Electrobiocorrosion by microbes without outer-surface cytochromes

Anaerobic microbial corrosion of iron-containing metals causes extensive economic damage.Some microbes are capable of direct metal-to-microbe electron transfer(electrobiocorrosion),but the prevalence of electrobiocorrosion among diverse methanogens and acetogens is poorly understood because of a lack of tools for their genetic manipulation.Previous studies have suggested that respiration with 316L stainless steel as the electron donor is indicative of electrobiocorrosion,because,unlike pure Fe^(0),316L stainless steel does not abiotically generate H_(2) as an intermediary electron carrier.Here,we report that all of the methanogens(Methanosarcina vacuolata,Methanothrix soehngenii,and Methanobacterium strain IM1)and acetogens(Sporomusa ovata and Clostridium ljungdahli)evaluated respired with pure Fe^(0)as the electron donor,but only M.vacuolata,Mx.soehngeni,and S.ovata were capable of stainless steel electrobiocorrosion.The electrobiocorrosive methanogens re-quired acetate as an additional energy source in order to produce methane from stainless steel.Cocultures of S.ovata and Mx.soehngeni demonstrated how acetogens can provide acetate to methanogens during corrosion.Not only was Meth-anobacterium strain IM1 not capable of electrobiocorrosion,but it also did not accept electrons from Geobacter metal-lireducens,an effective electron-donating partner for direct interspecies electron transfer to all methanogens that can directly accept electrons from Fe^(0).The finding that M.vacuolata,Mx.soehngeni,and S.ovata are capable of electrobiocorrosion,despite a lack of the outer-surface c-type cytochromes previously found to be important in other electrobiocorrosive microbes,demonstrates that there are multiple microbial strategies for making electrical contact with Fe^(0).

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

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure.There is substantial debate over their corrosion mechanisms.We investigated Fe^(0) corrosion with Desulfovibrio vulgaris,the sulfate reducer most often employed in corrosion studies.Cultures were grown with both lactate and Fe^(0) as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes.Fe^(0) was corroded in cultures of a D.vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe^(0) loss and H_(2) accumulation expected for Fe^(0) oxidation coupled to H+reduction to H_(2).This result and the extent of sulfate reduction indicated that D.vulgaris was not capable of direct Fe^(0)-to-microbe electron transfer even though it was provided with a supplementary energy source in the presence of abundant ferrous sulfide.Corrosion in the hydrogenase-deficient mutant cultures was greater than in sterile controls,demonstrating that H_(2) removal was not necessary for the enhanced corrosion observed in the presence of microbes.The parental H_(2)-consuming strain corroded more Fe^(0) than the mutant strain,which could be attributed to H_(2) oxidation coupled to sulfate reduction,producing sulfide that further stimulated Fe^(0) oxidation.The results suggest that H_(2) consumption is not necessary for microbially enhanced corrosion,but H_(2) oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction.The finding that D.vulgaris was incapable of direct electron uptake from Fe^(0) reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes.

Bioelectronic protein nanowire sensors for ammonia detection

Electronic sensors based on biomaterials can lead to novel green technologies that are low cost,renewable,and eco-friendly.Here we demonstrate bioelectronic ammonia sensors made from protein nanowires harvested from the microorganism Geobacter sulfurreducens.The nanowire sensor responds to a broad range of ammonia concentrations(10 to 10^6 ppb),which covers the range relevant for industrial,environmental,and biomedical applications.The sensor also demonstrates high selectivity to ammonia compared to moisture and other common gases found in human breath.These results provide a proof-of-concept demonstration for developing protein nanowire based gas sensors for applications in industry,agriculture,environmental monitoring,and healthcare.

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

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

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.

Different outer membrane c‐type cytochromes are involved in direct interspecies electron transfer to Geobacter or Methanosarcina species

Direct interspecies electron transfer(DIET)may be most important in methanogenic environments,but mechanistic studies of DIET to date have primarily focused on cocultures in which fumarate was the terminal electron acceptor.To better understand DIET with methanogens,the transcriptome of Geobacter metallireducens during DIET‐based growth with G.sulfurreducens reducing fumarate was compared with G.metallireducens grown in coculture with diverse Methanosarcina.The transcriptome of G.metallireducens cocultured with G.sulfurreducens was significantly different from those with Methanosarcina.Furthermore,the transcriptome of G.metallireducens grown with Methanosarcina barkeri,which lacks outer‐surface c‐type cytochromes,differed from those of G.metallireducens cocultured with M.acetivorans or M.subterranea,which have an outer‐surface c‐type cytochrome that serves as an electrical connect for DIET.Differences in G.metallireducens expression patterns for genes involved in extracellular electron transfer were particularly notable.Cocultures with c‐type cytochrome deletion mutant strains,ΔGmet_0930,ΔGmet_0557 andΔGmet_2896,never became established with G.sulfurreducens but adapted to grow with all three Methanosarcina.Two porin–cytochrome complexes,PccF and PccG,were important for DIET;however,PccG was more important for growth with Methanosarcina.Unlike cocultures with G.sulfurreducens and M.acetivorans,electrically conductive pili were not needed for growth with M.barkeri.Shewanella oneidensis,another electroactive microbe with abundant outer‐surface c‐type cytochromes,did not grow via DIET.The results demonstrate that the presence of outer‐surface c‐type cytochromes does not necessarily confer the capacity for DIET and emphasize the impact of the electron‐accepting partner on the physiology of the electron‐donating DIET partner.