open new options when MFC scaling-up is the objective, necessary for practical applications.

💡 Research Summary

This study investigates the performance of microbial fuel cells (MFCs) that employ two halophilic archaea—Haloferax volcanii and Natrialba magadii—as anodic biocatalysts under high‑ionic‑strength (high‑salt) conditions. The central hypothesis is that increasing the conductivity of the electrolyte by raising its ionic strength will lower the internal resistance (R_int) of the cell, thereby improving power output. Both archaea thrive in media with molar concentrations of NaCl (≈2.7 M for H. volcanii and ≈3.6 M for N. magadii), which is an order of magnitude higher than the typical low‑salt media used for bacterial MFCs.

The experimental platform consists of a two‑compartment, 1‑L MFC separated by a 4 cm² Nafion® 115 membrane. Identical plain Toray carbon‑paper electrodes (10 cm²) serve as anode and cathode. The catholyte contains ferricyanide as an electron acceptor, while the anolyte is the respective growth medium of each microorganism. For comparison, a conventional Escherichia coli K‑12 strain is tested in a low‑ionic‑strength (≈0.09 M) medium. All tests are performed at 37 °C with continuous air bubbling at the cathode and nitrogen purging at the anode to maintain anaerobic conditions.

Electrical performance is evaluated by recording polarization and power curves while varying the external load from 100 kΩ down to 2.3 Ω. Current density (j) and power density (P) are calculated on a per‑area basis, and internal resistance is derived from the linear region of the V‑I plot. Two operational modes are examined: (i) non‑mediated, where no exogenous redox mediator is added, and (ii) mediated, where 0.1 mM neutral red (NR) is introduced as an artificial electron shuttle.

Key findings include:

1. Baseline (non‑mediated) performance: H. volcanii delivers a maximum power density of 11.87 ± 0.54 µW cm⁻² and a current density of 49.67 ± 0.81 mA cm⁻² at its peak power point. This represents roughly a 100‑fold increase in both power and current relative to the E. coli control (P_max ≈ 0.12 µW cm⁻², j ≈ 0.58 mA cm⁻²). N. magadii, under the same conditions, achieves P_max = 4.57 µW cm⁻² and j = 22.03 mA cm⁻², indicating that the specific physiology of H. volcanii is more conducive to extracellular electron transfer in high‑salt media. Statistical analysis (two‑sample t‑test, p ≤ 0.01) confirms that the differences between the archaea and the bacterial strain are significant.

2. Effect of neutral red: Adding NR boosts H. volcanii’s power density to 50.98 µW cm⁻² (≈5‑fold increase) and raises N. magadii’s power to 5.39 µW cm⁻². The mediator also improves E. coli performance, but absolute values remain far below those of the archaea. The enhancement is attributed to NR’s redox potential (−325 mV vs. SHE), which aligns closely with the NADH/NAD⁺ couple, facilitating electron extraction from metabolic pathways that are otherwise poorly coupled to the anode.

3. Internal resistance reduction: High‑ionic‑strength electrolytes dramatically lower R_int (to ≈150 Ω for the H. volcanii system), confirming that electrolyte resistance dominates the total cell resistance in conventional low‑salt MFCs. By increasing conductivity, the voltage drop across the electrolyte is minimized, allowing the cell’s open‑circuit voltage to be more fully converted into usable power.

4. Electrochemical characterization: Cyclic voltammetry of the H. volcanii culture medium shows no intrinsic redox peaks, indicating that the observed current is not due to secreted mediators but rather to direct electron transfer or short‑range mediators produced at the cell surface. The high salt concentration does not impede the redox activity of added NR, as demonstrated by clear reversible peaks in the CVs recorded with the Toray electrode.

5. Comparative context: When benchmarked against literature values for mediated and non‑mediated MFCs (e.g., Pseudomonas aeruginosa at 88 mW m⁻², E. coli with mediators at 91 mW m⁻²), the archaea‑based system’s power density (≈50 µW cm⁻², i.e., 500 mW m⁻²) is competitive, especially considering the simple, non‑biofilm electrode configuration used here. The authors caution that direct comparisons are limited by differences in reactor geometry, electrode materials, and membrane properties.

6. Implications for scale‑up: The ability to operate MFCs at molar salt concentrations opens pathways for integrating these devices with saline waste streams, seawater, or brine from desalination plants. High conductivity reduces the need for additional supporting electrolytes, potentially lowering capital costs for large‑scale stacks. However, the authors acknowledge challenges such as membrane durability in aggressive ionic environments, corrosion of metallic components, and the economic feasibility of maintaining high‑salt media.

7. Future directions: The study suggests further work on (a) long‑term stability of archaea‑based MFCs under continuous operation, (b) optimization of electrode surface chemistry to promote direct electron transfer without mediators, (c) exploration of other extremophiles (e.g., thermophiles, acidophiles) that may synergize with high‑ionic‑strength conditions, and (d) engineering of modular stack designs that exploit the low internal resistance achieved here.

In conclusion, this paper demonstrates that halophilic archaea can serve as highly effective anodic biocatalysts in microbial fuel cells when the electrolyte is formulated at high ionic strength. The resulting reduction in internal resistance translates into power outputs that far exceed those of conventional bacterial systems under comparable hardware configurations. These findings broaden the design space for MFCs, suggesting that coupling extremophile biology with engineered high‑conductivity electrolytes may be a viable route toward practical, scalable bioelectrochemical power generation, particularly for applications involving saline or brackish water streams.