Methanogenic Archaea and their role in the corrosion of iron

Microbiologically influenced corrosion (MIC) is one of the main causes leading to extreme deterioration of the materials. For a long time, methanogenic archaea (MA) were considered as insignificant contributors to MIC, mainly due to relatively low corrosion rates under laboratory conditions. However, in recent years, MA have been increasingly recognized as important originators of MIC by the withdrawal of electrons from the iron surface, resulting in metal dissolution. This proposed mechanism is thought to be mediated by a special type of [NiFe] hydrogenases (MIC hydrogenases), located on an unstable genomic island named the ‘MIC Island’. This genomic island was originally discovered in the genome of the corrosive strain Methanococcus maripaludis OS7. Only methanogenic strains harboring the MIC Island show corrosive properties. In this work, a combinatorial approach of genetics, biofilm analysis, corrosion studies and protein-based experiments were chosen, to gain deeper insights into the corrosion mechanism of MA. With the sequencing of the complete genome of the corrosive M. maripaludis Mic1c10, we were able to demonstrate that this strain is closely related to M. maripaludis strains KA1 and OS7. Furthermore, genetic studies revealed that M. maripaludis Mic1c10 and Methanobacterium IM1 possess the MIC Island as well. Interestingly, the MIC Island of Methanobacterium IM1 was shown to be stably integrated into the genome while the MIC Island of M. maripaludis Mic1c10 was lost upon growth on hydrogen as shown for M. maripaludis OS7. Additionally, biofilms analysis of M. maripaludis Mic1c10 and Methanobacterium IM1 demonstrated that under the tested conditions both strains show significant differences in the biofilm structure and their affinity to iron. Together with the genomic difference in the MIC Island, these results indicate that the corrosion mechanism of those two genera might be slightly different. These results emphasize the importance of the fundamental understanding of the MIC Island and the underlying mechanism. Furthermore, it was demonstrated that only cell-free spent culture filtrates (stationary phase cultures that were passed through 0.22 µm pore size filters) of methanogens harboring the MIC Island were able to produce hydrogen at significantly higher levels in the presence of iron than abiotic controls. In contrast, hydrogen production by non-corrosive methanogen strains did not show differences compared to an abiotic control. Both corrosive and non-corrosive methanogens express up to six different hydrogenases for their methane metabolism. Corrosive MA harbor the MIC hydrogenase on the MIC Island as additional hydrogenase that might be involved in the enhanced hydrogen formation upon growth on iron. So far, we have not been able to receive a purified MIC hydrogenase for further corrosion studies, but in this study, some alternative approaches for the expression and purification of a functional enzyme are offered.