AbstractThe Candidate Phyla Radiation (CPR, or superphylum Patescibacteria) is a very large group of bacteria with few cultivated representatives first discovered by culture-independent metagenomic analyses. Within the CPR, the candidate phylum Parcubacteria (previously OD1) is prevalent in anoxic lake sediments and groundwater. We identified a bacterium belonging to the Parcubacteria in a methanogenic benzene-degrading enrichment culture originally derived from oil-contaminated sediments. Candidatus Nealsonbacteria DGGOD1a is the only bacterium other than a previously identified benzene-degrading fermenter (Deltaproteobacteria Candidate Sva0485 clade ORM2) consistently and abundantly detected in all active benzene-degrading transfers of this culture. Therefore, we hypothesized that DGGOD1a must serve an important role in sustaining anaerobic benzene metabolism in the consortium. Growth experiments using a variety of possible substrates suggested that it is involved in biomass recycling. Microscopic observations supported by molecular analyses and a closed genome revealed an epibiont lifestyle with very small Ca. Nealsonbacteria DGGOD1a closely associated with much larger Methanosaeta. The images reveal a first example of cross-domain episymbiosis that may apply to other Ca. Nealsonbacteria found in diverse environments.