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, 316L  stainless steel does not abiotically generate H as an intermediary electron carrier. Here, we report that all of the methanogens (, and strain IM1) and acetogens ( and ) evaluated respired with pure Fe as the electron donor, but only , and were capable of stainless steel electrobiocorrosion. The electrobiocorrosive methanogens required acetate as an additional energy source in order to produce methane from stainless steel. Cocultures of and demonstrated how acetogens can provide acetate to methanogens during corrosion. Not only was strain IM1 not capable of electrobiocorrosion, but it also did not accept electrons from , an effective electron-donating partner for direct interspecies electron transfer to all methanogens that can directly accept electrons from Fe. The finding that , and are capable of electrobiocorrosion, despite a lack of the outer-surface -type cytochromes previously found to be important in other electrobiocorrosive microbes, demonstrates that there are multiple microbial strategies for making electrical contact with Fe.