Culture-independent DNA sequencing approaches have explored diverse yet-to-be-cultured microorganisms and have significantly expanded the tree of life in recent years. One major lineage of the domain Bacteria,
. Patescibacteria (also known as candidate phyla radiation), is widely distributed in natural and engineered ecosystems and has been thought to be dependent on host bacteria due to the lack of several biosynthetic pathways and small cell/genome size. Although bacteria-parasitizing or bacteria-preying
. Patescibacteria have been described, our recent studies revealed that some lineages can specifically interact with archaea. In this study, we provide strong evidence that the relationship is parasitic, shedding light on overlooked roles of
. Patescibacteria in anaerobic habitats.