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AI summary

82% confidence

This study demonstrates that Pseudomonas stutzeri A1501 performs bioelectrochemical nitrogen fixation to extracellular NH4+ without chemical inhibitors or expensive catalysts, using a poised cathode potential of −0.3 V (SHE). The organism secretes phenazine-1-carboxylic acid as an endogenous electron shuttle, enabling indirect extracellular electron uptake from the electrode. Faradaic efficiency reached 20% ± 3% for total nitrogen reduction, 2–4 times higher than prior H2-mediated e-BNF systems. The work reveals a sustainable pathway for ammonia production and identifies the electron transfer mechanism through the respiratory chain and nitrogenase complex.

Key insights

  • Mechanism Extracellular NH4+ production (2.32 ± 0.25 mg/L) occurred spontaneously without assimilation inhibitors, because P. stutzeri fixes 3× more N2 than required for biomass growth, exceeding the capacity of AmtB transporters to recapture it.

    So what. Operators should not rely on chemical inhibitors; instead, select or engineer diazotrophs with high N2 fixation rates relative to growth demand to achieve extracellular ammonia secretion.

  • Mechanism Planktonic cells (85% of biomass) are responsible for the majority of electron uptake, not electrode-attached biofilm; current density dropped only 12% when electrodes were shielded by dialysis membrane, and 60% ± 4% was restored when biofilm was replaced with sterile electrode.

    So what. Design e-BNF reactors to maximize planktonic cell–electrode coupling via soluble electron shuttles rather than biofilm formation; avoid biofilm-centric electrode designs.

  • Threshold Phenazine-1-carboxylic acid concentration of ~100 nM is sufficient to sustain maximum current density (30 mA/m²); 50 nM enabled rapid startup within 2 h, but endogenous production was below 200 nM detection limit.

    So what. Monitor or supplement phenazine concentration to 50–100 nM range to accelerate startup and maintain stable electron transfer; endogenous production may be limiting in some conditions.

  • Mechanism Complex III and the quinone pool are essential for inward electron transfer; antimycin A (blocking complex III) inhibited 100% of current uptake with minimal effect on lactate-driven metabolism, whereas complex I is not indispensable (rotenone inhibition was transient and recovered).

    So what. Target complex III and quinone pool as control points for regulating electron uptake rate; complex I inhibition will not block the pathway.

  • Trade-off Inward electron uptake is more energy-intensive than lactate-driven metabolism; dicyclohexylcarbodiimide at 300 µM blocked 100% of electrode-driven current but only 1% of lactate-driven activity, and 600 µM was needed to suppress lactate metabolism by 98%.

    So what. Expect higher ATP/proton-motive-force demands for electrode-driven N2 fixation; optimize cathode potential and current density to balance energy cost against faradaic efficiency.

Opportunity areas6

Specific gaps in this paper, with an experiment that would close each.

  • High Cathode potential sweep: only −0.3 V (SHE) was tested. No data on how potential between −0.1 and −0.6 V affects current density, faradaic efficiency, or phenazine production rate.

    Potential is the primary control variable in electrochemical systems; the paper states this is a 'poised potential' experiment but does not explore the operating window or optimal potential for balancing power input against N2 fixation rate.

    Try. Vary cathode potential from −0.1 to −0.6 V (SHE) in steps of 0.1 V; measure current density, NH4+ production rate, phenazine concentration, and nitrogenase gene expression at each potential.

  • High Current density or applied voltage ramp: no data on how increasing current demand (e.g., via lower potential or larger electrode) affects phenazine shuttle saturation, N2O accumulation, or nitrite formation.

    The paper identifies phenazine as rate-limiting (100 nM endogenous, 50–100 nM exogenous needed) but does not test whether higher current densities cause shuttle depletion, side reactions, or nitrogen oxide byproducts.

    Try. Operate at 10, 20, 30, 40 mA/m² (via potential control); measure phenazine concentration, N2O, NO2−, NO3−, and faradaic efficiency at each current density.

  • High Dissolved oxygen and pH effects: no mention of DO or catholyte pH control. Nitrogenase is oxygen-sensitive; phenazine redox potential may shift with pH.

    The paper does not state whether the system was anaerobic or aerobic, nor does it report pH or DO during the 13-day run. These are critical for nitrogenase activity and electron shuttle function.

    Try. Operate under strict anaerobic conditions (DO < 0.1 mg/L) and test pH 6.5, 7.0, 7.5, 8.0; measure current, NH4+ production, and phenazine redox potential at each pH.

  • Medium Hydraulic retention time (HRT) and continuous operation: only batch experiments (13 days, static culture). No data on steady-state performance, washout, or long-term phenazine stability in continuous flow.

    The paper demonstrates proof-of-concept in batch; practical e-BNF systems require continuous operation. Phenazine may degrade, accumulate, or be washed out.

    Try. Operate in continuous mode with HRT 1, 2, 4, 8 days; measure steady-state NH4+ production rate, phenazine concentration, and current density over 30 days.

  • Medium Nitrogen source limitation and C/N ratio: no variation of N2 partial pressure, gas flow rate, or organic carbon (lactate) availability. The paper uses N2 gas but does not quantify mass transfer or test whether N2 or electron availability is limiting.

    The paper shows N2 dependence (Fig. S1: current drops when N2 is diluted by Ar) but does not characterize the kinetics or saturation point. No organic carbon is added; it is unclear whether heterotrophic growth or N2 fixation is carbon-limited.

    Try. Vary N2 partial pressure (10%, 50%, 100% in headspace) and add lactate at 0, 0.5, 1, 2 g/L; measure current, NH4+ production rate, and biomass yield.

  • Medium Nitrite and N2O accumulation: no data on whether NO2−, NO3−, or N2O are produced as side products or intermediates. Denitrification or nitrification pathways are not ruled out.

    The paper focuses on N2 fixation but does not measure nitrogen oxides. In mixed or biofilm systems, nitrification or denitrification can occur; N2O is a potent greenhouse gas.

    Try. Measure NO2−, NO3−, N2O, and N2 in headspace and liquid using ion chromatography and gas chromatography over the 13-day incubation; quantify nitrogen balance.

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Abstract

Diazotrophs can produce bioavailable nitrogen from inert N 2 gas by bioelectrochemical nitrogen fixation ( e -BNF), which is emerging as an energy-saving and highly selective strategy for agriculture and industry. However, current e -BNF technology is impeded by requirements for NH 4 + assimilation inhibitors to facilitate intracellular ammonia secretion and precious metal catalysts to generate H 2 as the energy-carrying intermediate. Here, we initially demonstrate inhibitor- and catalystless extracellular NH 4 + production by the diazotroph Pseudomonas stutzeri A1501 using an electrode as the sole electron donor. Multiple lines of evidence revealed that P. stutzeri produced 2.32 ± 0.25 mg/liter extracellular NH 4 + at a poised potential of -0.3 V (versus standard hydrogen electrode [SHE]) without the addition of inhibitors or expensive catalysts. The electron uptake mechanism was attributed to the endogenous electron shuttle phenazine-1-carboxylic acid, which was excreted by P. stutzeri and mediated electron transfer from electrodes into cells to directly drive N 2 fixation. The faradaic efficiency was 20% ± 3%, which was 2 to 4 times that of previous e -BNF attempts using the H 2 -mediated pathway. This study reports a diazotroph capable of producing secretable NH 4 + via extracellular electron uptake, which has important implications for optimizing the performance of e -BNF systems and exploring the novel nitrogen-fixing mode of syntrophic microbial communities in the natural environment. IMPORTANCE Ammonia greatly affects global ecology, agriculture, and the food industry. Diazotrophs with an enhanced capacity of extracellular NH 4 + excretion have been proven to be more beneficial to the growth of microalgae and plants, whereas most previously reported diazotrophs produce intracellular organic nitrogen in the absence of chemical suppression and genetic manipulation. Here, we demonstrate that Pseudomonas stutzeri A1501 is capable of extracellular NH 4 + production without chemical suppression or genetic manipulation when the extracellular electrode is used as the sole electron donor. We also reveal the electron uptake pathway from the extracellular electron-donating partner to P. stutzeri A1501 via redox electron shuttle phenazines. Since both P. stutzeri A1501 and potential electron-donating partners (such as electroactive microbes and natural semiconductor minerals) are abundant in diverse soils and sediments, P. stutzeri A1501 has broader implications on the improvement of nitrogen fertilization in the natural environment.

Keywords

Pseudomonas stutzeriDiazotrophExtracellularNitrogen fixationElectron transport chainEnvironmental chemistry

Identifiers

DOI
10.1128/aem.01998-20
PubMed
33310714
Journal
Applied and Environmental Microbiology
Year
2020
Extraction

Reported parameters

20 extracted values

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