Page 74 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 3. Metal corrosion protection potential of methanogenic communities
sediments. Some hydrogenotrophic methanogens can also directly reduce ferrihydrite, whereas some acetoclastic methanogens can switch between methanogenesis and iron reduction (Sivan, Shusta and Valentine 2016; Palacios et al. 2019; Prakash, Chauhan and Ferry 2019; Tang et al. 2019).
In addition, in a study on anaerobic benzene-oxidizing, iron-reducing cultures, Kunapuli et al. (2007) hypothesized that electrons from benzene degradation can be directly transferred to ferric iron or shared between community members, including methanogens (Kunapuli, Lueders and Meckenstock 2007). The interactions between methanogenesis and iron reduction are highly complex, and there are strong indications that both processes can occur simultaneously (Reiche, Torburg and Küsel 2008; Rissanen et al. 2017; Marquart et al. 2019).
Besides biotic processes, iron dissolves slowly and forms H2 with protons (H+) from water (Fe + 2H2O ↔ Fe2+ H2 +2OH-; E0’=-0.03 V). Although this is a slow process, it can be accelerated by microbial hydrogen consumption, such as hydrogenotrophic methanogenesis. This could be relevant in shaping the initial microbial community over the life span of iron sheet piles.
A possible reason for the subsequent formation of the CPLs could be the change of pH elicited by the activity of the ferrotrophic/hydrogenotrophic methanogens. In contrast, the degradation of complex organic compounds under lower methanogenic activity leads to an equilibrium at more acidic pH that is less optimal for mineral precipitation (McCauley, Jones and Jacobsen 2017). At the iron sheet piles, the presence of methanogens leads to the consumption of CO2 that is in equilibrium with carbonate and will result in a local pH increase at the iron sheet pile surface. Because of calcium- and magnesium-rich groundwater fluxes, a locally increased pH can facilitate the precipitation of iron, calcium, and magnesium minerals.
This deposit layer additionally protects the iron sheet pile from corrosion by buffering the sheet pile surface from direct acidification. Since the microbial community in and surrounding the DL stays active, the CPL can be maintained (Fig. 1). This significantly increases the life span of iron sheet piles, reducing safety risks and the economic costs of replacement. Our current understanding of natural CPLs, however, remains limited. More research confirming the role
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