This Microbe Challenges the Definition of Cellular Life

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SThe Cientists recently discovered a microbe with one of the smallest genomes on the earth. More surprisingly, the creature depends almost entirely on its host: its genes do not support any of the functions of metabolism, one of the main processes of life. As such, he calls into question the fundamental notions of what a living organism means.

The discovery was “a pure serendipity”, explains Takuro Nakayama, an evolving microbiologist at the University of Tsukuba in Japan. Takayama wanted to study the many microbes that live in a unique cellular dinoflallate, Regius leafA kind of plankton. But when he and his colleagues sequenced the genes of this microbial community, they continued to present themselves with tiny strange pieces of DNA.

It turns out that these DNA pieces belong to an unusual archaea – a branch on the tree of life populated by cell microbes which can often survive in extreme environments. (Archaea are similar to bacteria, but distinct in their structure, genetics and metabolism.)

How did Sukunaarchaeum end up with such a tiny genome?

Nakayama and his colleagues proposed the name of Sukunaarchaeum Mirabile for the newly discovered microbe: Sukunaarchaeum after the Japanese dwarf deity Sukuna-Biko-Naand mirabile for wonderful. Only 238,000 base pairs, the number of genes in the DNA of Sukunaarchaeum is smaller than that of any other known archée. Scientists described their conclusion in a Biorxiv pre -impression earlier this year.

So how did Sukunaarchaeum end up with such a surprisingly small genome? During evolution, genetic instructions for life often become more and more complex. But evolution can also go in the other direction, leading to greater simplicity in the genome. This so-called genomic reduction, where organisms are found with fewer genes than their ancestors, is generally observed in the fields of bacteria and archaea. What struck Nakayama and his colleagues about Sukunaarchaeum is the extent of reduction and specialization in its genes.

With its stripped genome, Sukunaarchaeum seems to depend completely on its host, C. RegiusFor energy and essential nutrients. “He probably cannot produce his own cell construction blocks,” notes Nakayama. “No microbe discovered previously has shown a degree of metabolic dependence so extreme.”

Sukunaarchaeum seems to live almost a new category of life, suspended somewhere between the archaea and the virus. It is like viruses – which are generally not considered “alive” – in that it has a small genome and depends completely on its host for metabolism. But unlike a virus, Sukunaarchaeum has its own ribosomes, cellular structures that synthesize proteins, and it can reproduce without the help of a host.

To get an idea of the unusual sukunararchaeum, the researchers decided to scan the oceans for potential parents. They analyzed the data of environmental genetic sequence of marine environments around the world, focusing on spots where C. Regius is known to live. By using a database called the Tara Oceans project, they discovered a wide range of sequences comparable to those of Sukunaarchaeum, which, according to them, could represent a new deeply branched archaeal line.

For Nakayama, this additional observation suggests that many more microbes that question the definition of life can be there, living in what Nakayama calls “microbial dark matter” or microbes that cannot be cultivated in the laboratory. “The extreme lifestyle and virus that we emit for Sukunaarchaeum is a perfect example of the surprising results found in this” natural laboratory of evolution “, he says.

Mart Krupovic, virologist and microbiologist at the Pasteur Institute in France which was not involved in the study, described the observation of “remarkable”. Krupovic studied giant viruses which, like Sukunaarchaeum, defy categorization. These giant viruses have evolved larger and more complex genomes which include some of the genes of DNA translation, a characteristic thought reserved for cell life. “I think it’s fascinating,” says Krupovic, “how we still know the world around us.”

Then Nakayama wants to culture and isolate Sukunaarchaeum in the laboratory, not just its genes, and take an image or a video. He hopes that this will help him to better understand his biology, his ecology and his way of building – how it achieves this exploit curious to live on the brink of life.

Lead image: Elizabeth Ann Stevens / Shutterstock

• Alice Sun

  Published on August 19, 2025

  Alice Sun is a scientific journalist based in Brooklyn, NY, her work frequently covers biology, the environment, the social sciences, mental health, etc. More of his work can be found on his website and X (formerly known as Twitter).