Page 261 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 5, is striking due to their expected low metabolic activity and growth, as was demonstrated in an estuarine sediment enrichment culture on lignin (Yu et al. 2018). The potential growth of Bathyarchaeota subgroup 8 (Bathy-8) enriched from estuarine sediments on lignin furthermore suggested that they can play a key role in the degradation of less labile plant organic matter fractions that can be found in e.g. permafrost, peat deposits, and coal deposits (Yu et al. 2018). If they can indeed grow on lignin, this supports their high relative abundances in these ecosystems.
Interestingly, in a recent metagenomics study of the Polar Fox Lagoon at the Bykovsky Peninsula in north Siberia, we could recover six high-quality Bathyarchaeotal MAGs (unpublished data). Although we did not find CH4 cycle-related genes, preliminary findings suggest their potential in complex organic matter turnover. As of today, no Bathyarchaeotal species have been successfully enriched or obtained as pure culture, despite their omnipresence in organic-rich sediments. The recovery of high-quality MAGs that allow for metabolic pathway reconstructions currently provides the best insights into their metabolism and ecosystem function.
Anaerobic oxidation of methane as the first biological methane filter
In the 2000s, the discovery of archaea catalyzing the anaerobic oxidation of methane (AOM) strongly altered our view on the methane cycle (Boetius et al. 2000). After the discovery of sulfate-AOM, also AOM processes using nitrite, nitrate and metal-oxides have been discovered (Ettwig et al. 2010; Haroon et al. 2013; Ettwig et al. 2016; Cai et al. 2018). Although the contribution of AOM in many pristine terrestrial ecosystems appears to be small, several discoveries suggest they might play a more important role than has so far been estimated (Gupta et al. 2013; Vaksmaa et al. 2017a; Winkel et al. 2018). Especially the potential for metal- dependent AOM (here abbreviated as Metal-AOM) has recently been described in more detail, and although there are no pure cultures, there are several highly enriched cultures available for metabolic studies (Cai et al. 2018; He et al. 2019b; Leu et al. 2020). Evidence that Metal-AOM can occur in permafrost is currently appearing, but it is too early to draw any conclusions on its potential as CH4 sink (Winkel et al. 2019).
Iron is generally present in permafrost environments (Rivkina et al. 1998; Hultman et al. 2015; Mu et al. 2016b). In our permafrost study of Chapter 9, we also observed relatively high
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