Electrochemical characterization of direct electron uptake in electrical microbially influenced corrosion of iron by the lithoautotrophic SRB Desulfopila corrodens strain IS4

Effects of marine eutrophication environment on microbial corrosion: A review

Metal materials undergo severe corrosion in eutrophic environments. The effect of DO decay stimulated by high concentrations of nitrogen and phosphorus pollutants on microorganisms leads to the coupling of electrochemical and microbial corrosion processes. However, there are few studies on microbial corrosion mechanisms in eutrophic environments. This article discusses the corrosive factors of marine eutrophication, summarizes the impact of marine eutrophication on microbial corrosion and the potential mechanisms, including aerobic biofilm corrosion, aerobic & anaerobic mixed biofilm corrosion, and anaerobic microbial electron transfer corrosion, and expounds on the research methods for microbial corrosion of materials serving in estuarine areas prone to pollution. Microbial prevention and control, such as nutrient restriction and microbial interspecies competition, are of research value in the field of green protection. Microbial corrosion mechanisms studies in marine eutrophication environments are significant for environment monitor development, water intake and algae control technologies, and corrosion protection in polluted environments.

Effect of yeast extract on microbiologically influenced corrosion of X70 pipeline steel by Desulfovibrio bizertensis SY-1

Microbiologically influenced corrosion (MIC) is a complicated process that happens ubiquitously and quietly in many fields. As a useful nutritional ingredient in microbial culture media, yeast extract (YE) is a routinely added in the MIC field. However, how the YE participated in MIC is not fully clarified. In the present work, the effect of YE on the growth of sulfate reducing prokaryotes (SRP) Desulfovibrio bizertensis SY-1 and corrosion behavior of X70 pipeline steel were studied. It was found that the weight loss of steel coupons in sterile media was doubled when YE was removed from culture media. However, in the SRP assays without YE the number of planktonic cells decreased, but the attachment of bacteria on steel surfaces was enhanced significantly. Besides, the corrosion rate of steel in SRP assays increased fourfold after removing YE from culture media. MIC was not determined for assays with planktonic SRP but only for biofilm assays. The results confirm the effect of YE on D. bizertensis SY-1 growth and also the inhibitory role of YE on MIC.

Iron corrosion concomitant with nitrate reduction by Iodidimonas nitroreducens sp. nov. isolated from iodide-rich brine associated with natural gas

Microbially influenced corrosion (MIC) may contribute significantly to corrosion-related failures in injection wells and iron pipes of iodine production facilities. In this study, the iron (Fe0) corroding activity of strain Q-1 isolated from iodide-rich brine in Japan and two Iodidimonas strains phylogenetically related to strain Q-1 were investigated under various culture conditions. Under aerobic conditions, the Fe0 foil in the culture of strain Q-1 was oxidized in the presence of nitrate and yeast extract, while those of two Iodidimonas strains were not. The amount of oxidized iron in this culture was six times higher than in the aseptic control. Oxidation of Fe0 in aerobic cultures of nitrate-reducing bacterium Q-1 was dependent on the formation of nitrite from nitrate. This Fe0 corrosion by nitrate-reducing bacterium Q-1 started after initial nitrite accumulation by day 4. Nitrate reduction in strain Q-1 is a unique feature that distinguishes it from two known species of Iodidimonas. Nitrite accumulation was supported by the encoding of genes for nitrate reductase and the missing of genes for nitrite reduction to ammonia or nitrogen gas in its genome sequence. Phylogenetic position of strain Q-1 based on the 16S rRNA gene sequence was with less than 96.1% sequence similarity to two known Iodidimonas species, and digital DNA-DNA hybridization (dDDH) values of 17.2-19.3%, and average nucleotide identity (ANI) values of 73.4-73.7% distinguished strain Q-1 from two known species. In addition of nitrate reduction, the ability to hydrolyze aesculin and gelatin hydrolysis and cellular fatty acid profiles also distinguished strain Q-1 from two known species. Consequently, a new species, named Iodidimonas nitroreducens sp. nov., is proposed for the nitrate-reducing bacterium strain Q-1T.

Putting the electro-bugs to work: A systematic review of 22 years of advances in bio-electrochemical systems and the parameters governing their performance

Wastewater treatment using bioelectrochemical systems (BESs) can be considered as a technology finding application in versatile areas such as for renewable energy production and simultaneous reducing environmental problems, biosensors, and bioelectrosynthesis. This review paper reports and critically discusses the challenges, and advances in bio-electrochemical studies in the 21st century. To sum and critically analyze the strides of the last 20+ years on the topic, this study first provides a comprehensive analysis on the structure, performance, and application of BESs, which include Microbial Fuel Cells (MFCs), Microbial Electrolysis Cells (MECs) and Microbial Desalination Cells (MDCs). We focus on the effect of various parameters, such as electroactive microbial community structure, electrode material, configuration of bioreactors, anode unit volume, membrane type, initial COD, co-substrates and the nature of the input wastewater in treatment process and the amount of energy and fuel production, with the purpose of showcasing the modes of operation as a guide for future studies. The results of this review show that the BES have great potential in reducing environmental pollution, purifying saltwater, and producing energy and fuel. At a larger scale, it aspires to facilitate the path of achieving sustainable development and practical application of BES in real-world scenarios.

Interactions among microorganisms functionally active for electron transfer and pollutant degradation in natural environments

Compared to single microbial strains, complex interactions between microbial consortia composed of various microorganisms have been shown to be effective in expanding ecological functions and accomplishing biological processes. Electroactive microorganisms (EMs) and degradable microorganisms (DMs) play vital roles in bioenergy production and the degradation of organic pollutants hazardous to human health. These microorganisms can strongly interact with other microorganisms and promote metabolic cooperation, thus facilitating electricity production and pollutant degradation. In this review, we describe several specific types of EMs and DMs based on their ability to adapt to different environments, and summarize the mechanism of EMs in extracellular electron transfer. The effects of interactions between EMs and DMs are evaluated in terms of electricity production and degradation efficiency. The principle of the enhancement in microbial consortia is also introduced, such as improved biomass, changed degradation pathways, and biocatalytic potentials, which are directly or indirectly conducive to human health.

Bioconversion of Glycerol to 1,3-Propanediol Using Klebsiella pneumoniae L17 with the Microbially Influenced Corrosion of Zero-Valent Iron

The bacterial redox state is essential for controlling the titer and yield of the final metabolites in most bioconversion processes. Glycerol conversion to 1,3-propanediol (PDO) requires a large amount of reducing equivalent and the expression of reductive pathways. Zero-valent iron (ZVI) was used in the glycerol bioconversion of Klebsiella pneumoniae L17. The level of 1,3-PDO production increased with the oxidation of ZVI (31.8 ± 1.2 vs. 25.7 ± 0.5, ZVI vs. no ZVI) while the cellular NADH/NAD+ level increased (0.6 vs. 0.3, ZVI vs. no ZVI). X-ray diffraction showed that the iron oxide (Fe2O3) was formed during glycerol fermentation. L17 obtained electrons from ZVI and dissolved the iron continuously to form cracks on the surface, suggesting microbially influenced corrosion (MIC) was involved on the surface of ZVI. The ZVI-implemented fermentation shifted bioconversion to a more glycerol-reductive pathway. The qPCR-presented glycerol dehydratase (DhaB) with ZVI implementation was strongly expressed compared to the control. These results suggest that ZVI can contribute to the biotransformation of PDO by inducing intracellular metabolic shifts. This study could also suggest a novel microbial fermentation strategy with the application of MIC.

Soluble Iron Enhances Extracellular Electron Uptake by Shewanella oneidensis MR-1

Extracellular electron transfer (EET) is a process that microorganisms use to reduce or oxidize external insoluble electron acceptors or donors. Much of our mechanistic understanding of this process is derived from studies of transmembrane cytochrome complexes and extracellular redox shuttles that mediate outward EET to anodes and external electron acceptors. In contrast, there are knowledge gaps concerning the reverse process of inward EET from external electron donors to cells. Here, we describe a role for soluble iron (exogenous FeCl2) in enhancing EET from cathodes to the model EET bacterium Shewanella oneidensis MR-1, with fumarate serving as the intracellular electron acceptor. This iron concentration-dependent electron uptake was eradicated upon addition of an iron chelator and occurred only in the presence of fumarate reductase, confirming an electron pathway from cathodes to this periplasmic enzyme. Moreover, S. oneidensis mutants lacking specific outer membrane and periplasmic cytochromes exhibited significantly decreased current levels relative to wild-type. These results indicate that soluble iron can function as an electron carrier to the EET machinery of S. oneidensis.

Text-Mining to Identify Gene Sets Involved in Biocorrosion by Sulfate-Reducing Bacteria: A Semi-Automated Workflow

A significant amount of literature is available on biocorrosion, which makes manual extraction of crucial information such as genes and proteins a laborious task. Despite the fast growth of biology related corrosion studies, there is a limited number of gene collections relating to the corrosion process (biocorrosion). Text mining offers a potential solution by automatically extracting the essential information from unstructured text. We present a text mining workflow that extracts biocorrosion associated genes/proteins in sulfate-reducing bacteria (SRB) from literature databases (e.g., PubMed and PMC). This semi-automatic workflow is built with the Named Entity Recognition (NER) method and Convolutional Neural Network (CNN) model. With PubMed and PMCID as inputs, the workflow identified 227 genes belonging to several Desulfovibrio species. To validate their functions, Gene Ontology (GO) enrichment and biological network analysis was performed using UniprotKB and STRING-DB, respectively. The GO analysis showed that metal ion binding, sulfur binding, and electron transport were among the principal molecular functions. Furthermore, the biological network analysis generated three interlinked clusters containing genes involved in metal ion binding, cellular respiration, and electron transfer, which suggests the involvement of the extracted gene set in biocorrosion. Finally, the dataset was validated through manual curation, yielding a similar set of genes as our workflow; among these, hysB and hydA, and sat and dsrB were identified as the metal ion binding and sulfur metabolism genes, respectively. The identified genes were mapped with the pangenome of 63 SRB genomes that yielded the distribution of these genes across 63 SRB based on the amino acid sequence similarity and were further categorized as core and accessory gene families. SRB's role in biocorrosion involves the transfer of electrons from the metal surface via a hydrogen medium to the sulfate reduction pathway. Therefore, genes encoding hydrogenases and cytochromes might be participating in removing hydrogen from the metals through electron transfer. Moreover, the production of corrosive sulfide from the sulfur metabolism indirectly contributes to the localized pitting of the metals. After the corroboration of text mining results with SRB biocorrosion mechanisms, we suggest that the text mining framework could be utilized for genes/proteins extraction and significantly reduce the manual curation time.

Toxicity of tire wear particles and the leachates to microorganisms in marine sediments

Tire wear particles (TWPs), which are among the microplastic pollutants in the environment, can inevitably accumulate in coastal sediments. The present study comprehensively investigated the effect of pristine TWPs on bacterial community structure in coastal sediments and compared the effect of pristine TWPs and aged TWPs on nine strains of bacteria in sediments. In addition, the effect of the TWP leachate was studied with all the nine bacterial strains and the toxicity-causing substances in the leachate was investigated using Bacillus subtilis. Exposure to TWPs could lead to a shift in bacteria community and affect nitrogen metabolism in marine sediments. Aged TWPs were more toxic than pristine TWPs due to changes in particle surface characteristics. The leachate exhibited greater toxicity than TWPs as well, and Zn was identified to be the major toxicity-causing substance. The overall results of this study are important for understanding the effects of TWPs and the leachates on microorganisms in marine sediments.

Photocatalytic performance of an α-Fe2O3 electrode and its effects on the growth and metabolism of Citrobacter freundii

Electronic exchanges occur between semiconductor minerals and microorganisms. However, researchers have focused on the photocatalytic degradation of pollutants by semiconductor minerals, and there is a limited amount of studies on semiconductor photogenerated electrons that influence the growth and energetic mechanisms of bacteria. Bioelectrochemical systems (BES) are important new bioengineering technologies for investigating the mechanisms by which bacteria absorb electrons. In this work, we built a BES that used α-Fe₂O₃ nanorods as a photoanode and Citrobacter freundii as bio-cathode bacteria to explore the effect of photoelectrons on C. freundii growth and metabolism. The photoanode was prepared by a hydrothermal synthesis method. As confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the photoanode was made of α-Fe₂O₃. Corresponding scanning electron microscope (SEM) images showed that α-Fe₂O₃ nanorod arrays formed with a diameter of 50 nm, and the band gap was 2.03 eV, as indicated by UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The C. freundii growth metabolism changed significantly because of photoelectrons; under light conditions, the growth rate of C. freundii significantly accelerated, and as inferred from the three-dimensional fluorescence spectrum, the protein, humic acid, and NADH concentrations were significantly higher at 72 h. According to the changes in the organic acid content, photoelectrons participated in the reductive tricarboxylic acid cycle (rTCA) to enhance growth and metabolism. The results of the study have broad implications for advancing fields that study the effects of semiconductor minerals on electroactive microorganisms and the semiconductor-photoelectronic transport mechanisms of electroautotrophic microorganisms. KEY POINTS: • For the first time, A BES was built that used α-Fe2O3 nanorods as a photoanode and C. freundii as a bio-cathode bacteria. • Photoelectrons produced by α-Fe2O3 photoelectrode promote the growth of C. freundii. • Effects of photoelectrons on C. freundii metabolism were conjectured by the changes of organic acids and NADH.

Desulfoferrobacter suflitae gen. nov., sp. nov., a novel sulphate-reducing bacterium in the Deltaproteobacteria capable of autotrophic growth with hydrogen or elemental iron

A mesophilic sulphate-reducing micro-organism, able to grow chemolithoautotrophically with H2/CO2 (20 : 80) and with elemental iron as a sole electron donor, was isolated from a consortium capable of degrading long-chain paraffins and designated strain DRH4T. Cells were oval shaped often with bright refractile cores and occurred singly or in pairs. The cells formed pili. Strain DRH4T could grow chemolithoautotrophically with H2/CO2 or elemental iron and chemoorganotrophically utilizing a number of organic substrates, such as fatty acids from formate to octanoate (C1–C8). Sulphate and thiosulphate served as terminal electron acceptors, but sulphite and nitrate did not. Optimal growth was observed from 37 to 40 °C and pH from 6.5 to 7.2. Strain DRH4T did not require NaCl for growth and could proliferate under a broad range of salinities from freshwater (1 g l−1 NaCl) to seawater (27 g l−1 NaCl) conditions. The genomic DNA G+C content was 54.46 mol %. Based on 16S rRNA gene sequence analysis. strain DRH4T was distinct from previously described Deltaproteobacteria species exhibiting the closest affiliation to Desulforhabdus amnigena ASRB1T, Syntrophobacterium sulfatireducens TB8106T and Desulfovirga adipica 12016T with 93.35, 93.42 and 92.85 % similarity, respectively. Strain DRH4T showed significant physiological differences with the aforementioned organisms. Based on physiological differences and phylogenetic comparisons, we propose to classify DRH4T as the type strain (=DSM 113 455T=JCM 39 248T) of a novel species of a new genus with the name Desulfoferrobacter suflitae gen. nov., sp. nov.

Enhancing the resistance to H2S toxicity during anaerobic digestion of low-strength wastewater through granular activated carbon (GAC) addition

Low-strength wastewater was treated using two laboratory-scale up-flow anaerobic sludge blankets (UASB) for 130 days under sulfate-reducing conditions. Granular activated carbon (GAC) was added to one of the reactors. The GAC addition increased the total chemical oxygen demand removal by 21-28% and total methane production by 32-78%. The sludge from the GAC-amended UASB showed higher specific methanogenic activities (SMA) and higher activities in the presence of H₂S, indicating that the GAC addition enhanced the resistance of methanogens to H₂S toxicity. Further, the microbial communities showed that the GAC addition shifted microbial communities. A robust syntrophic partnership between bacteria (i.e., Bacteroidetes_vadinHA17 and Trichococcus) and methanogens was established in the GAC-amended UASB. Sulfate-reducing bacteria (SRB) were enriched in the GAC biofilm, indicating the coexistence of competition and cooperation between SRB and methanogens. These findings provide significant insights regarding microbial community dynamics, especially SRB and methanogens, in a GAC-amended anaerobic digestion process under sulfate-reducing conditions.

Defined and unknown roles of conductive nanoparticles for the enhancement of microbial current generation: A review

The ability of various bacteria to make use of solid substrates through extracellular electron transfer (EET) or extracellular electron uptake (EEU) has enabled the development of valuable biotechnologies such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). It is common practice to use metallic and semiconductive nanoparticles (NPs) for microbial current enhancement. However, the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. In the present review, the proposed mechanisms for enhancing current production in MFCs and MES are summarized, and the critical factors for NPs to enhance microbial current generation are discussed. Implications for microbially induced iron corrosion, where iron sulfide NPs are proposed to enhance the rate of EEU, photochemically driven MES, and several future research directions to further enhance microbial current generation, are also discussed.

Electrotrophy: Other microbial species, iron, and electrodes as electron donors for microbial respirations

Electrotrophy, the growth of microbes on extracellular electron donors, drives important biogeochemical cycles and has practical applications. Studies of Fe(II)-based electrotrophy have provided foundational cytochrome-based mechanistic models for electron transport into cells. Direct electron uptake from other microbial species, Fe(0), or cathodes is of intense interest due to its potential roles in the production and anaerobic oxidation of methane, corrosion, and bioelectrochemical technologies. Other cells or Fe(0) can serve as the sole electron donor supporting the growth of several Geobacter and methanogen strains that are unable to use H₂ as an electron donor, providing strong evidence for electrotrophy. Additional evidence for electrotrophy in Geobacter strains and Methanosarcina acetivorans is a requirement for outer-surface c-type cytochromes. However, in most instances claims for electrotrophy in anaerobes are based on indirect inference and the possibility that H₂ is actually the electron donor supporting growth has not been rigorously excluded.

Extracellular Electron Exchange Capabilities of Desulfovibrio ferrophilus and Desulfopila corrodens

Microbial extracellular electron transfer plays an important role in diverse biogeochemical cycles, metal corrosion, bioelectrochemical technologies, and anaerobic digestion. Evaluation of electron uptake from pure Fe(0) and stainless steel indicated that, in contrast to previous speculation in the literature, Desulfovibrio ferrophilus and Desulfopila corrodens are not able to directly extract electrons from solid-phase electron-donating surfaces. D. ferrophilus grew with Fe(III) as the electron acceptor, but Dp. corrodens did not. D. ferrophilus reduced Fe(III) oxide occluded within porous alginate beads, suggesting that it released a soluble electron shuttle to promote Fe(III) oxide reduction. Conductive atomic force microscopy revealed that the D. ferrophilus pili are electrically conductive and the expression of a gene encoding an aromatics-rich putative pilin was upregulated during growth on Fe(III) oxide. The expression of genes for multi-heme c-type cytochromes was not upregulated during growth with Fe(III) as the electron acceptor, and genes for a porin-cytochrome conduit across the outer membrane were not apparent in the genome. The results suggest that D. ferrophilus has adopted a novel combination of strategies to enable extracellular electron transport, which may be of biogeochemical and technological significance.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Metagenomic and Metatranscriptomic Characterization of a Microbial Community That Catalyzes Both Energy-Generating and Energy-Storing Electrode Reactions

Electroactive bacteria are living catalysts, mediating energy-generating reactions at anodes or energy storage reactions at cathodes via extracellular electron transfer (EET). The Cathode-ANode (CANode) biofilm community was recently shown to facilitate both reactions; however, the identities of the primary constituents and underlying molecular mechanisms remain unknown. Here, we used metagenomics and metatranscriptomics to characterize the CANode biofilm. We show that a previously uncharacterized member of the family Desulfobulbaceae, Desulfobulbaceae-2, which had <1% relative abundance, had the highest relative gene expression and accounted for over 60% of all differentially expressed genes. At the anode potential, differential expression of genes for a conserved flavin oxidoreductase (Flx) and heterodisulfide reductase (Hdr) known to be involved in ethanol oxidation suggests a source of electrons for the energy-generating reaction. Genes for sulfate and carbon dioxide reduction pathways were expressed by Desulfobulbaceae-2 at both potentials and are the proposed energy storage reactions. Reduction reactions may be mediated by direct electron uptake from the electrode or from hydrogen generated at the cathode potential. The Desulfobulbaceae-2 genome is predicted to encode at least 85 multiheme (≥3 hemes) c-type cytochromes, some with as many as 26 heme-binding domains, that could facilitate reversible electron transfer with the electrode. Gene expression in other CANode biofilm species was also affected by the electrode potential, although to a lesser extent, and we cannot rule out their contribution to observed current. Results provide evidence of gene expression linked to energy storage and energy-generating reactions and will enable development of the CANode biofilm as a microbially driven rechargeable battery. IMPORTANCE Microbial electrochemical technologies (METs) rely on electroactive bacteria to catalyze energy-generating and energy storage reactions at electrodes. Known electroactive bacteria are not equally capable of both reactions, and METs are typically configured to be unidirectional. Here, we report on genomic and transcriptomic characterization of a recently described microbial electrode community called the Cathode-ANode (CANode). The CANode community is able to generate or store electrical current based on the electrode potential. During periods where energy is not needed, electrons generated from a renewable source, such as solar power, could be converted into energy storage compounds to later be reversibly oxidized by the same microbial catalyst. Thus, the CANode system can be thought of as a living "rechargeable battery." Results show that a single organism may be responsible for both reactions demonstrating a new paradigm for electroactive bacteria.

Gene Sets and Mechanisms of Sulfate-Reducing Bacteria Biofilm Formation and Quorum Sensing With Impact on Corrosion

Sulfate-reducing bacteria (SRB) have a unique ability to respire under anaerobic conditions using sulfate as a terminal electron acceptor, reducing it to hydrogen sulfide. SRB thrives in many natural environments (freshwater sediments and salty marshes), deep subsurface environments (oil wells and hydrothermal vents), and processing facilities in an industrial setting. Owing to their ability to alter the physicochemical properties of underlying metals, SRB can induce fouling, corrosion, and pipeline clogging challenges. Indigenous SRB causes oil souring and associated product loss and, subsequently, the abandonment of impacted oil wells. The sessile cells in biofilms are 1,000 times more resistant to biocides and induce 100-fold greater corrosion than their planktonic counterparts. To effectively combat the challenges posed by SRB, it is essential to understand their molecular mechanisms of biofilm formation and corrosion. Here, we examine the critical genes involved in biofilm formation and microbiologically influenced corrosion and categorize them into various functional categories. The current effort also discusses chemical and biological methods for controlling the SRB biofilms. Finally, we highlight the importance of surface engineering approaches for controlling biofilm formation on underlying metal surfaces.

Synergistic effect of carbon starvation and exogenous redox mediators on corrosion of X70 pipeline steel induced by Desulfovibrio singaporenus

In microbiologically influenced corrosion (MIC) induced by sulfate-reducing bacteria (SRB), the electrons released from iron were transferred via extracellular electron transfer (EET) to the inner cells. Electron mediators and carbon starvation have also been found to promote steel corrosion. This study aimed to investigate the synergistic effects of electron mediators and carbon starvation on MIC and their effect on biofilm catalytic activity. The results demonstrated that the weight losses of X70 steel were 0.68 and 1.03 mg/cm² in 100% and 10% carbon source (CS) SRB solution, respectively. The addition of riboflavin and cytochrome c increased the corrosion rate by 1.76 and 1.87 times, respectively, in the 100% CS SRB medium compared to the medium without exogenous redox mediators. For the 10% CS SRB medium, the corrosion rate increased by 1.40 and 1.89 times, respectively, when riboflavin and cytochrome c were added. The addition of riboflavin and cytochrome c also enhanced the biocatalytic activity of the SRB biofilm in both the 100% and 10% CS SRB media.

Nitrite as a causal factor for nitrate-dependent anaerobic corrosion of metallic iron induced by Prolixibacter strains

Microbially influenced corrosion (MIC) may contribute significantly to overall corrosion risks, especially in the gas and petroleum industries. In this study, we isolated four Prolixibacter strains, which belong to the phylum Bacteroidetes, and examined their nitrate respiration- and Fe0 -corroding activities, together with two previously isolated Prolixibacter strains. Four of the six Prolixibacter strains reduced nitrate under anaerobic conditions, while the other two strains did not. The anaerobic growth of the four nitrate-reducing strains was enhanced by nitrate, which was not observed in the two strains unable to reduce nitrate. When the nitrate-reducing strains were grown anaerobically in the presence of Fe0 or carbon steel, the corrosion of the materials was enhanced by more than 20-fold compared to that in aseptic controls. This enhancement was not observed in cultures of the strains unable to reduce nitrate. The oxidation of Fe0 in the anaerobic cultures of nitrate-reducing strains occurred concomitantly with the formation of nitrite. Since nitrite chemically oxidized Fe0 under anaerobic and aseptic conditions, the corrosion of Fe0 - and carbon steel by the nitrate-reducing Prolixibacter strains was deduced to be mainly enhanced via the biological reduction of nitrate to nitrite, followed by the chemical oxidation of Fe0 to Fe2+ and Fe3+ coupled to the reduction of nitrite.

Analysis of a Methanogen and an Actinobacterium Dominating the Thermophilic Microbial Community of an Electromethanogenic Biocathode

Electromethanogenesis refers to the bioelectrochemical synthesis of methane from CO₂ by biocathodes. In an electromethanogenic system using thermophilic microorganisms, metagenomic analysis along with quantitative real-time polymerase chain reaction and fluorescence in situ hybridization revealed that the biocathode microbiota was dominated by the methanogen Methanothermobacter sp. strain EMTCatA1 and the actinobacterium Coriobacteriaceae sp. strain EMTCatB1. RNA sequencing was used to compare the transcriptome profiles of each strain at the methane-producing biocathodes with those in an open circuit and with the methanogenesis inhibitor 2-bromoethanesulfonate (BrES). For the methanogen, genes related to hydrogenotrophic methanogenesis were highly expressed in a manner similar to those observed under H₂-limited conditions. For the actinobacterium, the expression profiles of genes encoding multiheme c-type cytochromes and membrane-bound oxidoreductases suggested that the actinobacterium directly takes up electrons from the electrode. In both strains, various stress-related genes were commonly induced in the open-circuit biocathodes and biocathodes with BrES. This study provides a molecular inventory of the dominant species of an electromethanogenic biocathode with functional insights and therefore represents the first multiomics characterization of an electromethanogenic biocathode.

Microbial corrosion of metals: The corrosion microbiome

Microbially catalyzed corrosion of metals is a substantial economic concern. Aerobic microbes primarily enhance Fe⁰ oxidation through indirect mechanisms and their impact appears to be limited compared to anaerobic microbes. Several anaerobic mechanisms are known to accelerate Fe⁰ oxidation. Microbes can consume H₂ abiotically generated from the oxidation of Fe⁰. Microbial H₂ removal makes continued Fe⁰ oxidation more thermodynamically favorable. Extracellular hydrogenases further accelerate Fe⁰ oxidation. Organic electron shuttles such as flavins, phenazines, and possibly humic substances may replace H₂ as the electron carrier between Fe⁰ and cells. Direct Fe⁰-to-microbe electron transfer is also possible. Which of these anaerobic mechanisms predominates in model pure culture isolates is typically poorly documented because of a lack of functional genetic studies. Microbial mechanisms for Fe⁰ oxidation may also apply to some other metals. An ultimate goal of microbial metal corrosion research is to develop molecular tools to diagnose the occurrence, mechanisms, and rates of metal corrosion to guide the implementation of the most effective mitigation strategies. A systems biology approach that includes innovative isolation and characterization methods, as well as functional genomic investigations, will be required in order to identify the diagnostic features to be gleaned from meta-omic analysis of corroding materials. A better understanding of microbial metal corrosion mechanisms is expected to lead to new corrosion mitigation strategies. The understanding of the corrosion microbiome is clearly in its infancy, but interdisciplinary electrochemical, microbiological, and molecular tools are available to make rapid progress in this field.

Interplay of microbial communities with mineral environments in coralline algae

Coralline algae are worldwide carbonate builders, considered to be foundational species and biodiversity hotspots. Coralline habitats face increasing pressure from human activities and effects related to Global Change, yet their ecological properties and adaptive responses remain poorly understood. The relationships of the algal microbiota with the mineral bioconstructions, as well as plasticity and resilience of coralline holobionts in a changing environment, are of particular interest. In the Gulf of California, Neogoniolithon trichotomum (Rhodophyta) is the main carbonate builder in tidal pools. We performed a multi-disciplinary assessment of the N. trichotomum microstructure using XRD, SEM microscopy and SR-FTIR spectromicroscopy. In the algal perithallus, magnesium-calcite and aragonite were spatially segregated and embedded in a polysaccharide matrix (rich in sulfated polysaccharides). Mg-calcites (18-19 mol% Mg) were the main mineral components of the thallus overall, followed by iron carbonates related to dolomite (ankerite) and siderite. Minerals of late evaporitic sequences (sylvite and bischofite) were also present, suggesting potential halophilic microenvironments within the algal thalli. The diverse set of abundant halophilic, halotolerant and oligotrophic taxa, whose abundance increase in the summer, further suggests this condition. We created an integrated model, based on environmental parameters and the microbiota distribution, that identified temperature and nutrient availability (particularly nitrate and silicate) as the main parameters related to specific taxa patterns. Among these, Hahella, Granulossicoccus, Ferrimonas, Spongiibacteraceae and cyanobacterial Xenococcaceae and Nostocaceae change significantly between seasons. These bacterial components might play relevant roles in algal plasticity and adaptive responses to a changing environment. This study contributes to the understanding of the interplay of the prokaryotic microbiota with the mineral microenvironments of coralline algae. Because of their carbonates with potential resistance to dissolution in a higher pCO₂ world and their seasonally dynamic bacteria, coralline algae are relevant targets to study coastal resilience and carbonated systems responses to changing environments.

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces

Electrochemically active bacteria can transport their metabolically generated electrons to anodes, or accept electrons from cathodes to synthesize high-value chemicals and fuels, via a process known as extracellular electron transfer (EET). Harnessing of this microbial EET process has led to the development of microbial bio-electrochemical systems (BESs), which can achieve the interconversion of electrical and chemical energy and enable electricity generation, hydrogen production, electrosynthesis, wastewater treatment, desalination, water and soil remediation, and sensing. Here, the focus is on the current understanding of the microbial EET process occurring at both the bacteria-electrode interface and the biotic interface, as well as some attempts to improve the EET by using various nanomaterials. The behavior of nanomaterials in different EET routes and their influence on the performance of BESs are described. The inherent mechanisms will guide rational design of EET-related materials and lead to a better understanding of EET mechanisms.

Oil field microorganisms cause highly localized corrosion on chemically inhibited carbon steel

Carbon steel pipelines, a means for crude oil transportation, occasionally experience highly localized perforation caused by microorganisms. While microorganisms grown in laboratory culture tend to corrode steel specimens unevenly, they rarely inflict a corrosion morphology consistent with that of pipelines, where centimetre-sized corrosion features are randomly distributed within vast stretches of otherwise pristine metal surface. In this study, we observed that corrosion inhibitors (CIs), widely used for the control of acid gas (H2 S, CO2 ) corrosion in oil fields, also affect microbial growth and activity. Inhibited carbon steel resisted biofilm formation and underwent negligible corrosion (< 0.002 mm Fe0 year-1 ), despite 15 months of exposure to oil field waters harbouring a diverse microbiome. In contrast, physical scavenging of CI in these waters led to severe and highly localized corrosion (up to 0.93 mm Fe0 year-1 ) underneath biofilms dominated by methanogenic archaea and sulfate-reducing bacteria. A sharp decline in CI concentration, as well as its active components, quaternary ammonium compounds (QACs), correlated with microbial sulfidogenesis. CIs are ubiquitously present in oil field waters and play an underappreciated role in microbial corrosion mitigation. Physical and biological scavenging of CIs may create local differences in steel inhibition effectiveness and thus result in highly localized corrosion.

Tungstate Control of Microbial Sulfidogenesis and Souring of the Engineered Environment

Sulfide accumulation in oil reservoir fluids (souring) from the activity of sulfate-reducing microorganisms (SRM) is of grave concern because of the associated health and facility failure risks. Here, we present an assessment of tungstate as a selective and potent inhibitor of SRM. Dose-response inhibitor experiments were conducted with a number of SRM isolates and enrichments at 30-80 °C and an increase in the effectiveness of tungstate treatment at higher temperatures was observed. To explore mixed inhibitor treatment modes, we tested synergy or antagonism between several inhibitors with tungstate, and found synergism between WO₄^2- and NO₂^-, while additive effects were observed with ClO₄^- and NO₃^-. We also evaluated SRM inhibition by tungstate in advective upflow oil-sand-packed columns. Although 2 mM tungstate was initially sufficient to inhibit sulfidogenesis, subsequent temporal CaWO₄ precipitation resulted in loss of the bioavailable inhibitor from solution and a concurrent increase in effluent sulfide. Mixing 4 mM sodium carbonate with the 2 mM tungstate was enough to promote tungstate solubility to reach inhibitory concentrations, without precipitation, and completely inhibit SRM activity. Overall, we demonstrate the effectiveness of tungstate as a potent SRM inhibitor, particularly at higher temperatures, and propose a novel carbonate-tungstate formulation for application to soured oil reservoirs.

Galvanic Corrosion Due to a Heterogeneous Sulfate Reducing Bacteria Biofilm

In this work, the galvanic corrosion behavior of sulfate reducing Desulfotomaculum nigrificans biofilm-covered and uncovered carbon steel was investigated using various electrochemical measurements. The results showed that the bare specimen in the abiotic solution functions as the anode; whereas the biofilm-covered specimen in the SRB-containing solution functions as the cathode after two electrodes being coupled. The anodic reaction of specimen in the biotic solution containing SRB was inhibited; whereas the cathodic reaction was considerably promoted after coupling. Hence, localized corrosion of specimen in the abiotic solution was observed due to the galvanic corrosion effect. SRB could still accelerate steel corrosion even after coupling, but the results indicate that the contribution of SRB to steel corrosion decreased. The localized corrosion of steel in the SRB-containing environments not only involved the SRB biofilm, but also a galvanic corrosion effect. The flow of electrons from the anodic dissolution of Fe in the abiotic solution to the SRB cells of cathodic area decreased the acceptance capacity of electrons by SRB from steel beneath biofilm. As a result, the steel corrosion beneath SRB biofilm decreased after coupling.

Enhancement of cell growth by uncoupling extracellular electron uptake and oxidative stress production in sediment sulfate-reducing bacteria

Microbial extracellular electron uptake (EEU) from solid electron donors has critical implications for microbial energy acquisition in energy-limited environments as well as electrochemical microbial technologies. Although EEU supplies sufficient energy to support cellular growth, additional soluble electron donors are required for most microbes to grow on electrode surfaces. Here, we demonstrated that the minimization of exogenous and endogenous oxidative stress greatly enhanced the growth rate of the sediment EEU-capable sulfate-reducing bacterium Desulfovibrio ferrophilus IS5 on an electrode without the addition of a soluble electron donor. Single-cell activity analysis by nanoscale secondary ion mass spectrometry showed that the metabolic activity of IS5 cells on the electrode was significantly enhanced following incubation in an H-type reactor, which was configured to reduce the exposure of cells to the potential oxidative stress source of the Pt counter electrode (CE). Additionally, the highest metabolic activity was observed at an electrode potential of -0.4 V (versus the standard hydrogen electrode), where electron uptake rate was not at peak. Compared to a single-chamber reactor, incubation in an H-type reactor at -0.4 V shortened the cell doubling time by 50-fold, which resulted in sufficient anabolism for cell replication (¹⁵N/N_total > 50%). The production of strongly oxidizing species at the CE was confirmed by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry analyses. Transcriptome analysis revealed overexpression of antioxidative genes in cells incubated at a potential with higher current production. These results suggested that higher levels of endogenous oxidative species were produced by a more reduced electron-transport chain from trace amounts of oxygen in the reactor, thereby lowering cell activity. In conclusion, EEU may enable sediment microbes to undergo enhanced cell growth and to find niches on minerals under anaerobic energy-limited conditions, where oxidative stress is much less likely to be present.

Extracellular electron uptake by autotrophic microbes: physiological, ecological, and evolutionary implications

Microbes exchange electrons with their extracellular environment via direct or indirect means. This exchange is bidirectional and supports essential microbial oxidation–reduction processes, such as respiration and photosynthesis. The microbial capacity to use electrons from insoluble electron donors, such as redox-active minerals, poised electrodes, or even other microbial cells is called extracellular electron uptake (EEU). Autotrophs with this capability can thrive in nutrient and soluble electron donor-deficient environments. As primary producers, autotrophic microbes capable of EEU greatly impact microbial ecology and play important roles in matter and energy flow in the biosphere. In this review, we discuss EEU-driven autotrophic metabolisms, their mechanism and physiology, and highlight their ecological, evolutionary, and biotechnological implications.

Insights into Advancements and Electrons Transfer Mechanisms of Electrogens in Benthic Microbial Fuel Cells

Benthic microbial fuel cells (BMFCs) are a kind of microbial fuel cell (MFC), distinguished by the absence of a membrane. BMFCs are an ecofriendly technology with a prominent role in renewable energy harvesting and the bioremediation of organic pollutants through electrogens. Electrogens act as catalysts to increase the rate of reaction in the anodic chamber, acting in electrons transfer to the cathode. This electron transfer towards the anode can either be direct or indirect using exoelectrogens by oxidizing organic matter. The performance of a BMFC also varies with the types of substrates used, which may be sugar molasses, sucrose, rice paddy, etc. This review presents insights into the use of BMFCs for the bioremediation of pollutants and for renewable energy production via different electron pathways.

Microbial ingress and in vitro degradation enhanced by glucose on bioabsorbable Mg-Li-Ca alloy

Biodegradable magnesium alloys are challenging to be implanted in patients with hyperglycemia and diabetes. A hypothesis is suggested that glucose accelerates microbial ingress and in vitro degradation of Mg-Li-Ca implants. Corrosion resistance and mechanical properties was demonstrated using electrochemical, hydrogen evolution and tensile tests. The bacteria from Hank's solution were isolated via 16S rRNA gene analysis. The results revealed that Mg-1Li-1Ca alloy exhibited different responses to Hank's solution with and without glucose. The solution acidity was ascribed to Microbacterium hominis and Enterobacter xiangfangensis, indicating that glucose promoted microbial activity and degradation and deterioration in mechanical property of Mg-1Li-1Ca alloy.

Electrophysiology of the Facultative Autotrophic Bacterium Desulfosporosinus orientis

Electroautotrophy is a novel and fascinating microbial metabolism, with tremendous potential for CO₂ storage and valorization into chemicals and materials made thereof. Research attention has been devoted toward the characterization of acetogenic and methanogenic electroautotrophs. In contrast, here we characterize the electrophysiology of a sulfate-reducing bacterium, Desulfosporosinus orientis, harboring the Wood-Ljungdahl pathway and, thus, capable of fixing CO₂ into acetyl-CoA. For most electroautotrophs the mode of electron uptake is still not fully clarified. Our electrochemical experiments at different polarization conditions and Fe⁰ corrosion tests point to a H₂- mediated electron uptake ability of this strain. This observation is in line with the lack of outer membrane and periplasmic multi-heme c-type cytochromes in this bacterium. Maximum planktonic biomass production and a maximum sulfate reduction rate of 2 ± 0.4 mM day^–1 were obtained with an applied cathode potential of −900 mV vs. Ag/AgCl, resulting in an electron recovery in sulfate reduction of 37 ± 1.4%. Anaerobic sulfate respiration is more thermodynamically favorable than acetogenesis. Nevertheless, D. orientis strains adapted to sulfate-limiting conditions, could be tuned to electrosynthetic production of up to 8 mM of acetate, which compares well with other electroacetogens. The yield per biomass was very similar to H₂/CO₂ based acetogenesis. Acetate bioelectrosynthesis was confirmed through stable isotope labeling experiments with Na-H¹³CO₃. Our results highlight a great influence of the CO₂ feeding strategy and start-up H₂ level in the catholyte on planktonic biomass growth and acetate production. In serum bottles experiments, D. orientis also generated butyrate, which makes D. orientis even more attractive for bioelectrosynthesis application. A further optimization of these physiological pathways is needed to obtain electrosynthetic butyrate production in D. orientis biocathodes. This study expands the diversity of facultative autotrophs able to perform H₂-mediated extracellular electron uptake in Bioelectrochemical Systems (BES). We characterized a sulfate-reducing and acetogenic bacterium, D. orientis, able to naturally produce acetate and butyrate from CO₂ and H₂. For any future bioprocess, the exploitation of planktonic growing electroautotrophs with H₂-mediated electron uptake would allow for a better use of the entire liquid volume of the cathodic reactor and, thus, higher productivities and product yields from CO₂-rich waste gas streams.

Extracellular Electron Uptake by Acetogenic Bacteria: Does H2 Consumption Favor the H2 Evolution Reaction on a Cathode or Metallic Iron?

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H2 partial pressures by microbial H2 consumption, which thermodynamically favors the H2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H2 partial pressure affects the H2 evolution onset potential and the H2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H2 consumption characteristics, i.e., H2 threshold (thermodynamic limit for H2 consumption) and H2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H2 partial pressures (lower H2 threshold, higher H2 affinity) more strongly increase the H2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Effect of Quorum Sensing on the Ability of Desulfovibrio vulgaris To Form Biofilms and To Biocorrode Carbon Steel in Saline Conditions

Sulfate-reducing bacteria (SRB) are key contributors to microbe-induced corrosion (MIC), which can lead to serious economic and environmental impact. The presence of a biofilm significantly increases the MIC rate. Inhibition of the quorum-sensing (QS) system is a promising alternative approach to prevent biofilm formation in various industrial settings, especially considering the significant ecological impact of conventional chemical-based mitigation strategies. In this study, the effect of the QS stimulation and inhibition on Desulfovibrio vulgaris is described in terms of anaerobic respiration, cell activity, biofilm formation, and biocorrosion of carbon steel. All these traits were repressed when bacteria were in contact with QS inhibitors but enhanced upon exposure to QS signal molecules compared to the control. The difference in the treatments was confirmed by transcriptomic analysis performed at different time points after treatment application. Genes related to lactate and pyruvate metabolism, sulfate reduction, electron transfer, and biofilm formation were downregulated upon QS inhibition. In contrast, QS stimulation led to an upregulation of the above-mentioned genes compared to the control. In summary, these results reveal the impact of QS on the activity of D. vulgaris, paving the way toward the prevention of corrosive SRB biofilm formation via QS inhibition.IMPORTANCE Sulfate-reducing bacteria (SRB) are considered key contributors to biocorrosion, particularly in saline environments. Biocorrosion imposes tremendous economic costs, and common approaches to mitigate this problem involve the use of toxic and hazardous chemicals (e.g., chlorine), which raise health and environmental safety concerns. Quorum-sensing inhibitors (QSIs) can be used as an alternative approach to inhibit biofilm formation and biocorrosion. However, this approach would only be effective if SRB rely on QS for the pathways associated with biocorrosion. These pathways would include biofilm formation, electron transfer, and metabolism. This study demonstrates the role of QS in Desulfovibrio vulgaris on the above-mentioned pathways through both phenotypic measurements and transcriptomic approach. The results of this study suggest that QSIs can be used to mitigate SRB-induced corrosion problems in ecologically sensitive areas.

Microbiologically Influenced Corrosion of Carbon Steel Beneath a Deposit in CO2-Saturated Formation Water Containing Desulfotomaculum nigrificans

The corrosion mechanism of carbon steel under deposit in the presence of sulfate reducing bacterium (SRB) Desulfotomaculum nigrificans was studied using surface analysis, weight loss and electrochemical measurements. Results showed that both the general corrosion and localized corrosion were considerably promoted by SRB under deposit. The corrosion rate of steel in the presence of SRB was approximately 6 times of that for the control according to the weight loss measurements. The maximum corrosion pit depth in the presence of SRB was approximately 7.7 times of that of the control. Both the anodic and cathodic reactions were significantly accelerated by SRB. A galvanic effect in the presence of SRB due to the heterogeneous biofilm led to serious localized corrosion.

Iron Corrosion via Direct Metal-Microbe Electron Transfer

The concept that anaerobic microorganisms can directly accept electrons from Fe(0) has been controversial because direct metal-microbe electron transfer has previously only been indirectly inferred. Fe(0) oxidation was studied with Geobacter sulfurreducens strain ACL, an autotrophic strain that was previously shown to grow with electrons derived from a graphite cathode as the sole electron donor. Strain ACL grew with Fe(0) as the sole electron donor and fumarate as the electron acceptor. However, it appeared that at least a portion of the electron transfer was via H2 produced nonenzymatically from the oxidation of Fe(0) to Fe(II). H2, which accumulated in abiotic controls, was consumed during the growth of strain ACL, the cells were predominately planktonic, and genes for the uptake hydrogenase were highly expressed. Strain ACLHF was constructed to prevent growth on H2 or formate by deleting the genes for the uptake of hydrogenase and formate dehydrogenases from strain ACL. Strain ACLHF also grew with Fe(0) as the sole electron donor, but H2 accumulated in the culture, and cells heavily colonized Fe(0) surfaces with no visible planktonic growth. Transcriptomics suggested that the outer surface c-type cytochromes OmcS and OmcZ were important during growth of strain ACLHF on Fe(0). Strain ACLHF did not grow on Fe(0) if the gene for either of these cytochromes was deleted. The specific attachment of strain ACLHF to Fe(0), coupled with requirements for known extracellular electrical contacts, suggest that direct metal-microbe electron transfer is the most likely option for Fe(0) serving as an electron donor.IMPORTANCE The anaerobic corrosion of iron structures is expensive to repair and can be a safety and environmental concern. It has been known for over 100 years that the presence of anaerobic respiratory microorganisms can accelerate iron corrosion. Multiple studies have suggested that there are sulfate reducers, methanogens, and acetogens that can directly accept electrons from Fe(0) to support sulfate or carbon dioxide reduction. However, all of the strains studied can also use H2 as an electron donor for growth, which is known to be abiotically produced from Fe(0). Furthermore, no proteins definitely shown to function as extracellular electrical contacts with Fe(0) were identified. The studies described here demonstrate that direct electron transfer from Fe(0) can support anaerobic respiration. They also map out a simple genetic approach to the study of iron corrosion mechanisms in other microorganisms. A better understanding of how microorganisms promote iron corrosion is expected to lead to the development of strategies that can help reduce adverse impacts from this process.

Phototrophic extracellular electron uptake is linked to carbon dioxide fixation in the bacterium Rhodopseudomonas palustris

Extracellular electron uptake (EEU) is the ability of microbes to take up electrons from solid-phase conductive substances such as metal oxides. EEU is performed by prevalent phototrophic bacterial genera, but the electron transfer pathways and the physiological electron sinks are poorly understood. Here we show that electrons enter the photosynthetic electron transport chain during EEU in the phototrophic bacterium Rhodopseudomonas palustris TIE-1. Cathodic electron flow is also correlated with a highly reducing intracellular redox environment. We show that reducing equivalents are used for carbon dioxide (CO₂) fixation, which is the primary electron sink. Deletion of the genes encoding ruBisCO (the CO₂-fixing enzyme of the Calvin-Benson-Bassham cycle) leads to a 90% reduction in EEU. This work shows that phototrophs can directly use solid-phase conductive substances for electron transfer, energy transduction, and CO₂ fixation.

Extracellular electron uptake (EEU) is the ability of microbes to take up electrons from solid-phase conductive substances such as metal oxides. Here, Guzman et al. show that electrons enter the photosynthetic electron transport chain and are used for CO₂ fixation during EEU in a phototrophic bacterium.

Understanding the Impact of Extracellular Polymeric Substances on Lead Release in Drinking Water Systems

Lead release in a lead (Pb, anode)-iron oxide (α-Fe₂O₃, cathode) galvanic system was studied under the influence of synthetic extracellular polymeric substances (sEPS). Sodium alginate, bovine serum albumin (BSA), and cytochrome c represented extracellular polysaccharides, proteins, and electrochemically active components, respectively. Microbiologically influenced corrosion was investigated using sEPS and pelleted and resuspended Pseudomonas aeruginosa cells. Relative to the anaerobic inorganic control, Pb release increased by 156, 202, and 198 μg/L when sEPS was present on the cathode side at 200 mg/L (100 mg/L alginate + 100 mg/L BSA), 400 mg/L (200 mg/L alginate + 200 mg/L BSA), and 200 mg/L with 123.84 mg/L cytochrome c, respectively, under anaerobic conditions. When the cathode was aerated, Pb release increased by 75, 260, and -71 μg/L under the aforementioned conditions, all relative to the aerated inorganic control. When sEPS was instead present on the anode side, sEPS caused localized corrosion on Pb and resulted in higher Pb release than predicted by electric current. P. aeruginosa generally enhanced corrosion; when cells were dosed in the anode side, part of the oxidized Pb was immobilized by cells or organic compounds adhered to the electrodes.

Electrode Potential Dependency of Single-Cell Activity Identifies the Energetics of Slow Microbial Electron Uptake Process

Electrochemical measurements have been widely applied to study microbial extracellular electron transport processes. However, because electrochemistry detects not only microbial electron transport but also other reactions, background signals comparable to or larger than microbial ones hamper the identification of microbial electrochemical properties. This problem is crucial especially for the detection of electron uptake processes by slow-growing microbes in low-energy subsurface sediments, as the environmental samples contain electrochemically active humus and mineral particles. In this study, we report a cell-specific stable isotope analysis to quantify the electrode potential dependency of anabolic activity in individual cells for identifying the electron uptake energetics of slow-growing bacteria. Followed by the incubation of Desulfovibrio ferrophilus IS5 cells with isotopic ¹⁵N-ammonium as the sole N source on electrodes poised at potentials of -0.2, -0.3, -0.4, and -0.5 V [vs. standard hydrogen electrode (SHE)], we conducted nanoscale secondary ion mass spectroscopy (NanoSIMS) to quantify ¹⁵N assimilation in more than 100 individual cells on the electrodes. We observed significant ¹⁵N assimilation at potentials of -0.4 and more ¹⁵N assimilation at -0.5 V, which is consistent with the onset potential for electron uptake via outer-membrane cytochromes (OMCs). The activation of cell energy metabolism was further examined by transcriptome analysis. Our results showed a novel methodology to study microbial electron uptake energetics. The results also serve as the first direct evidence that energy acquisition is coupled to the electron uptake process in sulfate-reducing bacteria that are ubiquitous in the subsurface environments, with implications on the electron-fueled subsurface biosphere hypothesis and other microbial processes, such as anaerobic iron corrosion and anaerobic methane oxidation.

An extracellular [NiFe] hydrogenase mediating iron corrosion is encoded in a genetically unstable genomic island in Methanococcus maripaludis

Certain methanogens deteriorate steel surfaces through a process called microbiologically influenced corrosion (MIC). However, the mechanisms of MIC, whereby methanogens oxidize zerovalent iron (Fe⁰), are largely unknown. In this study, Fe⁰-corroding Methanococcus maripaludis strain OS7 and its derivative (strain OS7mut1) defective in Fe⁰-corroding activity were isolated. Genomic analysis of these strains demonstrated that the strain OS7mut1 contained a 12-kb chromosomal deletion. The deleted region, termed "MIC island", encoded the genes for the large and small subunits of a [NiFe] hydrogenase, the TatA/TatC genes necessary for the secretion of the [NiFe] hydrogenase, and a gene for the hydrogenase maturation protease. Thus, the [NiFe] hydrogenase may be secreted outside the cytoplasmic membrane, where the [NiFe] hydrogenase can make direct contact with Fe⁰, and oxidize it, generating hydrogen gas: Fe⁰ + 2 H⁺ → Fe²⁺ + H₂. Comparative analysis of extracellular and intracellular proteomes of strain OS7 supported this hypothesis. The identification of the MIC genes enables the development of molecular tools to monitor epidemiology, and to perform surveillance and risk assessment of MIC-inducing M. maripaludis.

A Novel Shewanella Isolate Enhances Corrosion by Using Metallic Iron as the Electron Donor with Fumarate as the Electron Acceptor

The involvement of Shewanella spp. in biocorrosion is often attributed to their Fe(III)-reducing properties, but they could also affect corrosion by using metallic iron as an electron donor. Previously, we isolated Shewanella strain 4t3-1-2LB from an acetogenic community enriched with Fe(0) as the sole electron donor. Here, we investigated its use of Fe(0) as an electron donor with fumarate as an electron acceptor and explored its corrosion-enhancing mechanism. Without Fe(0), strain 4t3-1-2LB fermented fumarate to succinate and CO₂, as was shown by the reaction stoichiometry and pH. With Fe(0), strain 4t3-1-2LB completely reduced fumarate to succinate and increased the Fe(0) corrosion rate (7.0 ± 0.6)-fold in comparison to that of abiotic controls (based on the succinate-versus-abiotic hydrogen formation rate). Fumarate reduction by strain 4t3-1-2LB was, at least in part, supported by chemical hydrogen formation on Fe(0). Filter-sterilized spent medium increased the hydrogen generation rate only 1.5-fold, and thus extracellular hydrogenase enzymes appear to be insufficient to explain the enhanced corrosion rate. Electrochemical measurements suggested that strain 4t3-1-2LB did not excrete dissolved redox mediators. Exchanging the medium and scanning electron microscopy (SEM) imaging indicated that cells were attached to Fe(0). It is possible that strain 4t3-1-2LB used a direct mechanism to withdraw electrons from Fe(0) or favored chemical hydrogen formation on Fe(0) through maintaining low hydrogen concentrations. In coculture with an Acetobacterium strain, strain 4t3-1-2LB did not enhance acetogenesis from Fe(0). This work describes a strong corrosion enhancement by a Shewanella strain through its use of Fe(0) as an electron donor and provides insights into its corrosion-enhancing mechanism.IMPORTANCE Shewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes.

Nature's conductors: what can microbial multi-heme cytochromes teach us about electron transport and biological energy conversion?

Microorganisms can acquire energy from the environment by extending their electron transport chains to external solid electron donors or acceptors. This process, known as extracellular electron transfer (EET), is now being heavily pursued for wiring microbes to electrodes in bioelectrochemical renewable energy technologies. Recent studies highlight the crucial role of multi-heme cytochromes in facilitating biotic-abiotic EET both for cellular electron export and uptake. Here we explore progress in understanding the range and function of these biological electron conduits in the context of fuel-to-electricity and electricity-to-bioproduct conversion. We also highlight emerging topics, including the role of multi-heme cytochromes in inter-species electron transfer and in inspiring the design and synthesis of a new generation of protein-based bioelectronic components.

Electrotrophic activity and electrosynthetic acetate production by Desulfobacterium autotrophicum HRM2

Electroautotrophic microorganisms accept electrons from a cathode as source of reducing equivalents to drive CO₂ fixation by poorly understood mechanisms. Acetogenic bacteria were the first group found to possess the capability for electroautotrophic metabolism in pure culture with associated electrosynthesis of acetate as primary metabolite. Identification of additional electrotrophic species can contribute to our understanding of this unusual form of metabolism. Here, bioelectrochemical techniques, chemical analysis and microscopy were used to determine electrotrophic metabolism of Desulfobacterium autotrophicum HRM2. Chronoamperometry showed increasing current uptake over 21 days of incubation in duplicate bioelectrochemical system sets. Linear sweep voltammetry indicated peak current uptake at -243 mV. High performance liquid chromatography (HPLC) analysis quantified acetate accumulation in anaerobic minimal media containing inorganic carbon as sole carbon source, consistent with electrosynthesis. Scanning electron microscopy and live/dead staining by epifluorescence microscopy analysis indicated viable 1-2 μm cells after 76 days of cultivation under electroautotrophic conditions. The genome of Db. autotrophicum HRM2 is fully sequenced and, thus, could provide insight into the biochemical and physiological mechanisms by which electrotrophic cells utilize cathode-derived electrons. This research expands the diversity of facultative autotrophs capable of electrotrophic metabolism to include the sulfate-reducing marine bacterium Db. autotrophicum HRM2.