Researchers have identified a microscopic organism with the smallest genome reported for any member of its domain, yet its genetic material retains the core machinery for DNA replication and protein production.
The discovery, published in Current Biology by an international team including researchers from the University of Nottingham and the University of Tsukuba, raises new questions about the minimum genetic requirements for cellular life.
A Tiny God, an Even Smaller Genome
The organism is a member of the archaea, a group of single-celled microbes genetically distinct from bacteria. It was discovered by researchers during genetic sequencing of individual marine plankton and has been provisionally named Candidatus Sukunaarchaeum mirabile, or Sukunaarchaeum. This name refers to Sukunabikona, a deity from Japanese mythology whose name translates to “renowned little lord”. Sukunabikona is often described as a tiny dwarf about the size of a human thumb and is commonly known as the god of healing, hot springs, and magic.
The Sukunaarchaeum genome is less than half the size of the smallest previously identified archaeal genome and contains only 189 protein-coding genes. Genetic analysis indicates that Sukunaarchaeum is distinct from all previously recognized archaeal groups, while related sequences in marine samples suggest that a broader, previously overlooked lineage exists.
The Genes It Kept and the Ones It Lost
Many organisms with highly reduced genomes often lose their independence as they shed genes. Mitochondria and chloroplasts, for example, were once free-living bacteria but now rely on their host cells for essential functions. In contrast, Sukunaarchaeum has lost almost all genes needed to produce its own nutrients and energy, yet has retained the core genetic machinery necessary for genome replication and protein synthesis.
“The key to defining life is whether something can replicate itself, and whether it can do this autonomously,” said Professor Thorsten Allers of the University of Nottingham’s School of Life Sciences and co-author of the study. “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.”
About a quarter of the remaining genome encodes unusually large membrane proteins with functions that are not yet fully understood. Similar proteins are found in some parasitic archaea, leading researchers to suspect that Sukunaarchaeum may live as a parasite within or alongside another organism, although its specific host has not been identified.
A Provisional Designation
“Although it has lost almost all of the genes needed to make nutrients and energy, it has kept much of the machinery required to copy its genome and produce proteins from its genetic information,” Allers said. “This suggests that Sukunaarchaeum may be close to the minimum level of genetic information needed for an organism to remain an independent cell.”
While researchers have sequenced the genome of Sukunaarchaeum in great detail, they have never viewed the organism under a microscope. Since researchers have not yet cultured the organism, it carries the provisional designation Candidatus Sukunaarchaeum mirabile. A version of the genome was previously documented in a preprint that circulated for over a year, attracting biologists’ attention before the peer-reviewed paper was actually published.
The Limits of Cellular Life
The team’s next steps will be to determine where Sukunaarchaeum lives, what it uses as a source of nutrients and energy, and how it interacts with a potential host. This could, in turn, help answer the more fundamental question of what constitutes the minimal genetic repertoire for an independent cell to be considered alive.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.