Functional insights of salinity-stress related pathways in metagenome-resolved Methanothrix genomes. Salinity adapted methanothrix

Recently, methanogenic archaea belonging to the genus Methanothrix were reported to have a fundamental role to stable ecosystem function under different environmental conditions and anaerobic bioreactor configurations. In this study, we reconstructed three Methanothrix genomes from granular sludge developed within saline upflow anaerobic sludge blanket (UASB) reactors, were Methanothrix harundinacea was already shown to be pivotal to obtain compact and stable granules under elevated salinity levels (> 5 g/L Na+). One of these genomes was identified as Methanothrix harundinacea and named MAG_279, which was confirmed to be dominant during the UASB process and outcompeting other methanogens when growing at 20 g/L Na+. Genome annotation of the reconstructed metagenome-assembled genome (MAG) revealed a genetic repertoire enabling M. harundinacea MAG_279 to grow at high salinity. In particular, when the salinity increases M. harundinacea MAG_279 has the potential to implement its salinity resistance through build-up and excretion of different glycoconjugates via the N-glycosilation process, and via the production of the compatible solutes Nε-acetyl-β-lysine and ectoine. An additional potential stress-related pathway identified is the stabilization and reinforcement of the cell membrane via the production of isoprenoids and carotenoids. The understanding of the salinity stress related mechanisms of M. harundinacea unravel a further key-role in an extreme habitat for a member of this genus, opening new perspectives for high-efficiency methanisation of organic waste at high salinities.