The aerobic halophilic archaeon Natronorubrum tibetense accelerates pitting corrosion of 304 stainless steel in saline media: within 14 days it drives passive-film degradation (increased Fe2+\mathrm{Fe}^{2+} and Cr6+\mathrm{Cr}^{6+} fractions, falling passive-film resistance, rising Mott–Schottky doping densities) and produces micrometre-deep pits absent in sterile controls, and energy-starvation experiments indicate the cells extract energy from the steel itself. The proposed mechanism — oxygen-concentration cells beneath the biofilm, electron exchange mediated by biofilm-associated catalase, and uptake of electrons from cathodic areas by the cells — was presented as a hypothesis without direct measurement; the redox-active agent or agents carrying that exchange have not been identified; the observations are limited to one alloy, one medium, one temperature and 14-day incubations; and no quantitative relationship connects biofilm properties to pitting kinetics. That mechanistic and quantitative core remains open: the proposed electron-transfer mechanism has never been directly measured, and no quantitative relationship between biofilm state and pitting kinetics has been formulated.

Background

Microbiologically influenced corrosion (MIC) is the acceleration or initiation of metal corrosion by microorganisms, typically growing as biofilms — surface-attached microbial communities embedded in a matrix of extracellular polymeric substances (EPS) — on the metal surface. Stainless steels resist corrosion through a nanometre-scale chromium/iron-oxide passive film; localized breakdown of this film in chloride-bearing environments causes pitting, the dominant localized failure mode of stainless components. MIC by anaerobic and facultative bacteria is comparatively well characterized: sulfate-reducing bacteria and other microbes extract electrons from the metal, either by direct cell-surface contact or via soluble redox mediators such as flavins and phenazines, in a process called extracellular electron transfer (EET). The electrochemical fingerprints of passive-film degradation are conventionally quantified by electrochemical impedance spectroscopy (EIS), from which a fitted passive-film resistance RpR_p is extracted; by potentiodynamic polarization (corrosion and pitting potentials); and by Mott–Schottky analysis, which treats the passive film as a semiconductor and yields its donor and acceptor (doping) densities NDN_D and NAN_A.

Archaeal MIC is far less understood than its bacterial counterpart, although halophilic (salt-loving) archaea dominate the hypersaline environments of salt-production plants, saline lakes and high-salinity oilfield brines where stainless equipment is widely deployed. Qian and co-workers established that the haloarchaeon Natronorubrum tibetense accelerates corrosion of Q235 carbon steel (2018), with dissolved oxygen modulating the effect (2019), and, in the source study for this problem, of 304 stainless steel (2020). In the 2020 study, 14-day immersion at 37 °C in inoculated culture medium produced pits of average depth ≈4.8 µm (maximum 5.0 µm) where sterile medium produced none; EIS low-frequency impedance fell from ≈1.0×10⁵ to ≈3.2×10⁴ Ω·cm²; X-ray photoelectron spectroscopy (XPS) showed increased Fe2+\mathrm{Fe}^{2+} and Cr6+\mathrm{Cr}^{6+} fractions in the passive film; Mott–Schottky doping densities rose; and energy-starvation experiments showed slow growth in medium lacking organic nutrients only when stainless-steel coupons were present, implying that the cells obtain usable energy from the metal itself. To explain these observations the authors proposed an integrated mechanism: (i) oxygen-concentration cells, with anodic zones under biofilm-covered metal and cathodic zones on adjacent bare metal; (ii) electron exchange across the biofilm/passive-film interface mediated by biofilm-associated catalase — a heme enzyme that disproportionates hydrogen peroxide (2H₂O₂ → O₂ + 2H₂O) and whose Fe(II)/Fe(III) center can shuttle electrons, catalase being known from electrode studies to catalyze cathodic oxygen reduction (Lai and Bergel, 2000, cited by the source itself), an effect comparable to the hydrogenase-catalyzed cathodic reactions demonstrated on stainless steel in other anaerobic systems (Da Silva et al., 2004); and (iii) uptake of electrons from cathodic areas by the archaeal cells, either by direct EET or via dissolved mediators.

Crucially, steps (ii) and (iii) were inferences, not measurements. Whether halophilic archaea can perform EET at all became a directly demonstrated capability only in 2026, for a different haloarchaeon (Haloferax volcanii reducing ferrihydrite) and in the electron-donating direction, while electron uptake from stainless steel has been demonstrated for an archaeon only for a methanogen acting through indirect transfer (Hou et al., 2023). Which redox-active constituent or constituents of the N. tibetense biofilm actually carry electron exchange at the interface — catalase being the candidate named in the source, with specific EPS constituents or additional enzymes untested possibilities — has not been established. The study was also limited to 14 days, one medium, one alloy (304 SS), one temperature and pure culture, and no quantitative relationship between biofilm state and pit initiation or growth was formulated. Reviews of MIC electrochemistry (Cristiani et al., 2025) and of archaeal EET (Gao and Lu, 2021) confirm that electron-transfer mechanisms in archaeal corrosion remain an open frontier.

Problem Statement

As left open by Qian et al. (2020) and still standing, the problem is to replace the inferred MIC mechanism of Natronorubrum tibetense on passive stainless steel with a demonstrated and quantitative one, for the system studied in the source: an N. tibetense biofilm on the passive film of stainless steel in saline, aerated medium. The following aspects serve that single objective — a mechanistically validated, quantitative account of how the archaeal biofilm destroys the passive film and drives pitting:

(1) Directly demonstrate and quantify the electron-transfer pathways: measure, rather than infer, whether biofilm-associated catalase mediates electron exchange at the biofilm/passive-film interface, and establish by what route or routes N. tibetense cells take up electrons from cathodic areas (for example direct extracellular electron transfer, transfer mediated by dissolved redox carriers, or other pathways).

(2) Identify and localize the redox-active biofilm constituent or constituents actually responsible for the measured electron exchange and for the observed modification of the passive film — catalase being the candidate named in the source, with specific EPS constituents or additional enzymes among the possibilities to be tested — and establish their role.

(3) Establish the time-resolved kinetics of pit initiation and growth for this system, extending beyond the source's 14-day laboratory window.

(4) Develop quantitative rate laws and predictive descriptions connecting biofilm properties (coverage, thickness, catalytic activity), measured independently of the electrochemical observables and pit statistics they are used to predict, to the measured electrochemical degradation (for example rates of RpR_p decline or doping-density increase) and to pitting kinetics.

These aspects concern the same biofilm–metal system as the source study and express one objective: a demonstrated and quantitative mechanism of passive-film destruction and pitting by the archaeal biofilm.

References

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