Scientists have described a microbe from coastal ocean plankton whose genome is so small that it is pushing researchers to rethink how little genetic material a living cell can get by on. The organism, named Candidatus Sukunaarchaeum mirabile, carries a single loop of DNA just 238,000 base pairs long, the smallest genome ever found in the group of microbes known as archaea.

The finding was published Oct. 9 in Current Biology by an international team that includes researchers from the University of Tsukuba in Japan and the University of Nottingham in the United Kingdom. According to the University of Nottingham, the genome is less than half the size of the smallest archaeal genome previously known and holds just 189 protein-coding genes.

For ocean science, the discovery is a reminder of how much life in seawater remains unmapped. Related genetic sequences turn up in marine samples, the researchers say, suggesting Sukunaarchaeum belongs to a much larger branch of ocean microbes that has largely gone unnoticed.

Archaea, the Third Branch of Life

Archaea are single-celled organisms that, like bacteria, keep their DNA loose inside the cell rather than packed into a nucleus. For decades, they were mistaken for bacteria, but they have a separate evolutionary history and their own set of traits, which is why biologists treat them as one of the three main domains of life alongside bacteria and the eukaryotes that include plants, animals, and people.

Archaea were once thought to live mostly in extreme places such as hot springs and salt flats. Researchers now know they are common in ordinary seawater. A project summary from the Biological and Chemical Oceanography Data Management Office notes that one group, the marine Thaumarchaea, ranks among the most abundant microbes in the oceans and drives the globally important process of ammonia oxidation.

To put 238,000 base pairs in perspective, the preprint version of the study identified the 490,000-base-pair genome of Nanoarchaeum equitans as the smallest complete archaeal genome previously known. Some viruses carry larger genomes, including examples that reach 730,000 or even 2.5 million base pairs, even though viruses are not generally counted as living because they lack their own metabolism.

A Genome Stripped Down to Its Copy Machine

The team was not looking for a record. They were studying the single-celled plankton Citharistes regius, collected from surface seawater off Shimoda, Japan, and the photosynthesizing cyanobacteria that live with it. While sequencing genetic material from those plankton cells, they found an unusually small circular genome that matched nothing in existing reference databases.

What stands out is what the genome lacks. Sukunaarchaeum has lost almost all of the genes needed to make its own nutrients and energy. "The near-complete absence of recognizable metabolic pathways in Sukunaarchaeum suggests that it can neither sustain itself independently nor provide an obvious metabolic benefit to its host," the researchers wrote in the paper, as reported by ScienceAlert.

Yet the microbe kept much of the machinery needed to copy its own DNA and turn genetic instructions into proteins. That combination is what makes it unusual — mitochondria and chloroplasts, which descended from free-living bacteria, have handed much of that copying machinery to the cells that host them.

"This suggests that Sukunaarchaeum may be close to the minimum level of genetic information needed for an organism to remain an independent cell," said Thorsten Allers, a molecular biologist at the University of Nottingham's School of Life Sciences and a co-author of the study.

Parasite, Partner, or Something New

About a quarter of the genome codes for unusually large membrane proteins whose jobs are not yet known. Similar proteins appear in some parasitic archaea, where they appear to help the microbe attach to or invade a host. That raises the possibility that Sukunaarchaeum lives as a parasite, which would make it only the second parasitic archaeon on record.

The relationship remains a hypothesis. Because the microbe offers no obvious nutrients in return, the researchers argue a cooperative partnership is less likely than a one-sided one. But the University of Nottingham said the team has not yet directly observed the organism or confirmed which organism serves as its host.

The question matters beyond taxonomy. Plankton such as dinoflagellates sit near the base of ocean food webs, and the hidden communities of microbes living inside and alongside them are still poorly understood. A newly recognized group of stripped-down archaea could change how scientists read genetic surveys of seawater, where such tiny genomes may have been overlooked or misclassified.

Limits of a Genome-Only Discovery

This is a peer-reviewed genomic study, built on single-cell sequencing rather than microscope observation or laboratory culture. It identifies a complete circular genome, describes what that genome contains, and places the organism on the tree of life. It does not show what the microbe looks like, how it behaves, or exactly how it interacts with the plankton it was found with.

Two cautions are worth noting. First, the record applies to archaea specifically. Some bacteria that live permanently inside insects or other hosts carry genomes as small or smaller, so the university release's description of the smallest genome ever found should be read with that context in mind. Second, the study first circulated as a 2025 preprint, and the published version dropped the earlier comparison to viruses, likely to avoid confusion about how the organism is classified.

"The key to defining life is whether something can replicate itself, and whether it can do this autonomously," Allers said. By that measure, Sukunaarchaeum appears to sit right at the edge.

The team said its next steps are to find where the microbe lives, what it depends on and how it interacts with other organisms, including efforts to image it directly. For readers, the finding has no direct effect on human health or seawater safety — it is basic science about how life is built. The bottom line is that ocean plankton are still hiding entire lineages of life, and one of them is now forcing biologists to ask how small a cell can get before it stops being one.

What Readers Want to Know

What did scientists discover?

They described an archaeon called Candidatus Sukunaarchaeum mirabile with a genome of about 238,000 base pairs and 189 protein-coding genes, the smallest genome known for any archaeon.

Where was the microbe found?

It was detected in genetic material from the plankton Citharistes regius, collected in surface seawater off Shimoda, Japan.

What are archaea?

Archaea are single-celled microbes that form one of the three main domains of life. They resemble bacteria in size and structure but have a separate evolutionary history, and many are common in the ocean.

Is this the smallest genome of any living thing?

No. It is the smallest known for archaea. Some host-dependent bacteria have genomes of similar or smaller size.

Is the microbe a parasite?

Possibly. A quarter of its genome codes for large membrane proteins similar to those used by parasitic archaea, but researchers have not yet observed the microbe or confirmed its host.

Does this affect ocean safety or human health?

No. The study is basic research about the limits of cellular life and does not identify any risk to people or seawater quality.