A clear definition of life has always been elusive. Most biology classes agree that living things must grow, make energy, and reproduce on their own. This definition leaves viruses out, as they only come alive when inside a host and go dormant when alone.
However, scientists at Dalhousie University, led by Ryo Harada, have discovered something that challenges our understanding. They found a microbe called Sukunaarchaeum mirabile in plankton DNA off Japan’s coast. This finding makes us rethink what it means to be alive.
Traditionally, biologists said that “an organism is made of cells,” which is why viruses were viewed as distinct particles. Yet, the rise of giant viruses in the early 2000s started to blur those lines. Now Sukunaarchaeum presents an even bigger puzzle. It’s cellular but uses many strategies seen in viruses. While it has the genes to build vital components like ribosomes, it delegates nearly every function to a host cell.
Surprisingly, Sukunaarchaeum’s entire genome is just 238,000 base pairs long—akin to a medium magazine article. For context, the previously record-holding archaeon had around 490,000 base pairs. Viruses can vary in size, but none match Sukunaarchaeum for the full toolkit required for protein synthesis.
The research team found that this organism’s genome is stripped down, mainly encoding the components necessary for DNA replication, transcription, and translation. This means it resembles a viral blueprint more than a fully self-sufficient microbe.
Interestingly, Sukunaarchaeum holds a unique place within the Archaea domain, far from known groups. It might even warrant the creation of a new phylum due to its distinct lineage.
While conducting DNA sequencing, the researchers discovered a loop of unfamiliar DNA in the dinoflagellate Citharistes regius. This DNA shows that Sukunaarchaeum relies heavily on its host for survival, essentially shedding unnecessary genes and leaning on the dinoflagellate for vital functions.
The debate around what constitutes life—whether it is a strict binary or a spectrum—has been reignited by this find. Researchers believe many similar microorganisms may exist unnoticed in environmental data, previously misidentified as contaminants or viral anomalies.
This discovery has real-world implications. How we define “alive” influences funding, public health, and even planetary protection policies for space exploration. If organisms like Sukunaarchaeum are common, current biosecurity measures might fail to catch certain parasitic life forms.
Understanding the bare minimum genetic requirements for a living cell could also benefit synthetic biology by providing a framework for engineering minimal cells. The researchers suspect that Sukunaarchaeum’s extreme simplification arose because it thrived in nutrient-rich environments, allowing it to evolve beyond traditional survival needs.
From a historical perspective, this study could hint at how early life forms may have shared genes and resources more freely. This might suggest that today’s viruses and streamlined symbionts could reflect ancient survival strategies rather than merely being outliers.
Scientists plan to search for similar organisms in various marine ecosystems and analyze existing genetic databases for overlooked sequences resembling Sukunaarchaeum. Identifying the host organism is critical for fully understanding this microbe’s unique lifestyle.
You can read more about the study in bioRxiv. This discovery opens up a realm of questions about life and its many forms, offering a peek into the mystery of microbial interactions.
For further insights into marine biology and genetics, check out resources at EarthSnap and academic databases like Google Scholar.
