Methanosaeta and Candidatus Velamenicoccus archaeovorus

The Candidatus phylum Omnitrophica (candidate division OP3) occurs ubiquitous in anaerobic habitats, but is currently characterized only by draft genomes from metagenomes and single cells. We had visualized cells of the phylotype OP3 LiM in methanogenic cultures on limonene as small epibiontic cells. In this study, we enriched OP3 cells by double density centrifugation and obtained the first closed genome of an apparently clonal OP3 cell population applying metagenomics and PCR for gap closure. Filaments of acetoclastic Methanosaeta, the largest morphotype in limonene enrichment cultures, contained empty cells, dead cells and cells devoid of rRNA or both rRNA and DNA according to TEM, thin-section TEM, SEM, CARD-FISH and Live/Dead images. OP3 LiM cells were ultramicrobacteria (200-300 nm in diameter) and showed two physiological stages in CARD-FISH fluorescence signals: strong signals indicated many rRNA molecules and an active metabolism of OP3 LiM cells attached to Bacteria and to Archaea, whereas free-living OP3 cells had weak signals. Metaproteomics revealed that OP3 LiM lives with highly expressed secreted proteins involved in depolymerization and uptake of macromolecules, an active glycolysis and energy conservation by the utilization of pyruvate via a pyruvate:ferredoxin oxidoreductase and an RNF complex (Ferredoxin:NAD oxidoreductase). Besides sugar fermentation, a nucleotidyl transferase may contribute to energy conservation by phosphorolysis, the phosphate-dependent depolymerization of nucleic acids. Thin section TEM showed distinctive structures of predation that had been previously observed for “Velamenicoccus”. Our study demonstrated a predatory metabolism for OP3 LiM cells and we propose as name for OP3 LiM Candidatus Velamenicoccus archaeovorus gen. nov., sp. nov..