Page 257 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

protection of metal sheet piles. Surprisingly, methanogens formed a large part of these corrosion-protective layer microbial communities. These observations have been made before. A previous study on iron sheet piles in the Netherlands found such deposit layers with enrichments of hydrogenotrophic methanogens in five independent locations (Kip et al. 2017). Recently, such protective layers were also observed in brackish environments where iron sheet piles were exposed for 60 years to brackish water in the North Sea Canal at the IJmuiden locks (Rijkswaterstaat 2020). These deposit layers could significantly extend the sheet pile lifetime. In addition, our findings imply that the corrosion allowance, an added layer of steel that can safely corrode over a defined timeframe without safety risks, can be strongly reduced under the right conditions (Kraaijenbrink et al. 2014). This can potentially cause a reduction in the corrosion allowance thickness and an extension of sheet pile age, which strongly reduces costs,
risks, and environmental impacts of the placement and replacement procedures.
The corrosion protection is strongly linked to microbial activity, with a potential involvement of methanogens. On the one hand, methanogens can promote iron corrosion through the reduction of oxidized iron. A study by Dinh et al. (2004) found that Methanobacterium-like methanogens can directly use elemental iron as electron source. On the other hand, they might also be involved in corrosion protection. On the long term, mild corrosion induces CO2 consumption by hydrogenotrophic methanogens, which is linked to an increase in pH and mineral precipitation. In the metagenome-based study of Chapter 3, we found that an increase in pH is the potential protection mechanism for the sheet piles that were sampled in Gouderak, the Netherlands. In contrast, a recent study indicated that Methanosarcina barkeri can be involved in the production of zerovalent iron from Fe(III) (Shang et al. 2020). Corrosion reduction might therefore even be possible under the right conditions. Whether this can also occur in the field, and whether this can be used as a repair mechanism, is yet unknown. Future research should therefore also target the mimicking of these conditions in laboratory set-ups that allow for the tracking of protective layer formation and the associated changes in microbial diversity and activity. Furthermore, in situ tracking of geochemical conditions, such as in situ changes in the redox conditions, groundwater flow, and
changes in soil geochemistry, are essential to obtain a better understanding of the process.
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