Tiny genome discovered provides hints for limits of life
Scientists have identified a previously unknown microorganism with the smallest genome yet found in an archaeon, a type of single-celled organism distinct from bacteria. Named Candidatus Sukunaarchaeum mirabile, it appears to retain the genetic machinery needed to reproduce and make proteins despite having lost almost all the genes required to produce its own nutrients and energy.
The organism, known as Sukunaarchaeum, was discovered through genetic analysis of individual marine microorganisms and contains just 189 protein-coding genes. Its genome is less than half the size of the smallest archaeal genome previously known.
The designation Candidatus is used for organisms that have been proposed as new taxonomic groups but have not yet been fully characterised. In this case, researchers have identified Sukunaarchaeum's genetic material but have not directly observed the organism or identified a potential host.
An international team including researchers from the University of Nottingham and the University of Tsukuba in Japan found that Sukunaarchaeum is genetically distinct from all previously known groups of archaea. Related genetic sequences in marine samples suggest it may belong to a much larger group of organisms that has so far gone largely unnoticed.
Despite its unusually small genome, Sukunaarchaeum appears to have retained the machinery needed to copy its DNA and use its genetic information to produce proteins. However, it has lost almost all the genes needed to make nutrients and energy, suggesting it depends heavily on other organisms for the resources required to survive.
Around a quarter of its genome consists of genes encoding unusually large membrane proteins whose functions remain unknown. Similar proteins occur in some parasitic archaea, raising the possibility that Sukunaarchaeum lives inside or alongside another organism, although this has yet to be established.
Professor Thorsten Allers, from the University of Nottingham's School of Life Sciences and a co-author of the study, said the discovery offered new clues about how simple a living cell could become while retaining the ability to reproduce and maintain its genetic information.
The findings, published in Current Biology (DOI 10.1016/j.cub.2026.09.042), could help scientists better understand the limits of cellular life and the minimum genetic information needed to sustain it. The team plans to investigate where Sukunaarchaeum lives, what it depends on and how it interacts with other organisms.
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