Gene expression by the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough grown on an iron electrode under cathodic protection conditions

Transcriptome-wide marker gene expression analysis of stress-responsive sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) are terminal members of any anaerobic food chain. For example, they critically influence the biogeochemical cycling of carbon, nitrogen, sulfur, and metals (natural environment) as well as the corrosion of civil infrastructure (built environment). The United States alone spends nearly $4 billion to address the biocorrosion challenges of SRB. It is important to analyze the genetic mechanisms of these organisms under environmental stresses. The current study uses complementary methodologies, viz., transcriptome-wide marker gene panel mapping and gene clustering analysis to decipher the stress mechanisms in four SRB. Here, the accessible RNA-sequencing data from the public domains were mined to identify the key transcriptional signatures. Crucial transcriptional candidate genes of Desulfovibrio spp. were accomplished and validated the gene cluster prediction. In addition, the unique transcriptional signatures of Oleidesulfovibrio alaskensis (OA-G20) at graphene and copper interfaces were discussed using in-house RNA-sequencing data. Furthermore, the comparative genomic analysis revealed 12,821 genes with translation, among which 10,178 genes were in homolog families and 2643 genes were in singleton families were observed among the 4 genomes studied. The current study paves a path for developing predictive deep learning tools for interpretable and mechanistic learning analysis of the SRB gene regulation.

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.

From laboratory tests to field trials: a review of cathodic protection and microbially influenced corrosion

Cathodic protection (CP), an electrochemical method for managing corrosion, is widely used in many industries in both marine and buried environments. However, literature surrounding cathodic protection and its ability to prevent microbially influenced corrosion (MIC) is mixed. This review describes the mechanics of CP, how CP may influence MIC, and collates and summarises tests on CP and MIC reported in literature. The aim of the review is to identify any trends and knowledge gaps requiring further study. While the outcomes of CP testing are generally mixed, some trends can be seen and, overall, MIC is detrimental to the protective effects of CP, with CP being less effective when used according to current international standards. Tests conducted in the field or with mix communities of microbes showed that CP could be effective at preventing MIC, while tests with sulfate-reducing bacteria generally proved CP to be highly ineffective. It was commonly seen that the effectiveness of CP can be improved by increasing polarization, to potentials as low as -1000 mV (Ag/AgCl). However, a balance does need to be met via careful monitoring to ensure negative side effects of over protection do not become a major problem.

Desulfovibrio vulgaris as a model microbe for the study of corrosion under sulfate-reducing conditions

Corrosion of iron-containing metals under sulfate-reducing conditions is an economically important problem. Microbial strains now known as Desulfovibrio vulgaris served as the model microbes in many of the foundational studies that developed existing models for the corrosion of iron-containing metals under sulfate-reducing conditions. Proposed mechanisms for corrosion by D. vulgaris include: (1) H2 consumption to accelerate the oxidation of Fe0 coupled to the reduction of protons to H2; (2) production of sulfide that combines with ferrous iron to form iron sulfide coatings that promote H2 production; (3) moribund cells release hydrogenases that catalyze Fe0 oxidation with the production of H2; (4) direct electron transfer from Fe0 to cells; and (5) flavins serving as an electron shuttle for electron transfer between Fe0 and cells. The demonstrated possibility of conducting transcriptomic and proteomic analysis of cells growing on metal surfaces suggests that similar studies on D. vulgaris corrosion biofilms can aid in identifying proteins that play an important role in corrosion. Tools for making targeted gene deletions in D. vulgaris are available for functional genetic studies. These approaches, coupled with instrumentation for the detection of low concentrations of H2, and proven techniques for evaluating putative electron shuttle function, are expected to make it possible to determine which of the proposed mechanisms for D. vulgaris corrosion are most important.

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.

Exogenous autoinducer-2 inhibits biofilm development of Desulfovibrio sp. Huiquan2017

Sulfate-reducing bacteria (SRB) are culprits for microbiologically influenced corrosion, and biofilms are believed to play essential roles in the corrosion induced by SRB. However, little is known about the regulation of SRB biofilms. Quorum sensing signal molecules acyl-homoserine lactones (AHLs) and autoinducer-2 (AI-2) regulate biofilm formation of many bacteria. In this study, the production of AHLs and AI-2 by one SRB strain, Desulfovibrio sp. Huiquan2017, was detected, and the effect of exogenous AI-2 on bacterial biofilm formation was discussed. It was found that the cell-free supernatants of Desulfovibrio sp. Huiquan2017 induced luminescence in a ∆luxS mutant strain Vibrio harveyi BB170, indicating the production of functional AI-2 by the bacterium. In the presence of exogenous AI-2, the growth of Desulfovibrio sp. Huiquan2017 and early biofilm formation were not affected, but the later stage of biofilm development was inhibited significantly. The biofilms became looser, smaller, and thinner, and contained less bacteria and extracellular polymeric substances (EPS). The inhibition effect of AI-2 on the biofilm development of Desulfovibrio sp. Huiquan2017 was mainly achieved through reducing the amount of EPS in biofilms. These findings shed light on the biofilm regulation of SRB.

Photocathode optimization and microbial community in the solar-illuminated bio-photoelectrochemical system for nitrofurazone degradation

To further enhance the bio-photoelectrochemical system (BPES) performance for nitrofurazone (NFZ) degradation and current output, the g-C₃N₄/CdS photocathode was optimized, and microbial community shift from inoculation to the BPES was analyzed. Results showed that photocathode with g-C₃N₄/CdS (mass ratio of 1:9) loading of 7.5 mg/cm² exhibited the best performance, with NFZ removal of 83.14% (within 4 h) and current of ~9 mA in the BPES. Proteobacteria accounted for the largest proportion: 66.53% (inoculation), 71.89% (microbial electrolysis cell (MEC) anode), 74.67% (BPES anode) and 57.31% (BPES cathode), respectively. In addition, Geobacter was the most dominant genus in MEC and BPES anode and cathode, which occupied 31.64%, 67.73% and 41.34%, respectively. The microbial compositions of BPES anode and cathode were similar, but different from that of MEC anode. Notably, Rhodopseudomonas, a photosynthetic species, was detected in the BPES. Cognition of microbial community in the BPES is important for advancing its development.

Goethite Reduction by a Neutrophilic Member of the Alphaproteobacterial Genus Telmatospirillum

In tropical iron ore regions, biologically mediated reduction of crystalline iron oxides drives ongoing iron cycling that contributes to the stability of surface duricrusts. This represents a biotechnological opportunity with respect to post-mining rehabilitation attempts, requiring re-formation of these duricrusts. However, cultivated dissimilatory iron reducing bacteria typically reduce crystalline iron oxides quite poorly. A glucose-fermenting microbial consortium capable of reducing at least 27 mmol/L goethite was enriched from an iron duricrust region. Metagenome analysis led to the recovery of a metagenome assembled genome (MAG) of an iron reducer belonging to the alphaproteobacterial genus Telmatospirillum. This is the first report of iron reduction within the Telmatospirillum and the first reported genome of an iron-reducing, neutrophilic member of the Alphaproteobacteria. The Telmatospirillum MAG encodes putative metal transfer reductases (MtrA, MtrB) and a novel, multi-heme outer membrane cytochrome for extracellular electron transfer. In the presence of goethite, short chain fatty acid production shifted significantly in favor of acetate rather than propionate, indicating goethite is a hydrogen sink in the culture. Therefore, the presence of fermentative bacteria likely promotes iron reduction via hydrogen production. Stimulating microbial fermentation has potential to drive reduction of crystalline iron oxides, the rate limiting step for iron duricrust re-formation.

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.

Salinity-Mediated Increment in Sulfate Reduction, Biofilm Formation, and Quorum Sensing: A Potential Connection Between Quorum Sensing and Sulfate Reduction?

Biocorrosion in marine environment is often associated with biofilms of sulfate reducing bacteria (SRB). However, not much information is available on the mechanism underlying exacerbated rates of SRB-mediated biocorrosion under saline conditions. Using Desulfovibrio (D.) vulgaris and Desulfobacterium (Db.) corrodens as model SRBs, the enhancement effects of salinity on sulfate reduction, N-acyl homoserine lactone (AHL) production and biofilm formation by SRBs were demonstrated. Under saline conditions, D. vulgaris and Db. corrodens exhibited significantly higher specific sulfate reduction and specific AHL production rates as well as elevated rates of biofilm formation compared to freshwater medium. Salinity-induced enhancement traits were also confirmed at transcript level through reverse transcription quantitative polymerase chain reaction (RT-qPCR) approach, which showed salinity-influenced increase in the expression of genes associated with carbon metabolism, sulfate reduction, biofilm formation and histidine kinase signal transduction. In addition, by deploying quorum sensing (QS) inhibitors, a potential connection between sulfate reduction and AHL production under saline conditions was demonstrated, which is most significant during early stages of sulfate metabolism. The findings collectively revealed the interconnection between QS, sulfate reduction and biofilm formation among SRBs, and implied the potential of deploying quorum quenching approaches to control SRB-based biocorrosion in saline conditions.

Sulfated polysaccharide from sea cucumber modulates the gut microbiota and its metabolites in normal mice

Sulfated polysaccharide from sea cucumber (SCSP) has been demonstrated with various health effects, the mechanism of which, however, remains poorly understood. This study aimed to investigate the possible mechanism exhibited by gut microbiota in response to SCSP. BALB/c mice were fed diets supplemented with SCSP and depolymerized SCSP (d-SCSP) for 42 days. The microbiota composition, short chain fatty acids (SCFAs), lipopolysaccharide-binding protein (LBP), body weight and gut tissue index were analyzed. Results revealed that both SCSP and d-SCSP positively regulated the gut microbiota as indicated by the enriched microbiota diversity, SCFA-producing bacteria and sulfide-degrading bacteria, and decreased harmful bacteria. Moreover, SCSP and d-SCSP not only significantly improved the levels of microbial metabolites including SCFAs and LBP, but also effectively adjusted body weight and gut tissue index. The microbial metabolites were identified to strongly correlate with the growth performance using Pearson's correlation coefficient. We further showed that the modulating effect of SCSP on the gut microbiota was altered by free-radical depolymerization, while the microbial metabolites and related growth performance were not. These findings suggest that SCSP can be used as a gut microbiota manipulator for health promotion and alter the gut microbiota in a molecular weight (Mw) dependent manner.

Carrageenan-induced colitis is associated with decreased population of anti-inflammatory bacterium, Akkermansia muciniphila, in the gut microbiota of C57BL/6J mice

Carrageenan as a food additive has been used for years. However, controversy exists regarding to the safety of carrageenan and accumulating evidence indicates that it could induce colitis in experimental models. Here, to provide more information on this issue and solve the debate, we studied and compared in detail the toxic effects of different isomers of carrageenan (κ-, ι-, and λ-) on the colon of C57BL/6J mice. Interestingly, all isomers of carrageenan were found to induce colitis with a comparable activity. Given that carrageenan is unabsorbed after oral administration, and also in light of the fact that gut microbiota plays a pivotal role in the pathogenesis of colitis, we further investigated the effect of carrageenan on gut microbiota using high-throughput sequencing. Intriguingly, carrageenan-induced colitis was observed to be robustly correlated with changes in the composition of gut microbiota. Specifically, all carrageenans significantly decreased the abundance of a potent anti-inflammatory bacterium, Akkermansia muciniphila, in the gut, which is highly relevant for understanding the toxic effect of carrageenan. Altogether, our results corroborate previous studies demonstrating harmful gastrointestinal effect of carrageenan and, from a gut microbiota perspective, shed new light into the mechanism by which carrageenan induces colitis in experimental animals.

Polarity inversion of bioanode for biocathodic reduction of aromatic pollutants

The enrichment of specific pollutant-reducing consortium is usually required prior to the startup of biocathode bioelectrochemical system (BES) and the whole process is time consuming. To rapidly establish a non-specific functional biocathode, direct polar inversion from bioanode to biocathode is proposed in this study. Based on the diverse reductases and electron transfer related proteins of anode-respiring bacteria (ARB), the acclimated electrochemically active biofilm (EAB) may catalyze reduction of different aromatic pollutants. Within approximately 12 d, the acclimated bioanodes were directly employed as biocathodes for nitroaromatic nitrobenzene (NB) and azo dye acid orange 7 (AO7) reduction. Our results indicated that the established biocathode significantly accelerated the reduction of NB to aniline (AN) and AO7 to discolored products compared with the abiotic cathode and open circuit controls. Several microbes possessing capabilities of nitroaromatic/azo dye reduction and bidirectional electron transfer were maintained or enriched in the biocathode communities. Cyclic voltammetry highlighted the decreased over-potentials and enhanced electron transfer of biocathode as well as demonstrated the ARB Geobacter containing cytochrome c involved in the backward electron transfer from electrode to NB. This study offers new insights into the rapid establishment and modularization of functional biocathodes for the potential treatment of complicated electron acceptors-coexisting wastewaters.

Quorum Sensing and the Use of Quorum Quenchers as Natural Biocides to Inhibit Sulfate-Reducing Bacteria

Sulfate-reducing bacteria (SRB) are one of the main protagonist groups of biocorrosion in the seawater environment. Given their principal role in biocorrosion, it remains a crucial task to develop strategies to reduce the abundance of SRBs. Conventional approaches include the use of biocides and antibiotics, which can impose health, safety, and environmental concerns. This review examines an alternative approach to this problem. This is achieved by reviewing the role of quorum sensing (QS) in SRB populations and its impact on the biofilm formation process. Genome databases of SRBs are mined to look for putative QS systems and homologous protein sequences representative of autoinducer receptors or synthases. Subsequently, this review puts forward the potential use of quorum quenchers as natural biocides against SRBs and outlines the potential strategies for the implementation of this approach.

Insights into the Quorum Sensing Regulon of the Acidophilic Acidithiobacillus ferrooxidans Revealed by Transcriptomic in the Presence of an Acyl Homoserine Lactone Superagonist Analog

While a functional quorum sensing system has been identified in the acidophilic chemolithoautotrophic Acidithiobacillus ferrooxidans ATCC 23270(T) and shown to modulate cell adhesion to solid substrates, nothing is known about the genes it regulates. To address the question of how quorum sensing controls biofilm formation in A. ferrooxidans (T), the transcriptome of this organism in conditions in which quorum sensing response is stimulated by a synthetic superagonist AHL (N-acyl homoserine lactones) analog has been studied. First, the effect on biofilm formation of a synthetic AHL tetrazolic analog, tetrazole 9c, known for its agonistic QS activity, was assessed by fluorescence and electron microscopy. A fast adherence of A. ferrooxidans (T) cells on sulfur coupons was observed. Then, tetrazole 9c was used in DNA microarray experiments that allowed the identification of genes regulated by quorum sensing signaling, and more particularly, those involved in early biofilm formation. Interestingly, afeI gene, encoding the AHL synthase, but not the A. ferrooxidans quorum sensing transcriptional regulator AfeR encoding gene, was shown to be regulated by quorum sensing. Data indicated that quorum sensing network represents at least 4.5% (141 genes) of the ATCC 23270(T) genome of which 42.5% (60 genes) are related to biofilm formation. Finally, AfeR was shown to bind specifically to the regulatory region of the afeI gene at the level of the palindromic sequence predicted to be the AfeR binding site. Our results give new insights on the response of A. ferrooxidans to quorum sensing and on biofilm biogenesis.

Low temperature acclimation with electrical stimulation enhance the biocathode functioning stability for antibiotics detoxification

Improvement of the stability of functional microbial communities in wastewater treatment system is critical to accelerate pollutants detoxification in cold regions. Although biocathode communities could accelerate environmental pollutants degradation, how to acclimate the cold stress and to improve the catalytic stability of functional microbial communities are remain poorly understood. Here we investigated the structural and functional responses of antibiotic chloramphenicol (CAP) reducing biocathode communities to constant low temperature 10 °C (10-biocathode) and temperature elevation from 10 °C to 25 °C (S25-biocathode). Our results indicated that the low temperature acclimation with electrical stimulation obviously enhanced the CAP nitro group reduction efficiency when comparing the aromatic amine product AMCl2 formation efficiency with the 10-biocathode and S25-biocathode under the opened and closed circuit conditions. The 10-biocathode generated comparative AMCl maximum as the S25-biocathode but showed significant lower dehalogenation rate of AMCl2 to AMCl. The continuous low temperature and temperature elevation both enriched core functional community in the 10-biocathode and S25-biocathode, respectively. The 10-biocathode functioning stability maintained mainly through selectively enriching cold-adapted functional species, coexisting metabolically similar nitroaromatics reducers and maintaining the relative abundance of key electrons transfer genes. This study provides new insights into biocathode functioning stability for accelerating environmental pollutants degradation in cold wastewater system.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

The role of acetogens in microbially influenced corrosion of steel

Microbially influenced corrosion (MIC) of iron (Fe(0)) by sulfate-reducing bacteria (SRB) has been studied extensively. Through a mechanism, that is still poorly understood, electrons or hydrogen (H2) molecules are removed from the metal surface and used as electron donor for sulfate reduction. The resulting ferrous ions precipitate in part with the sulfide produced, forming characteristic black iron sulfide. Hydrogenotrophic methanogens can also contribute to MIC. Incubation of pipeline water samples, containing bicarbonate and some sulfate, in serum bottles with steel coupons and a headspace of 10% (vol/vol) CO2 and 90% N2, indicated formation of acetate and methane. Incubation of these samples in serum bottles, containing medium with coupons and bicarbonate but no sulfate, also indicated that formation of acetate preceded the formation of methane. Microbial community analyses of these enrichments indicated the presence of Acetobacterium, as well as of hydrogenotrophic and acetotrophic methanogens. The formation of acetate by homoacetogens, such as Acetobacterium woodii from H2 (or Fe(0)) and CO2, is potentially important, because acetate is a required carbon source for many SRB growing with H2 and sulfate. A consortium of the SRB Desulfovibrio vulgaris Hildenborough and A. woodii was able to grow in defined medium with H2, CO2, and sulfate, because A. woodii provides the acetate, needed by D. vulgaris under these conditions. Likewise, general corrosion rates of metal coupons incubated with D. vulgaris in the presence of acetate or in the presence of A. woodii were higher than in the absence of acetate or A. woodii, respectively. An extended MIC model capturing these results is presented.

Corrosion of carbon steel by bacteria from North Sea offshore seawater injection systems: laboratory investigation

Influence of sulfidogenic bacteria, from a North Sea seawater injection system, on the corrosion of S235JR carbon steel was studied in a flow bioreactor; operating anaerobically for 100days with either inoculated or filtrated seawater. Deposits formed on steel placed in reactors contained magnesium and calcium minerals plus iron sulfide. The dominant biofilm-forming organism was an anaerobic bacterium, genus Caminicella, known to produce hydrogen sulfide and carbon dioxide. Open Circuit Potentials (OCP) of steel in the reactors was, for nearly the entire test duration, in the range -800<E(OCP)/mV (vs. SCE)<-700. Generally, the overall corrosion rate, expressed as 1/(Rp/Ω), was lower in the inoculated seawater though they varied significantly on both reactors. Initial and final corrosion rates were virtually identical, namely initial 1/(Rp/Ω)=2×10(-6)±5×10(-7) and final 1/(Rp/Ω)=1.1×10(-5)±2.5×10(-6). Measured data, including electrochemical noise transients and statistical parameters (0.05<Localized Index<1; -5<Skewness<-5; Kurtosis>45), suggested pitting on steel samples within the inoculated environment. However, the actual degree of corrosion could neither be directly correlated with the electrochemical data and nor with the steel corrosion in the filtrated seawater environment. Further laboratory tests are thought to clarify the noticed apparent discrepancies.

Sulfate-reducing bacteria lower sulfur-mediated pitting corrosion under conditions of oxygen ingress

The effect of oxygen ingress into sour water containing dissolved sulfide on the production of sulfur and polysulfide (S-PS) and associated iron corrosion was investigated. Biotic (active SRB present), abiotic (autoclaved SRB present), and chemical (no bacteria present) conditions were compared. Under biotic conditions formation of S-PS was only seen at a high ratio of oxygen to sulfide (R(OS)) of 1 to 2.4. General corrosion rates increased 10-fold to 0.10 mm/yr under these conditions. Under abiotic and chemical conditions S-PS formation increased over the entire range of R(OS) with general corrosion rates reaching 0.06 mm/yr. Although general corrosion rates were thus highest under biotic conditions, biotically corroded coupons showed much less pitting corrosion. Maximum pit depth increased to 40-80 μm with increasing R(OS) for coupons incubated for 1 month under abiotic or chemical conditions but not for biotically incubated coupons (10 μm). This appeared to be related to the properties and size of the sulfur formed, which was hydrophobic and in excess of 10 μm under chemical or abiotic conditions and hydrophilic and 0.5 to 1 μm under biotic conditions. Hence, perhaps contrary to expectation, SRB lowered pitting corrosion rates under conditions of oxygen ingress due to their ability to respire oxygen and produce a less aggressive form of sulfur. Microbial control, which is usually required in sour systems, may be counterproductive under these conditions.

Linking bacterial metabolism to graphite cathodes: electrochemical insights into the H(2) -producing capability of Desulfovibrio sp

Microbial biocathodes allow converting and storing electricity produced from renewable sources in chemical fuels (e.g., H(2) ) and are, therefore, attracting considerable attention as alternative catalysts to more expensive and less available noble metals (notably Pt). Microbial biocathodes for H(2) production rely on the ability of hydrogenase-possessing microorganisms to catalyze proton reduction, with a solid electrode serving as direct electron donor. This study provides new chemical and electrochemical data on the bioelectrocatalytic activity of Desulfovibrio species. A combination of chronoamperometry, cyclic voltammetry, and impedance spectroscopy tests were used to assess the performance of the H(2) -producing microbial biocathode and to shed light on the involved electron transfer mechanisms. Cells attached onto a graphite electrode were found to catalyze H(2) production for cathode potentials more reducing than -900 mV vs. standard hydrogen electrode. The highest obtained H(2) production was 8 mmol L(-1) per day, with a Coulombic efficiency close to 100 %. The electrochemical performance of the biocathode changed over time probably due to the occurrence of enzyme activation processes induced by extended electrode polarization. Remarkably, H(2) (at least up to 20 % v/v) was not found to significantly inhibit its own production.

Transcriptomic and proteomic analyses of Desulfovibrio vulgaris biofilms: carbon and energy flow contribute to the distinct biofilm growth state

Background

Desulfovibrio vulgaris Hildenborough is a sulfate-reducing bacterium (SRB) that is intensively studied in the context of metal corrosion and heavy-metal bioremediation, and SRB populations are commonly observed in pipe and subsurface environments as surface-associated populations. In order to elucidate physiological changes associated with biofilm growth at both the transcript and protein level, transcriptomic and proteomic analyses were done on mature biofilm cells and compared to both batch and reactor planktonic populations. The biofilms were cultivated with lactate and sulfate in a continuously fed biofilm reactor, and compared to both batch and reactor planktonic populations.

Results

The functional genomic analysis demonstrated that biofilm cells were different compared to planktonic cells, and the majority of altered abundances for genes and proteins were annotated as hypothetical (unknown function), energy conservation, amino acid metabolism, and signal transduction. Genes and proteins that showed similar trends in detected levels were particularly involved in energy conservation such as increases in an annotated ech hydrogenase, formate dehydrogenase, pyruvate:ferredoxin oxidoreductase, and rnf oxidoreductase, and the biofilm cells had elevated formate dehydrogenase activity. Several other hydrogenases and formate dehydrogenases also showed an increased protein level, while decreased transcript and protein levels were observed for putative coo hydrogenase as well as a lactate permease and hyp hydrogenases for biofilm cells. Genes annotated for amino acid synthesis and nitrogen utilization were also predominant changers within the biofilm state. Ribosomal transcripts and proteins were notably decreased within the biofilm cells compared to exponential-phase cells but were not as low as levels observed in planktonic, stationary-phase cells. Several putative, extracellular proteins (DVU1012, 1545) were also detected in the extracellular fraction from biofilm cells.

Conclusions

Even though both the planktonic and biofilm cells were oxidizing lactate and reducing sulfate, the biofilm cells were physiologically distinct compared to planktonic growth states due to altered abundances of genes/proteins involved in carbon/energy flow and extracellular structures. In addition, average expression values for multiple rRNA transcripts and respiratory activity measurements indicated that biofilm cells were metabolically more similar to exponential-phase cells although biofilm cells are structured differently. The characterization of physiological advantages and constraints of the biofilm growth state for sulfate-reducing bacteria will provide insight into bioremediation applications as well as microbially-induced metal corrosion.

A molybdopterin oxidoreductase is involved in H2 oxidation in Desulfovibrio desulfuricans G20

Three mutants deficient in hydrogen/formate uptake were obtained through screening of a transposon mutant library containing 5,760 mutants of Desulfovibrio desulfuricans G20. Mutations were in the genes encoding the type I tetraheme cytochrome c(3) (cycA), Fe hydrogenase (hydB), and molybdopterin oxidoreductase (mopB). Mutations did not decrease the ability of cells to produce H(2) or formate during growth. Complementation of the cycA and mopB mutants with a plasmid carrying the intact cycA and/or mopB gene and the putative promoter from the parental strain allowed the recovery of H(2) uptake ability, showing that these specific genes are involved in H(2) oxidation. The mop operon encodes a periplasm-facing transmembrane protein complex which may shuttle electrons from periplasmic cytochrome c(3) to the menaquinone pool. Electrons can then be used for sulfate reduction in the cytoplasm.