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 , Clostridium ljungdahlii ) evaluated respired with pure Fe 0 as the electron donor, but only M. vacuolata , Mx soehngenii , and S. ovata were 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 of S. ovata and Mx. soehngenii demonstrated how acetogens can provide acetate to methanogens during corrosion. Not only was Methanobacterium strain IM1 not capable of electrobiocorrosion, but it also did not accept electrons from Geobacter metallireducens , 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. soehngenii , 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 . Impact Statement Understanding how anaerobic microbes receive electrons from Fe 0 is 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 Fe 0 oxidation was inhibited by deletion of genes for outer-surface c -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 that Methanobacterium strain IM1, is not electrobiocorrosive, in contrast to previous speculation. However, some methanogens and acetogens without known outer-surface c -type cytochromes do appear to be capable of electrobiocorrosion, suggesting that this corrosion mechanism may be more widespread than previously thought.