Researchers have discovered a tiny marine microorganism named Candidatus Sukunaarchaeum mirabile, which possesses the smallest genome ever found. Containing just 189 protein-coding genes, the newly described organism has lost almost all genes needed to produce energy, suggesting a highly dependent or parasitic existence.
Candidatus Sukunaarchaeum mirabile Discovery in Marine Plankton
Scientists exploring the genetic material of individual marine microorganisms discovered a previously unknown single-celled microbe belonging to the archaea. Researchers from the University of Nottingham and the University of Tsukuba in Japan participated in the international effort, with their findings published in the journal Current Biology. The newly identified organism was named Candidatus Sukunaarchaeum mirabile, or Sukunaarchaeum for short, in honor of a small Japanese god.
Plankton samples yielded genetic sequences indicating that Sukunaarchaeum belongs to a much broader group of organisms that has largely escaped notice until now. Although the microorganism belongs to archaea—which are fundamentally different from bacteria—it stands apart through its extreme genetic reduction.
Sukunaarchaeum Maintains Replication Tools Despite Minimal Genome Size
The genome of Sukunaarchaeum contains only 189 protein-coding genes, measuring less than half the size of the smallest archaeal genome previously documented. Despite this minimal genetic catalog, the organism preserves the essential machinery required to copy its DNA and synthesize proteins.
That specific retention pattern sets the organism apart from typical reduced cells. While cellular components like mitochondria and chloroplasts rely on host cells for replication machinery, Sukunaarchaeum maintained its own replication tools while discarding its metabolic pathways.
Lack of Metabolic Genes Suggests Parasitic Lifestyle
Because the archaeon lacks nearly all genes necessary to independently generate nutrients and energy, researchers conclude it relies heavily on other organisms to survive. Approximately a quarter of the Sukunaarchaeum genome is dedicated to encoding unusually large membrane proteins whose exact functions remain unidentified.
Similar oversized membrane proteins appear in certain parasitic archaea. That structural similarity leads the research team to suggest that the microorganism might live as a parasite inside or alongside another host organism.
“The key to defining life is whether something can replicate itself, and whether it can do this autonomously. This exciting discovery provides new clues about how simple a living cell can become while remaining capable of reproducing and maintaining its own genetic information.”
Professor Thorsten Allers, School of Life Sciences at the University of Nottingham
Unresolved Questions and Next Steps for the Research Team
Direct observation of Sukunaarchaeum has not yet occurred, and scientists have yet to determine its host organism. Investigators plan to trace where the archaeon lives, what specific resources it requires, and how it engages with neighboring marine life.
Those upcoming investigations aim to establish the absolute lower boundaries of genetic information required for an independent cell to persist and reproduce.