Defining the minimal genetic requirements for cellular life remains a fundamental question in biology. Genomic exploration continually reveals novel microbial lineages, often exhibiting extreme genome reduction, particularly within symbiotic relationships. Here, we report the discovery of Candidatus Sukunaarchaeum mirabile, a novel archaeon with an unprecedentedly small genome of only 238 kbp —less than half the size of the smallest previously known archaeal genome— from a dinoflagellate-associated microbial community. Phylogenetic analyses place Sukunaarchaeum as a deeply branching lineage within the tree of Archaea, representing a novel major branch distinct from established phyla. Environmental sequence data indicate that sequences closely related to Sukunaarchaeum form a diverse and previously overlooked clade in microbial surveys. Its genome is profoundly stripped-down, lacking virtually all recognizable metabolic pathways, and primarily encoding the machinery for its replicative core: DNA replication, transcription, and translation. This suggests an unprecedented level of metabolic dependence on a host, a condition that challenges the functional distinctions between minimal cellular life and viruses. The discovery of Sukunaarchaeum pushes the conventional boundaries of cellular life and highlights the vast unexplored biological novelty within microbial interactions, suggesting that further exploration of symbiotic systems may reveal even more extraordinary life forms, reshaping our understanding of cellular evolution. ![Figure][1] Graphical Abstract ### Competing Interest Statement The authors have declared no competing interest. Japan Society for the Promotion of ScienceJapan Society for the Promotion of Science, https://ror.org/00hhkn466, JP16H06280, 20H03305, 21K15131, 22KJ0401, 23K27226, 24K09587 Institute for FermentationInstitute for Fermentation, https://ror.org/05nq89q24, G-2025-2-067 the World Premier International Research Center Initiativethe World Premier International Research Center Initiative, , [1]: pending:yes

Defining the minimal genetic requirements for cellular life remains a fundamental question in biology…Here, we report the discovery of Candidatus Sukunaarchaeum mirabile, a novel archaeon with an unprecedentedly small genome of only 238 kbp —less than half the size of the smallest previously known archaeal genome…lacking virtually all recognizable metabolic pathways, and primarily encoding the machinery for its replicative core: DNA replication, transcription, and translation. This suggests an unprecedented level of metabolic dependence on a host, a condition that challenges the functional distinctions between minimal cellular life and viruses…

  • meyotch@slrpnk.netEnglish
    3·
    8 days ago

    As a junior biologist, I remember having the realization that viruses and other quasi-living things are just an epi-phenomenon of life. That was a level-up moment in my training, a lot of things clicked into place.

    Meaning, if one could magically remove all viruses from the biosphere, they would re-evolve very quickly. Let alone the fact there are significant numbers of viral fragments in our DNA, some of which have been co-opted and used for crucial purposes, such as the mammalian placenta.

    Plus the tendency of some of those fragments to move around the genome, providing a source of variation that is sometimes selected for.

    Viruses and maybe this archaea may not meet a binary definition of ‘life’, but any biosphere anywhere in the universe will have things like them in great abundance. Any evolving system will produce them.