Acyl homoserine lactone (AHL)-based quorum sensing commonly refers to cell density-dependent regulatory mechanisms found in bacteria. However, beyond bacteria, this cell-to-cell communication mechanism is poorly understood. Here we show that a methanogenic archaeon, Methanosaeta harundinacea 6Ac, encodes an active quorum sensing system that is used to regulate cell assembly and carbon metabolic flux. The methanogen 6Ac showed a cell density-dependent physiology transition, which was related to the AHL present in the spent culture and the filI gene-encoded AHL synthase. Through extensive chemical analyses, a new class of carboxylated AHLs synthesized by FilI protein was identified. These carboxylated AHLs facilitated the transition from a short cell to filamentous growth, with an altered carbon metabolic flux that favoured the conversion of acetate to methane and a reduced yield in cellular biomass. The transcriptomes of the filaments and the short cell forms differed with gene expression profiles consistent with the physiology. In the filaments, genes encoding the initial enzymes in the methanogenesis pathway were upregulated, whereas those for cellular carbon assimilation were downregulated. A luxI–luxR ortholog filI–filR was present in the genome of strain 6Ac. The carboxylated AHLs were also detected in other methanogen cultures and putative filI orthologs were identified in other methanogenic genomes as well. This discovery of AHL-based quorum sensing systems in methanogenic archaea implies that quorum sensing mechanisms are universal among prokaryotes.

Two strictly anaerobic, proteolytic bacterial strains, designated strain D3RC-2^Tand D3RC-3r, were isolated from a cellulose-degrading mixed culture enriched from yak rumen content. The strains were Gram-stain negative and non-spore-forming with cell sizes of 0.5–0.8×0.6–2.0 μm. The temperature range for growth was 24–46 °C (optimum 38–39 °C) and the pH range was between 5.6 and 8.7 (optimum 7.0–7.3). Both strains used soya peptone, tryptone,l-phenylalanine,l-leucine,l-methionine,l-serine,l-valine,l-threonine andl-histidine as carbon and nitrogen sources, but did not use any of the saccharides tested. The major fermentation products from PY medium were acetate, propionate and iso-butyrate. The DNA G+C contents of strains D3RC-2^Tand D3RC-3r were 41.0±0.1 mol% and 41.3±0.1 mol% (HPLC), respectively. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the two strains represented a new phyletic sublineage within the familyClostridiaceae, with <93.8 % 16S rRNA gene sequence similarity to recognized species. On the basis of the phenotypic, genotypic and physiological evidence, strains D3RC-2^Tand D3RC-3r are proposed as representing a novel species of a new genus, for which the nameProteiniclasticum ruminisgen. nov., sp. nov. is proposed. The type strain of the type species is D3RC-2^T(=AS 1.5057^T=JCM 14817^T).

Two methanogenic strains, 8-2^T and 4-1, with rod-shaped (0·4–0·5×3–5 μm), non-motile cells, sometimes observed in chains, were isolated from two anaerobic digesters in Beijing, China. The two strains used H₂/CO₂ and formate for growth and produced methane. The temperature range for growth was 25–50 °C, with fastest growth at 37 °C. The pH ranges for growth and methane production were 6·5–8·0 for strain 8-2^T and 6·8–8·6 for strain 4-1, with the fastest growth at pH 7·2 for strain 8-2^T and pH 7·5–7·7 for strain 4-1. The G+C content of genomic DNA for strain 8-2^T was 38·9 mol%. The similarity levels of the 16S rRNA sequence of strain 8-2^T with other species of the genus Methanobacterium ranged from 93·8 to 96·0 %. Based on the phylogenetic analysis and phenotypic characteristics, the novel species Methanobacterium beijingense sp. nov. is proposed, with the type strain 8-2^T (=DSM 15999^T=CGMCC 1.5011^T).

The rumen hosts one of the most efficient microbial systems for degrading plant cell walls, yet the predominant cellulolytic proteins and fibrolytic mechanism(s) remain elusive. Here we investigated the cellulolytic microbiome of the yak rumen by using a combination of metagenome-based and bacterial artificial chromosome (BAC)-based functional screening approaches. Totally 223 fibrolytic BAC clones were pyrosequenced and 10,070 ORFs were identified. Among them 150 were annotated as the glycoside hydrolase (GH) genes for fibrolytic proteins, and the majority (69%) of them were clustered or linked with genes encoding related functions. Among the 35 fibrolytic contigs of >10 Kb in length, 25 were derived from Bacteroidetes and four from Firmicutes. Coverage analysis indicated that the fibrolytic genes on most Bacteroidetes-contigs were abundantly represented in the metagenomic sequences, and they were frequently linked with genes encoding SusC/SusD-type outer-membrane proteins. GH5, GH9, and GH10 cellulase/hemicellulase genes were predominant, but no GH48 exocellulase gene was found. Most (85%) of the cellulase and hemicellulase proteins possessed a signal peptide; only a few carried carbohydrate-binding modules, and no cellulosomal domains were detected. These findings suggest that the SucC/SucD-involving mechanism, instead of one based on cellulosomes or the free-enzyme system, serves a major role in lignocellulose degradation in yak rumen. Genes encoding an endoglucanase of a novel GH5 subfamily occurred frequently in the metagenome, and the recombinant proteins encoded by the genes displayed moderate Avicelase in addition to endoglucanase activities, suggesting their important contribution to lignocellulose degradation in the exocellulase-scarce rumen.