Somewhere in the plankton off Japan’s Izu Peninsula drifts a cell that has thrown away almost everything except the ability to copy itself. Researchers from Japan’s University of Tsukuba, Britain’s University of Nottingham and partner institutions describe it in Current Biology, and its genetic inventory is startlingly short: 238,000 base pairs carrying 189 protein-coding genes.
Nobody went looking for it. The team was sequencing Citharistes regius, a single-celled plankton species from the waters near Shimoda that relies on cyanobacteria as live-in partners. Among the DNA they pulled out was a small circular genome that matched none of the expected guests. It turned out to be an archaeon, a member of the third great domain of life alongside bacteria and organisms like us. Its discoverers christened it Sukunaarchaeum mirabile in honour of Sukuna, a diminutive but powerful figure in Japanese myth.
The previous archaeal record holder had more than twice as much DNA, and some giant viruses carry more DNA. What it lacks is striking: virtually every pathway for building nutrients or generating energy is gone. What it keeps is just as telling, namely the machinery to replicate its DNA and turn genes into proteins.
Nottingham molecular biologist Thorsten Allers said this places the organism near “the minimum level of genetic information” a cell needs to stay independent. Roughly a quarter of its genes encode oversized membrane proteins, resembling proteins that a parasitic archaeon described earlier uses to break into its hosts. If the parasite hypothesis holds, Sukunaarchaeum would be just the second such case.
There are still big gaps. No one has seen the microbe through a microscope, and its host remains a mystery. Yet similar sequences turning up in ocean samples hint that it belongs to a much larger family of minimalists that biology has simply overlooked.
