Thorarchaeota are a new archaeal phylum within the Asg ard superphylum, whose ancestors have been proposed to play possible ecological roles in cellular evolution. However, little is known about the lifestyles of these uncultured archaea. To
provide a better resolution of the ecological roles and metabolic capacity of Thorarchaeota, we obtained Thorarchaeota genomes reconstructed from metagenomes of different depth layers in mangrove and mudﬂat sediments. These genomes from deep anoxic layers suggest the presence of Thorarchaeota with the potential to degrade organic matter, ﬁx inorganic carbon, reduce sulfur/sulfate and produce acetate. In particular, Thorarchaeota may be involved in ethanol production, nitrogen ﬁxation, nitrite reduction, and arsenic detoxiﬁcation. Inter estingly, these Thorarchaeotal genomes are inferred to
contain the tetrahydromethanopterin and tetrahydrofolate Wood–Ljungdahl (WL) pathways for CO2 reduction, and the latter WL pathway appears to have originated from bacteria. These archaea are predicted to be able to use various inorganic and organic carbon sources, possessing genes inferred to encode ribulose bisph osphate carboxy lase-like proteins (normally without RuBisCO activity) and a near-complete Calvin–Benson– Bassham cycle. The existence of eukaryotic selen ocysteine insertion sequences and many genes for proteins previously considered eukaryote-speciﬁc in Thorarchaeota genomes provide new insights into their evolutionary roles in the origin of eukaryo tic cellular complexity. Resolving the metabolic capacities of these enigmatic archaea and their origins will enhance our understanding of the origins of eukaryotes and their
roles in ecosystems.