Scientists have discovered a remarkably simple microbe, provisionally named Sukunaarchaeum mirabile, that lives inside plankton and behaves startlingly like a virus. This fascinating organism has such a stripped-down existence that it’s challenging our fundamental understanding of what it means for something to be a living cell. Its extreme dependence on its host makes the traditional boundary between life and non-life – where viruses often sit outside – seem even more fluid.
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A Strange Discovery in Ocean Plankton
The discovery happened almost by accident. Researchers at Dalhousie University, led by genomicist Ryo Harada, were busy cataloging the DNA of a plankton species called Citharistes regius and the bacteria living inside it. Amidst the expected genetic material, they found a small, unusual loop of DNA that didn’t seem to belong to anything they knew. It was a tiny organism with a lifestyle unlike most cells.
What Makes Sukunaarchaeum So Unique?
Sukunaarchaeum mirabile is incredibly small, genetically speaking. Its DNA contains only 238,000 base pairs. While some viruses have even larger genomes, this is minuscule for a cellular entity.
Like a virus, Sukunaarchaeum seems to “delegate” most of its essential life tasks to its host. Think of it like a minimalist guest who doesn’t bring anything but expects their host to provide all the food, shelter, and utilities. Sukunaarchaeum lacks the genes for almost all metabolic functions – the processes cells use to create energy and build necessary molecules.
Instead, like many viruses, the majority of Sukunaarchaeum‘s genes are dedicated to just one thing: making more copies of itself.
“Its genome is profoundly stripped-down, lacking virtually all recognizable metabolic pathways, and primarily encoding the machinery for its replicative core,” the researchers explained. This suggests an “unprecedented level of metabolic dependence on a host.”
Properties of Sukunaarchaeum mirabile, a cell with virus-like metabolic dependence on its hostProperties of the newly discovered Sukunaarchaeum mirabile. (Harada et al., BioRxiv, 2025)
The Key Difference: It Can Replicate Itself
Here’s where Sukunaarchaeum differs from a true virus and why scientists classify it as a cellular entity, albeit an extremely pared-down one. Viruses completely hijack their host’s cellular machinery to replicate their DNA and build new virus particles. They don’t have their own tools for this.
Sukunaarchaeum, however, still possesses the genes needed to create its own DNA replication proteins, including components for messenger RNA and transfer RNA – essential pieces of the cell’s protein-making machinery. It can handle the core replication process itself, even if it needs the host for everything else. It also appears to produce proteins to form its own membrane, which viruses typically do not.
Where Does It Fit on the Tree of Life?
Genetic analysis suggests Sukunaarchaeum belongs to the domain of life called archaea. These are single-celled microorganisms, distinct from bacteria and eukaryotes (the group that includes plants, animals, and fungi, and from which we evolved). The circular shape of its DNA also resembles that of bacteria and archaea.
Because it seems to take resources without giving anything back, Sukunaarchaeum appears to be a parasite of the plankton host.
Why This Discovery Matters
The question of “What is life?” is one of the oldest and most complex in science. Viruses have always occupied a controversial space, often considered non-living because they can’t reproduce or sustain themselves without a host cell.
Sukunaarchaeum complicates this picture. It can replicate itself using its own genetic instructions for the core machinery, which is a key characteristic of cellular life. But it cannot sustain itself metabolically, a key characteristic often lacking in viruses.
This new microbe exists in a fascinating grey area, right on the edge of our current definitions. Discoveries like Sukunaarchaeum push the boundaries of what we thought was necessary for a cellular organism to survive and force us to reconsider the minimum requirements for life itself.
This research is currently a preprint available on bioRxiv and is awaiting formal peer review by other scientists. It’s a reminder that even in seemingly simple organisms like plankton, the microscopic world holds incredible complexity and continues to challenge our understanding of life’s fundamental nature. You can read more about viruses and what it means to be alive in related articles.
