Page 260 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 11. Integration and outlook
term warming (Chapter 9), which suggests that a niche for hydrogen-dependent methylotrophic methanogenesis can be present in thawing permafrost. Accordingly, we also found rapid conversion of methylated substrates to methane, although these conversions were mainly linked to Methanosarcinaceae that are often not dependent on simultaneous H2 availability (Chapter 7 and 8).
These recent findings highlight that there is still an undiscovered world of methanogenic archaea that fulfill yet understudied metabolic functions. The hunt for novel CH4 cycling microbes provides important knowledge on microbial methane cycling in a wide variety of ecosystems.
Bathyarchaeota are likely important players in organic matter turnover
In 2015, the discovery of mcrA-containing archaea outside of the Euryarchaeota phylum sparked the discussion on whether methanogenesis could occur in other archaeal phyla (Evans et al. 2015; Vanwonterghem et al. 2016). With this observation, it was hypothesized that Bathyarchaeota are methylotrophic methanogens that are growing on methylated compounds present in e.g. coal and peat, and possessing the ability to ferment peptides, sugars, and fatty acids. However, there is no experimental evidence of CH4 production by Bathyarchaeota. Genomic and enzymatic analysis of several Bathyarchaeal lineages showed their capacity for acetogenesis and fermentation of a wide range of organic substrates, including cellulose and aromatic compounds (He et al. 2016; Lazar et al. 2016). Based on our current knowledge, these mcrA-related sequences seem indicative of a (semi)-fermentative lifestyle on non-CH4 alkanes (Borrel et al. 2019; Laso-Pérez et al. 2019; Wang et al. 2019).
We observed Bathyarchaeota in thermokarst lake sediments, permafrost soils, freshwater, and marine peat sediments. Only in the amended coal well study of Chapter 4 we did not detect Bathyarchaeota-related sequences, potentially due to the highly specialized conditions in situ, and limitations in the knowledge and reference databases back in 2015. In fact, Evans et al. obtained the first mcrA-containing Bathyarchaeota from coal-bed CH4 wells in the Surat Basin in Queensland, Australia (Evans et al. 2015). Follow-up studies have mainly detected them in carbon-rich sediments, which supports their potential role in organic matter turnover (Lazar et al. 2016; Xiang et al. 2017; Zhou et al. 2018; Juottonen et al. 2020). Their high relative abundance of sometimes over 90% of archaeal sequences, as was for example observed in
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