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Microbial extracellular electron transfer and its relevance to iron corrosion
Souichiro Kato
KI-Zusammenfassung
72 % KonfidenzThis minireview synthesizes research on extracellular electron transfer (EET) mechanisms in microorganisms and their role in microbiologically influenced corrosion (MIC) of iron. The paper distinguishes between chemical MIC (CMIC), where microbial metabolites accelerate corrosion, and electrical MIC (EMIC), where microorganisms directly extract electrons from metallic iron as an energy source. Four metabolic groups—sulfate-reducing bacteria, methanogenic archaea, acetogenic bacteria, and nitrate-reducing bacteria—are identified as EMIC-inducers. The review emphasizes that EMIC mechanisms remain poorly understood but likely involve direct EET via outer membrane proteins or nanowires, or indirect transfer via electron mediators including H₂, formate, and flavins.
Zentrale Erkenntnisse
Mechanismus Nitrate-reducing bacteria (NRB) produce nitrite as a metabolic end-product, which acts as a strong corrosive compound independent of their direct electron uptake from iron, creating a dual corrosion mechanism.
Was heißt das. If using nitrate injection for souring mitigation in oil reservoirs, monitor for NRB-assisted corrosion; nitrite accumulation may offset SRB suppression benefits.
Mechanismus EMIC-inducing SRB strains (Desulfopila corrodens IS4, Desulfovibrio ferrophilus IS5) reduce sulfate with metallic iron much faster than abiotic H₂ generation in organic matter-free medium, indicating direct electron uptake rather than H₂-mediated cathodic depolarization.
Was heißt das. Design corrosion mitigation strategies assuming direct iron-to-cell electron transfer, not just H₂ consumption; H₂-scavenging alone will not prevent EMIC.
Schwellenwert Corrosive methanogen Methanobacterium sp. IM1 produces methane at electrode potentials of −0.4 V, much more positive than the −0.41 V required for abiotic H₂ production on graphite, proving direct electron extraction without H₂ as intermediate.
Was heißt das. Electrochemical monitoring at potentials near −0.4 V can detect methanogenic EMIC activity; this is a diagnostic window distinct from H₂ evolution.
Mechanismus Prolixibacter sp. MIC1-1 (NRB) does not oxidize H₂ yet shows high EMIC activity coupled with nitrate reduction, confirming that H₂ is not a universal electron carrier for EMIC.
Was heißt das. Do not assume H₂ is the primary electron shuttle in all anaerobic iron-corroding systems; test for direct EET mechanisms in NRB-dominated communities.
Methode Mineral crust composition differs by metabolic pathway: black FeS-rich crust forms under sulfate reduction, while greyish FeCO₃ and iron phosphate crusts form under methanogenic, acetogenic, and nitrate-reducing conditions.
Was heißt das. Use mineral crust composition as a diagnostic indicator of which EMIC-inducing metabolic group is active; this guides targeted mitigation strategies.
Widersprüche2
Nur Konflikte, die die Publikation selbst zeigt, jeweils am Text geprüft.
Innerhalb der Publikation 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.
Eine SeiteH₂ 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”
Andere SeiteH₂ 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”
Einordnung der Autoren. 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.
Innerhalb der Publikation Nitrate injection for souring mitigation: expected to suppress SRB-driven corrosion, but discovery of NRB-induced EMIC creates risk of alternative corrosion pathway.
Eine SeiteNitrate 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”
Andere SeiteSupplementation 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”
Einordnung der Autoren. 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.
Ansatzpunkte6
Konkrete Lücken in dieser Publikation, jeweils mit einem Experiment, das sie schließen würde.
Hoch 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.
Versuch. 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.
Hoch 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.
Versuch. 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.
Mittel 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.
Versuch. 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.
Mittel 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.
Versuch. 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.
Mittel 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.
Versuch. 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.
Mittel 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.
Versuch. 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.
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Zusammenfassung
Extracellular electron transfer (EET) is a microbial metabolism that enables efficient electron transfer between microbial cells and extracellular solid materials. Microorganisms harbouring EET abilities have received considerable attention for their various biotechnological applications, including bioleaching and bioelectrochemical systems. On the other hand, recent research revealed that microbial EET potentially induces corrosion of iron structures. It has been well known that corrosion of iron occurring under anoxic conditions is mostly caused by microbial activities, which is termed as microbiologically influenced corrosion (MIC). Among diverse MIC mechanisms, microbial EET activity that enhances corrosion via direct uptake of electrons from metallic iron, specifically termed as electrical MIC (EMIC), has been regarded as one of the major causative factors. The EMIC-inducing microorganisms initially identified were certain sulfate-reducing bacteria and methanogenic archaea isolated from marine environments. Subsequently, abilities to induce EMIC were also demonstrated in diverse anaerobic microorganisms in freshwater environments and oil fields, including acetogenic bacteria and nitrate-reducing bacteria. Abilities of EET and EMIC are now regarded as microbial traits more widespread among diverse microbial clades than was thought previously. In this review, basic understandings of microbial EET and recent progresses in the EMIC research are introduced.
Schlagwörter
Kennungen
- Zeitschrift
- Microbial Biotechnology
- Jahr
- 2016
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