Abstract Each prokaryotic domain, Bacteria and Archaea, contains a large and diverse group of organisms characterized with ultrasmall cell size and symbiotic lifestyles – Patescibacteria (also known as Candidate Phyla Radiation/CPR) and DPANN archaea. Cultivation-based approaches have revealed that Patesibacteria and DPANN symbiotically interact with bacterial and archaeal partners/hosts respectively, but cross-domain symbiosis/parasitism has never been observed. Here, we discovered physical interaction between ultramicrobacterial Patescibacteria and methanogenic archaea using cultures from anaerobic wastewater treatment sludge. In the cultures, we observed physical attachment of ultramicrobial cells to cells resembling Methanothrix and Methanospirillum using transmission electron microscopy and successfully detected physical association of Ca . Yanofskybacteria and Methanothrix using fluorescence in situ hybridization (FISH) (other ultramicrosized bacterial cells, presumably Patescibacteria, were also observed to attach on Methanospirillum ). This was further confirmed to be a symbiosis rather than simple aggregation based on the observation that most ultramicrobacterial cells attached to Methanothrix were Ca . Yanofskybacteria and positive correlation ( p &lt; 0.05) between the relative abundance of Patescibacteria lineages and methanogenic archaea ( e.g., Ca . Yanofskybacteria– Methanothrix and uncultured clade 32-520– Methanospirillum ). The results shed light on a novel cross-domain symbiosis and inspire potential strategies for culturing CPR/DPANN.

Symbiosis between Patescibacteria and Archaea discovered in wastewater-treating bioreactors