Ecological drivers of Promethearchaeati's specialization for deep subsurface environments.

Promethearchaeati, a recently discovered kingdom of archaea, shares striking similarities with eukaryotes, including genes for cellular complexity previously thought unique to eukaryotes. Despite these features, Promethearchaeati thrive almost exclusively in stable subsurface environments, unlike their eukaryotic relatives that thrive on Earth's surface. Here we explore why these microbes remain confined to the subsurface, examining their slow growth, fragile cellular structures, and dependence on microbial partners. We discuss how their eukaryotic-like traits might function in subsurface ecosystems and why these traits may be selective in these types of ecosystems.