An Asgard archaeon from a modern analog of ancient microbial mats.

One of the most significant events in the evolution of life is the origin of the eukaryotic cell. Despite recent advances, the driving forces behind the emergence of complex eukaryotic attributes remain a gap in our knowledge. One model proposes that eukaryotic cells evolved via symbiosis between sulfate-reducing bacteria and hydrogen-producing archaea in ancient microbial mats. Here, we describe a highly enriched (89%) culture of a novel Asgard archaeon, Nerearchaeum marumarumayae, along with a bacterium Stromatodesulfovibrio nilemahensis from a modern microbial mat. The N. marumarumayae genome indicates that it has the capacity to produce H2, acetate, formate, and sulfite, while S. nilemahensis synthesizes amino acids and vitamins that could be exchanged in a syntrophic partnership. Electron cryotomography revealed that N. marumarumayae cells produce chains of budded envelope vesicles attached to the coccoid cell body by extracellular fibers, as well as intracellular tube- and cage-like structures. Furthermore, the two species were observed directly interacting via intercellular tubular fibers assembled by the bacterium. These characteristics and interactions may reflect an early step in the symbiotic evolution of eukaryotic cells.