Methanogenic archaea use a bacteria-like methyltransferase system to demethoxylate aromatic compounds

Methane-generating archaea drive the final step in anaerobic organic compound mineralization and dictate the carbon flow of Earth’s diverse anoxic ecosystems in the absence of inorganic electron acceptors. Although such Archaea were presumed to be restricted to life on simple compounds like hydrogen (H2), acetate or methanol, an archaeon, Methermicoccus shengliensis, was recently found to convert methoxylated aromatic compounds to methane. Methoxylated aromatic compounds are important components of lignin and coal, and are present in most subsurface sediments. Despite the novelty of such a methoxydotrophic archaeon its metabolism has not yet been explored. In this study, transcriptomics and proteomics reveal that under methoxydotrophic growth M. shengliensis expresses an O-demethylation/methyltransferase system related to the one used by acetogenic bacteria. Enzymatic assays provide evidence for a two step-mechanisms in which the methyl-group from the methoxy compound is (1) transferred on cobalamin and (2) further transferred on the C1-carrier tetrahydromethanopterin, a mechanism distinct from conventional methanogenic methyl-transfer systems which use coenzyme M as final acceptor. We further hypothesize that this likely leads to an atypical use of the methanogenesis pathway that derives cellular energy from methyl transfer (Mtr) rather than electron transfer (F420H2 re-oxidation) as found for methylotrophic methanogenesis.

An aerobic link between lignin degradation and C1 metabolism: growth on methoxylated aromatic compounds by members of the genus Methylobacterium

Microorganisms faces many barriers in the degradation of the polycyclic aromatic polymer lignin, one of which is an abundance of methoxy substituents. Demethoxylation of lignin-derived aromatic monomers in aerobic environments releases formaldehyde, a potent cellular toxin that organisms must eliminate in order to further degrade the aromatic ring. Here we provide the first comprehensive description of the ecology and evolution of the catabolism of methoxylated aromatics in the genus Methylobacterium, a plant-associated genus of methylotrophs capable of using formaldehyde for growth. Using comparative genomics, we found that the capacity for aromatic catabolism is ancestral to two clades, but has also been acquired horizontally by other members of the genus. Through laboratory growth assays, we demonstrated that several Methylobacterium strains can grow on p-hydroxybenzoate, protocatechuate, vanillate, and ferulate; furthermore, whereas non-methylotrophs excrete formaldehyde as a byproduct during growth on vanillate, Methylobacterium do not. Finally, we surveyed published metagenome data to find that vanillate-degrading Methylobacterium can be found in many soil and rhizosphere ecosystems but is disproportionately prominent in the phyllosphere, and the most highly represented clade in the environment (the root-nodulating species M. nodulans) is one with few cultured representatives.

Obligate Sulfide-Dependent Degradation of Methoxylated Aromatic Compounds and Formation of Methanethiol and Dimethyl Sulfide by a Freshwater Sediment Isolate,Parasporobacterium paucivorans gen. nov., sp. nov

ABSTRACT Methanethiol (MT) and dimethyl sulfide (DMS) have been shown to be the dominant volatile organic sulfur compounds in freshwater sediments. Previous research demonstrated that in these habitats MT and DMS are derived mainly from the methylation of sulfide. In order to identify the microorganisms that are responsible for this type of MT and DMS formation, several sulfide-rich freshwater sediments were amended with two potential methyl group-donating compounds, syringate and 3,4,5-trimethoxybenzoate (0.5 mM). The addition of these methoxylated aromatic compounds resulted in excess accumulation of MT and DMS in all sediment slurries even though methanogenic consumption of MT and DMS occurred. From one of the sediment slurries tested, a novel anaerobic bacterium was isolated with syringate as the sole carbon source. The strain, designated Parasporobacterium paucivorans, produced MT and DMS from the methoxy groups of syringate. The hydroxylated aromatic residue (gallate) was converted to acetate and butyrate. Like Sporobacterium olearium, another methoxylated aromatic compound-degrading bacterium, the isolate is a member of the XIVa cluster of the low-GC-contentClostridiales group. However, the new isolate differs from all other known methoxylated aromatic compound-degrading bacteria because it was able to degrade syringate in significant amounts only in the presence of sulfide.