Microbial extracellular electron transfer and its relevance to iron corrosion

Tensions the paper itself points to. Each side quotes the passage it comes from.

• Within the paper H₂ consumption as a mechanism for iron corrosion: initially believed to be a major driver (cathodic depolarization), but recent evidence shows it is insufficient alone.

  One side

  H₂ consumption was believed to induce cathodic depolarization and iron corrosion

  “Microorganisms consuming H₂ abiotically generated on iron surfaces (Eq. 3) coupled with sulfate reduction (Eq. 4) or methanogenesis (Eq. 6) were considered to induce cathodic depolarization”

  Other side

  H₂ consumption itself is not sufficient to induce fatal iron corrosion as often observed in actual fields

  “However, stimulation of anaerobic corrosion via H₂ consumption has been viewed critically (Hardy, 1983), and recent studies revealed that H₂ consumption itself is not sufficient to induce fatal iron corrosion as often observed in actual fie”

  Authors’ reading. The authors reconcile this by introducing EMIC as the dominant mechanism: only microorganisms with direct EET ability can stimulate sufficient cathodic reaction and iron corrosion. H₂ consumption alone (cathodic depolarization) is now viewed as a minor contributor.

• Within the paper Nitrate injection for souring mitigation: expected to suppress SRB-driven corrosion, but discovery of NRB-induced EMIC creates risk of alternative corrosion pathway.

  One side

  Nitrate injection promotes NRB growth and suppresses SRB growth, mitigating souring and MIC

  “Nitrate injection into oil and gas reservoirs has been used for mitigation of souring and MIC caused by SRB, since it promotes growth of NRB and in turn suppresses growth of SRB”

  Other side

  Supplementation of nitrate may risk inducing NRB-assisted corrosion via EMIC and nitrite production

  “However, the discovery of MIC-inducing NRB led to the revelation that supplementation of nitrate may risk inducing NRB-assisted corrosion”

  Authors’ reading. The authors acknowledge this as an unresolved trade-off: while NRB suppress SRB, they introduce a new corrosion risk. The net benefit depends on relative corrosion rates of SRB-CMIC versus NRB-EMIC, which is not quantified in this review.

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

• High No quantitative comparison of corrosion rates (e.g., iron dissolution rate, current density) between EMIC-inducing metabolic groups (SRB vs. methanogens vs. acetogens vs. NRB) under standardized conditions.

  The paper identifies four EMIC-inducing groups but provides no data on which poses the greatest corrosion risk or under what conditions one dominates. Section 'Microorganisms inducing EMIC' describes each group separately without comparative metrics.

  Try. Electrochemical cell with identical iron electrode, pH, temperature, and electron acceptor concentration (NO₃⁻, SO₄²⁻, HCO₃⁻) inoculated with each EMIC-inducing strain; measure cathodic current density (mA/m²) and iron dissolution rate (mg/cm²/day) over 30 days.

• High No investigation of how cathode potential (applied voltage) affects the relative activity of different EMIC-inducing microorganisms or the threshold potential for each metabolic pathway.

  The paper reports that Methanobacterium IM1 operates at −0.4 V and Prolixibacter MIC1-1 at unspecified potential, but does not test whether applied potential selectively enriches or inhibits specific EMIC-inducers. This is critical for bioelectrochemical system design.

  Try. Potentiostat-controlled iron cathode at −0.2, −0.4, −0.6, −0.8 V vs. SHE in anaerobic medium with mixed inoculum; measure current density and identify dominant microorganism by 16S rRNA sequencing at each potential over 60 days.

• Medium No data on the effect of pH on EMIC activity or on the stability of mineral crusts that form under different metabolic conditions.

  Mineral crust composition differs by pathway (FeS vs. FeCO₃), but pH is not mentioned as a variable. pH affects both microbial metabolism and mineral solubility/precipitation, yet no experiments vary pH.

  Try. Batch cultures of each EMIC-inducing strain with metallic iron at pH 6.0, 7.0, 8.0, 9.0; measure iron dissolution rate, crust composition (XRD), and microbial growth (OD₆₀₀) over 30 days.

• Medium No systematic test of whether exogenous electron mediators (riboflavin, FAD, humic substances) enhance EMIC activity in pure cultures of EMIC-inducing strains, or whether these strains produce their own mediators.

  Section 'How do corrosive microorganisms uptake electrons from metallic iron?' mentions that exogenous mediators accelerate corrosion in D. vulgaris and that some EET-harbouring microorganisms produce flavins, but no direct evidence links mediator production to EMIC in the isolated strains listed in Table 1.

  Try. Pure culture of each EMIC-inducing strain (Table 1) with metallic iron ± riboflavin (10 µM), ± FAD (10 µM), ± humic acid (50 mg/L); measure cathodic current density and iron dissolution rate. Analyze culture supernatant for flavin concentration by HPLC.

• Medium No investigation of how dissolved oxygen (DO) or microaerobic conditions affect EMIC activity, or whether EMIC-inducing microorganisms are strictly anaerobic or facultative.

  All EMIC studies described are under anaerobic conditions. Bacillus licheniformis ATCC 14580 is noted as 'facultative' but no data on its EMIC activity under microaerobic conditions. This is relevant for bioelectrochemical systems where oxygen may diffuse into the cathode compartment.

  Try. Batch cultures of facultative EMIC-inducers (B. licheniformis, Prolixibacter sp.) with metallic iron at DO levels of 0, 1, 5, 10% air saturation; measure cathodic current density and iron dissolution rate over 30 days.

• Medium No data on the effect of current density or applied potential on the accumulation of intermediate nitrogen species (NO₂⁻, N₂O, N₂) in NRB-driven EMIC systems, or on the stoichiometry of nitrate reduction coupled to iron oxidation.

  The paper identifies nitrite as a corrosive product of NRB-EMIC but does not quantify NO₂⁻ accumulation, nor does it address whether further reduction to N₂O or N₂ occurs. This is critical for understanding the full nitrogen cycle in bioelectrochemical systems.

  Try. Potentiostat-controlled iron cathode at −0.4, −0.6, −0.8 V vs. SHE with Prolixibacter sp. MIC1-1 or B. licheniformis in medium with 10 mM NO₃⁻; measure NO₂⁻, N₂O, and N₂ by ion chromatography and gas chromatography over 60 days; calculate electron balance.