Direct microbial electron uptake as a mechanism for stainless steel corrosion in aerobic environments

Acceleration mechanism of riboflavin on Fe0-to-microbe electron transfer in corrosion of EH36 steel by Pseudomonas aeruginosa

Riboflavin (RF), as a common electron mediator that can accelerate extracellular electron transfer (EET), is usually used as a probe to confirm EET-microbiologically influenced corrosion (MIC). However, the acceleration mechanism of RF on EET-MIC is still unclear, especially the effect on gene expression in bacteria. In this study, a 13-mer antimicrobial peptide E6 and tetrakis hydroxymethyl phosphonium sulfate (THPS) were used as new tools to investigate the acceleration mechanism of RF on Fe0-to-microbe EET in corrosion of EH36 steel caused by Pseudomonas aeruginosa. 60 min after 20 ppm (v/v) THPS and 20 ppm THPS & 100 nM E6 were injected into P. aeruginosa 1 and P. aeruginosa 2 (two glass bottles containing P. aeruginosa with different treatments) at the 3-d incubation, respectively, P. aeruginosa 1 and P. aeruginosa 2 had a similar planktonic cell count, whereas the sessile cell count in P. aeruginosa 1 was 1.3 log higher than that in P. aeruginosa 2. After the 3-d pre-growth and subsequent 7-d incubation, the addition of 20 ppm (w/w) RF increased the weight loss and maximum pit depth of EH36 steel in P. aeruginosa 1 by 0.7 mg cm-2 and 4.1 μm, respectively, while only increasing those in P. aeruginosa 2 by 0.4 mg cm-2 and 1.7 μm, respectively. This suggests that RF can be utilized by P. aeruginosa biofilms since the corrosion rate should be elevated by the same value if it only acts on the planktonic cells. Furthermore, the EET capacity of P. aeruginosa biofilm was enhanced by RF because the protein expression of cytochrome c (Cyt c) gene in sessile cells was significantly increased in the presence of RF, which accelerated EET-MIC by P. aeruginosa against EH36 steel.

Insights into the biocorrosion of Q235A steel influenced by the electron transfer process between iron and methanogenic microbiota

Anaerobic microbiologically influenced corrosion (MIC) of Fe (0) metals causes great harm to the environment and economy, which depends on the key electron transfer process between anaerobic microorganisms and Fe (0) metals. However, the key electron transfer process in microbiota dominating MIC remains unclear, especially for methanogenic microbiota wildly distributed in the environment. Herein, three different methanogenic microbiota (Methanothrix, Methanospirillum, and Methanobacterium) were acclimated to systematically investigate electron transfer pathways on corroding Q235A steel coupons. Results indicated that microbiota dominated by Methanothrix, Methanospirillum, or Methanobacterium accelerated the steel corrosion mainly through direct electron transfer (DET) pathway, H2 mediated electron transfer (HMET) pathway, and combined DET and HMET pathways, respectively. Compared with Methanospirillum dominant microbiota, Methanothrix or Methanobacterium dominant microbiota caused more methane production, higher weight loss, corrosion pits with larger areas, higher corrosion depth, and smaller corrosion pits density. Such results reflected that the DET process between microbiota and Fe (0) metals decided the biocorrosion degree and behavior of Fe (0) metals. This study insightfully elucidates the mechanisms of methanogenic microbiota on corroding steels, in turn providing new insights for anti-corrosion motives.

Screening, characterization and optimization of potential dichlorodiphenyl trichloroethane (DDT) degrading fungi

Dichlorodiphenyltrichloroethane is an organo-chlorine insecticide used for malaria and agricultural pest control, but it is the most persistent pollutant, endangering both human and environmental health. The primary aim of the research is to screen, characterize, and assess putative fungi that degrade DDT for mycoremediation. Samples of soil and wastewater were gathered from Addis Ababa, Koka, and Ziway. Fungi were isolated and purified using potato dextrose media. Matrix-Assisted Laser Desorption, Ionization, and Flight Duration The technique of mass spectrometry was employed to identify fungi. It was found that the finally selected isolate, AS1, was Aspergillus niger. Based on growth factor optimization at DDT concentrations (0, 3500, and 7000 ppm), temperatures (25, 30, and 35 °C), and pH levels (4, 7, and 10), the potential DDT-tolerant fungal isolates were investigated. A Box-Behnken experimental design was used to analyze and optimize fungal biomass and sporulation. The highest biomass (0.981 ± 0.22 g) and spore count (5.60 ± 0.32 log/mL) of A. niger were found through optimization assessment, and this fungus was chosen as a potential DDT-degrader. For DDT degradation investigations by A. niger in DDT-amended liquid media, gas chromatograph-electron capture detector technology was employed. DDT and its main metabolites, DDE and DDD, were eliminated from both media to the tune of 96-99 % at initial DDT concentrations of 1750, 3500, 5250, and 7000 ppm. In conclusion, it is a promising candidate for detoxifying and/or removing DDT and its breakdown products from contaminated environments.

Exploring phenazine electron transfer interaction with elements of the respiratory pathways of Pseudomonas putida and Pseudomonas aeruginosa

Pseudomonas aeruginosa phenazines contribute to survival under microaerobic and anaerobic conditions by extracellular electron discharge to regulate cellular redox balances. This electron discharge is also attractive to be used for bioelectrochemical applications. However, elements of the respiratory pathways that interact with phenazines are not well understood. Five terminal oxidases are involved in the aerobic electron transport chain (ETC) of Pseudomonas putida and P. aeruginosa. The latter bacterium also includes four reductases that allow for denitrification. Here, we explored if phenazine-1-carboxylic acid interacts with those elements to enhance anodic electron discharge and drive bacterial growth in oxygen-limited conditions. Bioelectrochemical evaluations of terminal oxidase-deficient mutants of both Pseudomonas strains and P. aeruginosa with stimulated denitrification pathways indicated no direct beneficial interaction of phenazines with ETC elements for extracellular electron discharge. However, the single usage of the Cbb3-2 oxidase increased phenazine production, electron discharge, and cell growth. Assays with purified periplasmic cytochromes NirM and NirS indicated that pyocyanin acts as their electron donor. We conclude that phenazines play an important role in electron transfer to, between, and from terminal oxidases under oxygen-limiting conditions and their modulation might enhance EET. However, the phenazine-anode interaction cannot replace oxygen respiration to deliver energy for biomass formation.

Low efficiency of cathodic protection in marine tidal corrosion of X80 steel in the presence of Pseudomonas sp

Owing to the effects of seawater erosion, dry/wet cycles, dissolved oxygen and microorganisms, the corrosion of steel in marine tidal environments is a serious threat to the safe and stable operation of marine equipment and facilities. Among them, microbiologically influenced corrosion (MIC) of steel has received increasing attention. Cathodic protection (CP) is frequently used to control the corrosion of offshore steel structures. However, in the presence of microorganisms, implementation of CP and its specific effects remain controversial. In this study, the influence of Pseudomonas sp. on the CP efficiency of Zn sacrificial anodes (ZnSAs) during the tidal corrosion of X80 steel was studied. The results showed that CP efficiency exceeded 92% in an abiotic tidal environment. However, in the biotic tidal environment, Pseudomonas sp. significantly reduced the CP efficiency. Pseudomonas sp. and its biofilm promoted the corrosion of steel under CP, inhibited the formation of a complete calcareous deposit layer, which weakened the CP efficiency of ZnSA in the marine tidal environment.

Elucidating microbial iron corrosion mechanisms with a hydrogenase-deficient strain of Desulfovibrio vulgaris

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure. There is substantial debate over their corrosion mechanisms. We investigated Fe0 corrosion with Desulfovibrio vulgaris, the sulfate reducer most often employed in corrosion studies. Cultures were grown with both lactate and Fe0 as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes. Fe0 was corroded in cultures of a D. vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe0 loss and H2 accumulation expected for Fe0 oxidation coupled to H+ reduction to H2. This result and the extent of sulfate reduction indicated that D. vulgaris was not capable of direct Fe0-to-microbe electron transfer even though it was provided with a supplementary energy source in the presence of abundant ferrous sulfide. Corrosion in the hydrogenase-deficient mutant cultures was greater than in sterile controls, demonstrating that H2 removal was not necessary for the enhanced corrosion observed in the presence of microbes. The parental H2-consuming strain corroded more Fe0 than the mutant strain, which could be attributed to H2 oxidation coupled to sulfate reduction, producing sulfide that further stimulated Fe0 oxidation. The results suggest that H2 consumption is not necessary for microbially enhanced corrosion, but H2 oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction. The finding that D. vulgaris was incapable of direct electron uptake from Fe0 reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes.

Enhanced corrosion of 2205 duplex stainless steel by Acetobacter aceti through synergistic electron transfer and organic acids acceleration

Acetobacter aceti is a microbe that produces corrosive organic acids, causing severe corrosion of industrial equipment. Previous studies have focused on the organic acid corrosion of A. aceti, but neglected the possibility that it has electron transfer corrosion. This study found that electron transfer and organic acids can synergistically promote the corrosion of 2205 duplex stainless steel (DSS). Electrochemical measurement results showed that corrosion of 2205 DSS was more severe in the presence of A. aceti. Surface analysis indicated a thick biofilm formed on the steel surface, with low pH and dissolved oxygen concentrations under the biofilm. Corrosion intensified when A. aceti lacked a carbon source, suggesting that A. aceti can corrode metals by using metallic substrates as electron donors, in addition to its acidic by-products.

Accelerated corrosion of 316L stainless steel in a simulated oral environment via extracellular electron transfer and acid metabolites of subgingival microbiota

316L stainless steel (SS) is widely applied as microimplant anchorage (MIA) due to its excellent mechanical properties. However, the risk that the oral microorganisms can corrode 316L SS is fully neglected. Microbiologically influenced corrosion (MIC) of 316L SS is essential to the health and safety of all patients because the accelerated corrosion caused by the oral microbiota can trigger the release of Cr and Ni ions. This study investigated the corrosion behavior and mechanism of subgingival microbiota on 316L SS by 16S rRNA and metagenome sequencing, electrochemical measurements, and surface characterization techniques. Multispecies biofilms were formed by the oral subgingival microbiota in the simulated oral anaerobic environment on 316L SS surfaces, significantly accelerating the corrosion in the form of pitting. The microbiota samples collected from the subjects differed in biofilm compositions, corrosion behaviors, and mechanisms. The oral subgingival microbiota contributed to the accelerated corrosion of 316L SS via acidic metabolites and extracellular electron transfer. Our findings provide a new insight into the underlying mechanisms of oral microbial corrosion and guide the design of oral microbial corrosion-resistant materials.

Corrosion of Pseudomonas aeruginosa toward a Cu-Zn-Ni alloy inhibited by the simulative tidal region

The corrosion of marine engineering equipment not only threatens human security and ecological environment but also increases energy consumption, restricting the sustainable development of marine economies and industries. The tidal region is a complex and challenging environment that can cause severe corrosion of facilities and affect microbial activities. However, the current understanding of the mechanisms underlying microbiologically influenced corrosion (MIC) of tidal region is insufficient. To address this issue, the effect of Pseudomonas aeruginosa on a Cu-Zn-Ni alloy in the simulative tidal region was investigated by chemical and molecular biological analysis in this study. The results demonstrated that P. aeruginosa formed thicker biofilms on the Cu-Zn-Ni alloy samples under the full exposure, accelerating corrosion compared to sterile controls. Interestingly, the corrosion of P. aeruginosa toward the Cu-Zn-Ni alloy was inhibited in the simulative tidal region. This inhibition behavior was relevant to the reduction in the quantity of sessile cells and cell activities. The expression down-regulation of genes encoding phenazines induced the decrease in electron transfer mediators and weakened the MIC of P. aeruginosa on alloy samples in the simulative tidal region. The research sheds light on the characteristics of P. aeruginosa and corrosion products on the Cu-Zn-Ni alloy, as well as their interaction mechanisms underlying corrosion in the simulative tidal region. The study will facilitate the evaluation and control of MIC in the tidal region, contributing to the development of sustainable strategies for preserving the integrity and safety of marine facilities.

Mitigation of EH36 ship steel biocorrosion using an antimicrobial peptide as a green biocide enhancer

In this study, a 13-mer antimicrobial peptide (RRWRIVVIRVRRC) named by E6 was used as an enhancer of a green biocide to mitigate the biocorrosion of EH36 ship steel. Results show that a low concentration of E6 (100 nM) alone was no-biocidal and could not resist the Desulfovibrio vulgaris adhesion on the EH36 steel surface. However, E6 enhanced the bactericidal effect of tetrakis hydroxymethyl phosphonium sulfate (THPS). When E6 and THPS were both added to the bacteria and steel system, both the sessile D. vulgaris cells and biocorrosion rate of EH36 steel decreased significantly. Compared with the 80 ppm THPS alone treatment, the combination of 100 nM E6 + 80 ppm THPS led to an extra 1.6-log reduction in the sessile cell count. Fewer sessile D. vulgaris cells led to a lower extracellular electron transfer (EET) rate, directly resulting in 78% and 83% decreases in weight loss and pit depth of EH36 steel, respectively. E6 saved more than 50% of THPS dosage in this work to achieve a similar biocorrosion mitigation effect on EH36 steel.

Burning question: Are there sustainable strategies to prevent microbial metal corrosion?

The global economic burden of microbial corrosion of metals is enormous. Microbial corrosion of iron-containing metals is most extensive under anaerobic conditions. Microbes form biofilms on metal surfaces and can directly extract electrons derived from the oxidation of Fe0 to Fe2+ to support anaerobic respiration. H2 generated from abiotic Fe0 oxidation also serves as an electron donor for anaerobic respiratory microbes. Microbial metabolites accelerate this abiotic Fe0 oxidation. Traditional strategies for curbing microbial metal corrosion include cathodic protection, scrapping, a diversity of biocides, alloys that form protective layers or release toxic metal ions, and polymer coatings. However, these approaches are typically expensive and/or of limited applicability and not environmentally friendly. Biotechnology may provide more effective and sustainable solutions. Biocides produced with microbes can be less toxic to eukaryotes, expanding the environments for potential application. Microbially produced surfactants can diminish biofilm formation by corrosive microbes, as can quorum-sensing inhibitors. Amendments of phages or predatory bacteria have been successful in attacking corrosive microbes in laboratory studies. Poorly corrosive microbes can form biofilms and/or deposit extracellular polysaccharides and minerals that protect the metal surface from corrosive microbes and their metabolites. Nitrate amendments permit nitrate reducers to outcompete highly corrosive sulphate-reducing microbes, reducing corrosion. Investigation of all these more sustainable corrosion mitigation strategies is in its infancy. More study, especially under environmentally relevant conditions, including diverse microbial communities, is warranted.

Microbially mediated metal corrosion

A wide diversity of microorganisms, typically growing as biofilms, has been implicated in corrosion, a multi-trillion dollar a year problem. Aerobic microorganisms establish conditions that promote metal corrosion, but most corrosion has been attributed to anaerobes. Microbially produced organic acids, sulfide and extracellular hydrogenases can accelerate metallic iron (Fe0) oxidation coupled to hydrogen (H2) production, as can respiratory anaerobes consuming H2 as an electron donor. Some bacteria and archaea directly accept electrons from Fe0 to support anaerobic respiration, often with c-type cytochromes as the apparent outer-surface electrical contact with the metal. Functional genetic studies are beginning to define corrosion mechanisms more rigorously. Omics studies are revealing which microorganisms are associated with corrosion, but new strategies for recovering corrosive microorganisms in culture are required to evaluate corrosive capabilities and mechanisms. Interdisciplinary studies of the interactions among microorganisms and between microorganisms and metals in corrosive biofilms show promise for developing new technologies to detect and prevent corrosion. In this Review, we explore the role of microorganisms in metal corrosion and discuss potential ways to mitigate it.

Behaviors and mechanisms of microbially-induced corrosion in metal-based water supply pipelines: A review

Microbially-induced corrosion (MIC) is unstoppable and extensively spread throughout drinking water distribution systems (DWDSs) as the cause of pipe leakage and deteriorating water quality. For maintaining drinking water safety and reducing capital inputs in pipe usage, the possible consequences from MIC in DWDSs is still a research hotspot. Although most studies have investigated the effects of changing environmental factors on MIC corrosion, the occurrence of MIC in DWDSs has not been discussed sufficiently. This review aims to fill this gap by proposing that the formation of deposits with microbial capture may be a source of MIC in newly constructed DWDSs. The microbes early attaching to the rough pipe surface, followed by chemically and microbially-induced mineral deposits which confers resistance to disinfectants is ascribed as the first step of MIC occurrence. MIC is then activated in the newly-built, viable, and accessible microenvironment while producing extracellular polymers. With longer pipe service, oligotrophic microbes slowly grow, and metal pipe materials gradually dissolve synchronously with electron release to microbes, resulting in pipe-wall damage. Different corrosive microorganisms using pipe material as a reaction substrate would directly or indirectly cause different types of corrosion. Correspondingly, the formation of scale layers may reflect the distribution of microbial species and possibly biogenic products. It is therefore assumed that the porous and loose layer is an ideal microbial-survival environment, capable of providing diverse and sufficient ecological niches. The usage and chelation of metabolic activities and metabolites, such as acetic, oxalic, citric and glutaric acids, may lead to the formation of a porous scale layer. Therefore, the microbial interactions within the pipe scale reinforce the stability of microbial communities and accelerate MIC. Finally, a schematic model of the MIC process is presented to interpret MIC from its onset to completion.

Carbon starvation considerably accelerated nickel corrosion by Desulfovibrio vulgaris

Carbon starvation can affect the activity of microbes, thereby affecting the metabolism and the extracellular electron transfer (EET) process of biofilm. In the present work, the microbiologically influenced corrosion (MIC) behavior of nickel (Ni) was investigated under organic carbon starvation by Desulfovibrio vulgaris. Starved D. vulgaris biofilm was more aggressive. Extreme carbon starvation (0% CS level) reduced weight loss due to the severe weakening of biofilm. The corrosion rate of Ni (based on weight loss) was sequenced as 10% CS level > 50% CS level > 100 CS level > 0% CS level. Moderate carbon starvation (10% CS level) caused the deepest pit of Ni in all the carbon starvation treatments, with a maximal pit depth of 18.8 μm and a weight loss of 2.8 mg·cm-2 (0.164 mm·y-1). The corrosion current density (icorr) of Ni for the 10% CS level was as high as 1.62 × 10-5 A·cm-2, which was approximately 2.9-fold greater than the full-strength medium (5.45 × 10-6 A·cm-2). The electrochemical data corresponded to the corrosion trend revealed by weight loss. The various experimental data rather convincingly pointed to the Ni MIC of D. vulgaris following the EET-MIC mechanism despite a theoretically low Ecell value (+33 mV).

Accelerated Microbial Corrosion by Magnetite and Electrically Conductive Pili through Direct Fe0 -to-Microbe Electron Transfer

Electrobiocorrosion, the process in which microbes extract electrons from metallic iron (Fe0 ) through direct Fe0 -microbe electrical connections, is thought to contribute to the costly corrosion of iron-containing metals that impacts many industries. However, electrobiocorrosion mechanisms are poorly understood. We report here that electrically conductive pili (e-pili) and the conductive mineral magnetite play an important role in the electron transfer between Fe0 and Geobacter sulfurreducens, the first microbe in which electrobiocorrosion has been rigorously documented. Genetic modification to express poorly conductive pili substantially diminished corrosive pitting and rates of Fe0 -to-microbe electron flux. Magnetite reduced resistance to electron transfer, increasing corrosion currents and intensifying pitting. Studies with mutants suggested that the magnetite promoted electron transfer in a manner similar to the outer-surface c-type cytochrome OmcS. These findings, and the fact that magnetite is a common product of iron corrosion, suggest a potential positive feedback loop of magnetite produced during corrosion further accelerating electrobiocorrosion. The interactions of e-pili, cytochromes, and magnetite demonstrate mechanistic complexities of electrobiocorrosion, but also provide insights into detecting and possibly mitigating this economically damaging process.

Accelerated role of exogenous riboflavin in selective Desulfovibrio desulfuricans corrosion of pipeline welded joints

Effect of exogenous riboflavin on sulfate-reducing bacteria (SRB) corrosion of a spirally welded joint (WJ) of X80 steel was investigated by SEM/EDS, XPS, 3D ultra-depth microscopy and electrochemical measurements. The main style of SRB corrosion of the WJ is local corrosion. The local corrosion sensitivity of the heating affected zone (HAZ) of the WJ was always lower than that of the weld zone (WZ) and base metal (BM) in all the SRB-inoculated mediums. SRB corrosion of the WJ is selective. With the dosage increase of riboflavin, the selective pitting corrosion of the WJ becomes more pronounced.

Recent advances in enrichment, isolation, and bio-electrochemical activity evaluation of exoelectrogenic microorganisms

Exoelectrogenic microorganisms (EEMs) catalyzed the conversion of chemical energy to electrical energy via extracellular electron transfer (EET) mechanisms, which underlay diverse bio-electrochemical systems (BES) applications in clean energy development, environment and health monitoring, wearable/implantable devices powering, and sustainable chemicals production, thereby attracting increasing attentions from academic and industrial communities in the recent decades. However, knowledge of EEMs is still in its infancy as only ~100 EEMs of bacteria, archaea, and eukaryotes have been identified, motivating the screening and capture of new EEMs. This review presents a systematic summarization on EEM screening technologies in terms of enrichment, isolation, and bio-electrochemical activity evaluation. We first generalize the distribution characteristics of known EEMs, which provide a basis for EEM screening. Then, we summarize EET mechanisms and the principles underlying various technological approaches to the enrichment, isolation, and bio-electrochemical activity of EEMs, in which a comprehensive analysis of the applicability, accuracy, and efficiency of each technology is reviewed. Finally, we provide a future perspective on EEM screening and bio-electrochemical activity evaluation by focusing on (i) novel EET mechanisms for developing the next-generation EEM screening technologies, and (ii) integration of meta-omics approaches and bioinformatics analyses to explore nonculturable EEMs. This review promotes the development of advanced technologies to capture new EEMs.

Effects of Pseudomonas aeruginosa on EH40 steel corrosion in the simulated tidal zone

Corrosion of metals in the tidal zone shortens the service life of facilities considerably and causes extensive economic losses each year. However, the contribution of microbiologically influenced corrosion (MIC) to this progress is usually ignored, and consequently the research on the mechanism of MIC in the tidal zone is highly desirable. In this study, the impact of the typical marine strain Pseudomonas aeruginosa on EH40 steel corrosion in the simulated tidal zone was evaluated. P. aeruginosa accelerated the corrosion of EH40 steel in the simulated tidal zone and its corrosion promotion efficiency rose over time. The environmental stress promoted the metabolism, energy production, and secretion of phenazines of P. aeruginosa, which promoted extracellular electron transfer between bacteria and steel, and accelerated MIC. The study proposes a possible mechanism of MIC in the tidal zone at the molecular biological level, which is of theoretical significance for evaluating the corrosion risks of marine equipment.

Effect of Pseudomonas sp. on simulated tidal corrosion of X80 pipeline steel

Microbiologically influenced corrosion of pipeline steel in seawater has long been concerned by scholars all over the world, but there were few reports on the microorganism effect on marine tidal corrosion of steels. In this work, the effect of Pseudomonas sp. on static tidal corrosion of X80 pipeline steel were systematically studied using weight-loss, Fourier transform infrared spectroscopy (FTIR), electrochemical measurements, scanning electron microscopy (SEM) and ultra-deep field 3D microscope. The results manifested that after 720 h exposure to the marine tidal environment, the sessile Pseudomonas sp. counts multiplied with the elevation increase. The corrosion style of the steel in the inoculated environment was mainly localized corrosion. As a consequence of the higher bacteria number, the corrosion rate, pit depth and corrosion product thickness collectively enhanced. Pseudomonas sp. significantly accelerated uniform and localized corrosion of the steel in the marine tidal zone, and the acceleration role enhanced with the steel elevation in the tidal zones.

H2 Is a Major Intermediate in Desulfovibrio vulgaris Corrosion of Iron

Desulfovibrio vulgaris has been a primary pure culture sulfate reducer for developing microbial corrosion concepts. Multiple mechanisms for how it accepts electrons from Fe⁰ have been proposed. We investigated Fe⁰ oxidation with a mutant of D. vulgaris in which hydrogenase genes were deleted. The hydrogenase mutant grew as well as the parental strain with lactate as the electron donor, but unlike the parental strain, it was not able to grow on H₂. The parental strain reduced sulfate with Fe⁰ as the sole electron donor, but the hydrogenase mutant did not. H₂ accumulated over time in Fe⁰ cultures of the hydrogenase mutant and sterile controls but not in parental strain cultures. Sulfide stimulated H₂ production in uninoculated controls apparently by both reacting with Fe⁰ to generate H₂ and facilitating electron transfer from Fe⁰ to H⁺. Parental strain supernatants did not accelerate H₂ production from Fe⁰, ruling out a role for extracellular hydrogenases. Previously proposed electron transfer between Fe⁰ and D. vulgaris via soluble electron shuttles was not evident. The hydrogenase mutant did not reduce sulfate in the presence of Fe⁰ and either riboflavin or anthraquinone-2,6-disulfonate, and these potential electron shuttles did not stimulate parental strain sulfate reduction with Fe⁰ as the electron donor. The results demonstrate that D. vulgaris primarily accepts electrons from Fe⁰ via H₂ as an intermediary electron carrier. These findings clarify the interpretation of previous D. vulgaris corrosion studies and suggest that H₂-mediated electron transfer is an important mechanism for iron corrosion under sulfate-reducing conditions. IMPORTANCE Microbial corrosion of iron in the presence of sulfate-reducing microorganisms is economically significant. There is substantial debate over how microbes accelerate iron corrosion. Tools for genetic manipulation have only been developed for a few Fe(III)-reducing and methanogenic microorganisms known to corrode iron and in each case those microbes were found to accept electrons from Fe⁰ via direct electron transfer. However, iron corrosion is often most intense in the presence of sulfate-reducing microbes. The finding that Desulfovibrio vulgaris relies on H₂ to shuttle electrons between Fe⁰ and cells revives the concept, developed in some of the earliest studies on microbial corrosion, that sulfate reducers consumption of H₂ is a major microbial corrosion mechanism. The results further emphasize that direct Fe⁰-to-microbe electron transfer has yet to be rigorously demonstrated in sulfate-reducing microbes.

Cytochrome-mediated direct electron uptake from metallic iron by Methanosarcina acetivorans

Methane-producing microorganisms accelerate the corrosion of iron-containing metals. Previous studies have inferred that some methanogens might directly accept electrons from Fe(0), but when this possibility was more intensively investigated, H2 was shown to be an intermediary electron carrier between Fe(0) and methanogens. Here, we report that Methanosarcina acetivorans catalyzes direct metal-to-microbe electron transfer to support methane production. Deletion of the gene for the multiheme, outer-surface c-type cytochrome MmcA eliminated methane production from Fe(0), consistent with the key role of MmcA in other forms of extracellular electron exchange. These findings, coupled with the previous demonstration that outer-surface c-type cytochromes are also electrical contacts for electron uptake from Fe(0) by Geobacter and Shewanella species, suggest that the presence of multiheme c-type cytochromes on corrosion surfaces might be diagnostic for direct metal-to-microbe electron transfer and that interfering with cytochrome function might be a strategy to mitigate corrosion.

Microbiologically Influenced Corrosion of Q235 Carbon Steel by Ectothiorhodospira sp

The biological sulfur cycle is closely related to iron corrosion in the natural environment. The effect of the sulfur-oxidising bacterium Ectothiorhodospira sp., named PHS-Q, on the metal corrosion behaviour rarely has been investigated. In this study, the corrosion mechanism of Q235 carbon steel in a PHS-Q-inoculated medium is discussed via the characterization of the morphology and the composition of the corrosion products, the measurement of local corrosion and the investigation of its electrochemical behaviour. The results suggested that, initially, PHS-Q assimilates sulfate to produce H₂S directly or indirectly in the medium without sulfide. H₂S reacts with Fe²⁺ to form an inert film on the coupon surface. Then, in localised areas, bacteria adhere to the reaction product and use the oxidation of FeS as a hydrogen donor. This process leads to a large cathode and a small anode, which incurs pitting corrosion. Consequently, the effect of PHS-Q on carbon steel corrosion behaviour is crucial in an anaerobic environment.

Special Issue: Environmental Corrosion of Metals and Its Prevention: An Overview and Introduction to the Special Issue

Corrosion is a natural process of deterioration and an extremely costly problem [...]

Extracellular Polymeric Substances and Biocorrosion/Biofouling: Recent Advances and Future Perspectives

Microbial cells secrete extracellular polymeric substances (EPS) to adhere to material surfaces, if they get in contact with solid materials such as metals. After phase equilibrium, microorganisms can adhere firmly to the metal surfaces causing metal dissolution and corrosion. Attachment and adhesion of microorganisms via EPS increase the possibility and the rate of metal corrosion. Many components of EPS are electrochemical and redox active, making them closely related to metal corrosion. Functional groups in EPS have specific adsorption ability, causing them to play a key role in biocorrosion. This review emphasizes EPS properties related to metal corrosion and protection and the underlying microbially influenced corrosion (MIC) mechanisms. Future perspectives regarding a comprehensive study of MIC mechanisms and green methodologies for corrosion protection are provided.