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).

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.

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.

Development of a whole-cell biosensor based on an ArsR-Pars regulatory circuit from Geobacter sulfurreducens

In this study,an Escherichia coli(E.coli)whole-cell biosensor for the specific detection of bioavailable arsenic was developed by placing a green fluorescent protein(GFP)reporter gene under the control of the ArsR1(GSU2952)regulatory circuit from Geobacter sulfurreducens.E.coli cells only emitted green fluorescence in the presence of arsenite and were more sensitive to arsenite when they were grown in M9 supplemented medium compared to LB medium.Under optimal test conditions,the Geobacter arsR1 promoter had a detection limit of 0.01 mM arsenite and the GFP expression was linear within a range of 0.03-0.1 mM(2.25-7.5 mg/l).These values were well below World Health Organization’s drinking water quality standard,which is 10 mg/l.The feasibility of using this whole-cell biosensor to detect arsenic in water samples,such as arsenic polluted tap water and landfill leachate was verified.The biosensor was determined to be just as sensitive as atomic fluorescence spectrometry.This study examines the potential applications of biosensors constructed with Geobacter ArsR-Pars regulatory circuits and provides a rapid and cost-effective tool that can be used for arsenic detection in water samples.