Anaerobic oxidation of propane by thermophilic archaea from marine hydrothermal sediments

Propane is a major constituent of natural gas and an important substrate in anoxic hydrocarbon-rich environments. While archaeal oxidation of the even-chain volatile alkanes ethane and butane have been demonstrated and mechanistically characterized, the organisms and mechanisms underlying archaeal oxidation of propane, the archetypal odd-chain alkane, have remained unknown. Here we describe Propane50, a thermophilic propane-oxidizing enrichment culture derived from Guaymas Basin hydrothermal sediments. Metagenomics and direct cell identification with lineage-specific oligonucleotide probes identified Candidatus Syntrophoarchaeum caldarium as the dominant archaeon, growing in association with the sulfate-reducing bacterium Candidatus Desulfofervidus auxilii. Targeted metabolomics identified both 1- and 2-propyl-coenzyme M, demonstrating propane activation through an archaeal alkyl-coenzyme M reductase (ACR). Proteogenomic analyses revealed a single highly expressed ACR complex among four complete acr operons encoded by Ca. S. caldarium, suggesting ACR1 as the enzyme catalyzing propane activation. Integrated genome, proteome and metabolite analyses suggest conversion of propyl-CoM to propionyl-CoA, followed by complete oxidation through a variant of the 3-hydroxypropionyl-CoA pathway and the reverse Wood-Ljungdahl pathway. Proteogenomics identified abundant multiheme cytochromes in both partners, supporting the involvement of a direct extracellular electron transfer mechanism during syntrophic propane oxidation. These findings establish archaeal propane oxidation and expand the known diversity of anaerobic archaeal volatile alkane metabolism.