Page 73 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Figure 4. Scheme illustrating the potential microbial food web of the deposit layer and the surrounding iron sheet pile (indicated by “Fe”) and organic-rich sediment. The microbial community of the surrounding sediment has the capacity to degrade organic matter (OM) to volatile fatty acids (VFA), alcohols, H2, and CO2. Syntrophobacterales can syntrophically oxidize VFA and alcohols to H2, CO2, and acetate. Methanogens can use substrates produced by the bacterial community and potentially directly obtain electrons from iron oxidation. Black arrows indicate microbial conversions, red arrows indicate redox reactions that involve extracellular electron transfer, and blue arrows indicate abiotic iron oxidation.
The background community was typical for organic-rich sediments and showed similarities between AS and DL. The sheet piles are initially placed in a mildly corrosive environment, potentially facilitating growth of methanogenic archaea. Closely related methanogens were shown to use elemental iron as a direct electron source (Dinh et al. 2004). Dinh et al. (2004) isolated a Methanobacterium-like species under low-sulfate conditions with supplementation of elemental iron. Interestingly, this species grew faster when iron was provided, and strictly hydrogenotrophic growth was slow and only obtained at pH >7.5. This growth strategy could also be facilitated in the initial phase of iron sheet pile placement in the anoxic methanogenic
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