A Micrarchaeon Isolate Is Covered by a Proteinaceous S-Layer

In previous publications, it was hypothesized that Micrarchaeota cells are covered by two individual membrane systems. This study proves that at least the recently cultivated "Candidatus Micrarchaeum harzensis A_DKE" possesses an S-layer covering its cytoplasmic membrane. The potential Slayer protein was found to be among the proteins with the highest abundance in "Ca. Micrarchaeum harzensis A_DKE," and in silico characterization of its primary structure indicated homologies to other known S-layer proteins. Homologues of this protein were found in other Micrarchaeota genomes, which raises the question of whether the ability to form an S-layer is a common trait within this phylum. The S-layer protein seems to be glycosylated, and the micrarchaeon expresses genes for N-glycosylation under cultivation conditions, despite not being able to synthesize carbohydrates. Electron micrographs of freeze-etched samples of a previously described coculture, containing "Ca. Micrarchaeum harzensis A_DKE" and a Thermopiasmatales member as its host organism, verified the hypothesis of an S-layer on the surface of "Ca. Micrarchaeum harzensis A_DKE." Both organisms are clearly distinguishable by cell size, shape, and surface structure. IMPORTANCE Our knowledge about the DPANN superphylum, which comprises several archaeal phyla with limited metabolic capacities, is mostly based on genomic data derived from cultivation-independent approaches. This study examined the surface structure of a recently cultivated member "Candidatus Micrarchaeum harzensis A_DKE," an archaeal symbiont dependent on an interaction with a host organism for growth. The interaction requires direct cell contact between interaction partners, a mechanism which is also described for other DPANN archaea. Investigating the surface structure of "Ca. Micrarchaeum harzensis A_DKE" is an important step toward understanding the interaction between Micrarchaeota and their host organisms and living with limited metabolic capabilities, a trait shared by several DPANN archaea.