Electrobiocorrosion by Microbes without Outer-Surface Cytochromes

Abstract

AbstractAnaerobic 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 Fe0, 316L stainless steel does not abiotically generate H2as an intermediary electron carrier. Here we report that all of the methanogens (Methanosarcina vacuolata,Methanothrix soehngenii, andMethanobacteriumstrain IM1) and acetogens (Sporomusa ovata,Clostridium ljungdahlii) evaluated respired with pure Fe0as the electron donor, but onlyM. vacuolata,Mx soehngenii, andS. ovatawere capable of stainless steel electrobiocorrosion. The electrobiocorrosive methanogens required acetate as an additional energy source in order to produce methane from stainless steel. Co-cultures ofS. ovataandMx. soehngeniidemonstrated how acetogens can provide acetate to methanogens during corrosion. Not only wasMethanobacteriumstrain IM1 not capable of electrobiocorrosion, but it also did not accept electrons fromGeobacter metallireducens, an effective electron- donating partner for direct interspecies electron transfer to all methanogens that can directly accept electrons from Fe0. The finding thatM. vacuolata,Mx. soehngenii, andS. ovataare capable of electrobiocorrosion, despite a lack of the outer-surfacec-type cytochromes previously found to be important in other electrobiocorrosive microbes, demonstrates that there are multiple microbial strategies for making electrical contact with Fe0.Impact StatementUnderstanding how anaerobic microbes receive electrons from Fe0is likely to lead to novel strategies for mitigating the corrosion of iron-containing metals, which has an enormous economic impact. Electrobiocorrosion, is a relatively recently recognized corrosion mechanism. It was previously demonstrated in pure cultures when Fe0oxidation was inhibited by deletion of genes for outer-surfacec-type cytochromes known to be involved in other forms of extracellular electron exchange. However, many methanogens and acetogens lack obvious outer-surface electrical connections and are difficult to genetically manipulate. The study reported here provides an alternative approach to evaluating whether microbes are capable of electrobiocorrosion that does not require genetic manipulation. The results indicate thatMethanobacteriumstrain IM1, is not electrobiocorrosive, in contrast to previous speculation. However, some methanogens and acetogens without known outer-surfacec-type cytochromes do appear to be capable of electrobiocorrosion, suggesting that this corrosion mechanism may be more widespread than previously thought.