Page 72 - 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
The ability to accept electrons via direct interspecies electron transfer (DIET) can be an important factor in shaping the methanogenic community. Methanosarcinales possess c-type cytochromes that play a role in DIET, as shown for co-cultures with Geobacter metallireducens (Rotaru et al. 2014a). A recent study by Yee et al. (2019) showed the ability of Methanosarcina species to directly accept electrons from insoluble electron donors, including conductive particles and biocathodes that show similarities to the metal sheet piles in our study. A study by Jaramillo et al. (2019) observed a competition of Methanosarcina species with acetogens in nonsulfidic environments. The study’s observations of higher methanogenesis rates without a mixed community that includes Clostridiales indicated a competition to obtain electrons from elemental iron. This finding supports a central role for elemental iron in structuring the community.
Methanomicrobiales and Methanobacteriales do not possess c-type cytochromes (Kühn et al. 1983). However, the archaellum of Methanobacteriales species Methanospirillum hungatei, which is structurally related to the type IV pilus, is electrically conductive (Walker et al. 2019). Similar observations have been well described for Methanococcus species (Albers and Jarrell 2015). In addition, iron minerals such as magnetite (Fe3O4) and, to a lesser extent, limonite and wustite were observed in natural CPLs by Kip et al. (Kip et al. 2017). These can compensate for the lack of pilin-associated c-type cytochromes as shown for Geobacter sulfurreducens (Liu et al. 2015). The methanogen diversity might be further explained by the absence of mixing and disturbances, which could result in the formation of small niches (Tang et al. 2011). At low temperatures, hydrogenotrophic methanogens with higher affinity for H2 than homoacetogens can win the competition in syntrophic reactions of fatty acid decomposition (Kotsyurbenko 2005a). This could explain the relatively high metabolic complexity and diversity of the methanogenic archaea in DL.
Role of iron under anoxic conditions
The microbial community in DL is distinctive from that in AS. It is therefore tempting to speculate that the microbial community is responsible for the formation of the CPLs. Several methanogenic archaeal MAGs were enriched in the metagenomes of DL. Genomic metabolic potential analysis of the MAGs unique to DL indicated that they form an anaerobic food chain degrading organic matter under methanogenic conditions (Fig. 4).
70