DPANN archaea.

Archaea are one of the two primary domains of life alongside Bacteria. Extant archaea play an important role in global nutrient cycles and comprise members that were crucial for the evolution of life on Earth including the origin of eukaryotic cells through a symbiotic integration of an archaeal and bacterial partner. Despite their importance in ecology and evolution, our knowledge of archaeal diversity and function remains limited in part because it has proven challenging to cultivate archaea in the laboratory. Over the last two decades, the use of novel cultivation-independent approaches such as metagenomics has not only led to the discovery of a vast diversity of previously unknown archaeal lineages but also provided a window into their genomic content, allowing researchers to make predictions about metabolic functions and lifestyles. For example, by combining genomics approaches with phylogenetic analyses (that is, the reconstruction of species trees) researchers have uncovered several phylum-level lineages of putative genome-reduced archaea referred to as the 'DPANN' archaea, whose members were shown to have limited metabolic capabilities, indicating their dependency on symbiotic partners. These findings are consistent with observations from cultivation-based studies that have succeeded in enriching some of these small-cell symbionts in co-cultures with their hosts. Although they were initially discovered in extreme environments, DPANN archaea have now been shown to be widespread across a variety of environments and may thus play an important role in not only host evolution but also ecology. Herein, we aim to highlight DPANN archaea by providing an overview of their diversity, genomic and metabolic features, unique cell biology and interactions, and evolutionary origins. We also underscore several fascinating topics that remain underexplored.