The importance of biofilm formation for cultivation of a Micrarchaeon and its interactions with its Thermoplasmatales host

Micrarchaeota is a distinctive lineage assigned to the DPANN archaea, which includes poorly
characterised microorganisms with reduced genomes that likely depend on interactions with
hosts for growth and survival. Here, we report the enrichment of a stable co-culture of a
member of the Micrarchaeota (Ca. Micrarchaeum harzensis) together with its Thermoplasmatales host (Ca. Scheffleriplasma hospitalis), as well as the isolation of the latter. We
show that symbiont-host interactions depend on biofilm formation as evidenced by growth
experiments, comparative transcriptomic analyses and electron microscopy. In addition,
genomic, metabolomic, extracellular polymeric substances and lipid content analyses indicate
that the Micrarchaeon symbiont relies on the acquisition of metabolites from its host. Our
study of the cell biology and physiology of a Micrarchaeon and its host adds to our limited
knowledge of archaeal symbioses.