The methanogenic degradation of oil hydrocarbons can proceed through syntrophic partnerships of hydrocarbon-degrading bacteria and methanogenic archaea [1][2][3] . However, recent culture-independent studies have suggested that the archaeon 'Candidatus Methanoliparum' alone can combine the degradation of long-chain alkanes with methanogenesis 4,5 . Here we cultured Ca. Methanoliparum from a subsurface oil reservoir. Molecular analyses revealed that Ca. Methanoliparum contains and overexpresses genes encoding alkyl-coenzyme M reductases and methyl-coenzyme M reductases, the marker genes for archaeal multicarbon alkane and methane metabolism. Incubation experiments with different substrates and mass spectrometric detection of coenzyme-M-bound intermediates confirm that Ca. Methanoliparum thrives not only on a variety of long-chain alkanes, but also on n-alkylcyclohexanes and n-alkylbenzenes with long n-alkyl (C ≥13 ) moieties. By contrast, short-chain alkanes (such as ethane to octane) or aromatics with short alkyl chains (C ≤12 ) were not consumed. The wide distribution of Ca. Methanoliparum 4-6 in oil-rich environments indicates that this alkylotrophic methanogen may have a crucial role in the transformation of hydrocarbons into methane.In subsurface oil reservoirs and marine oil seep sediments, microorganisms use hydrocarbons as a source of energy and carbon 7,8 . The microorganisms preferentially consume alkanes, cyclic and aromatic compounds, leaving an unresolved complex mixture as residue and thereby altering the quality of the oil 7,8 . In the absence of sulfate, microorganisms couple anaerobic hydrocarbon degradation to methane formation 1,9,10 . This reaction was originally demonstrated by Zengler et al 2 as methanogenic 'microbial alkane cracking', and a large number of studies have shown that it can be performed in syntrophic interactions of bacteria and archaea 11 . In this syntrophy, the bacteria ferment the oil to acetate, carbon dioxide and hydrogen, while hydrogenotrophic and/or acetotrophic methanogenic archaea use the products for methanogenesis 1,2,11 .Diverse anaerobic hydrocarbon activation mechanisms exist, including the well-studied fumarate addition pathway catalysed by glycyl radical enzymes 12 . This mechanism is widespread among bacteria that thrive on alkanes of various chain lengths and other hydrocarbons 12,13 . By contrast, several archaeal lineages activate gaseous alkanes with the help of a specific type of methyl-coenzyme M reductase (MCR), an enzyme that was originally described to catalyse the reduction of methyl-coenzyme M (methyl-CoM) to methane in methanogens 14 . Anaerobic methanotrophic archaea use canonical MCRs to activate methane into methyl-CoM, which is then oxidized to CO 2 . Short-chain alkane-oxidizing archaea contain divergent variants of this enzyme, which are known as alkyl-CoM reductases (ACRs). Analogous to the methane-activating MCR, ACRs activate multicarbon alkanes to form CoM-bound alkyl units [15][16][17] . The cultured alkane-oxidizing archaea oxidize sho...