Defining “life” is often trickier than it first seems. While sentient animals and single-celled organisms are easily slotted into biology’s grand hierarchy, strange cases like viruses have long tested the boundaries of what counts as “living.” Now, a new microscopic entity discovered by scientists in Canada and Japan adds to this ongoing debate. Named Sukunaarchaeum mirabile, this bizarre microbe could change how we classify life itself.
In a paper recently posted on the bioRxiv preprint server, molecular biologist Ryo Harada and his team at Dalhousie University revealed the discovery of Sukunaarchaeum mirabile, a mysterious cellular entity that doesn’t fit neatly into current biological categories. On one hand, it possesses genetic machinery typically associated with cellular organisms, such as the ability to produce ribosomes and messenger RNA—functions that viruses notably lack. On the other, it relies heavily on a host for survival and replication, much like a virus.
“It offloads many key biological tasks to its host,” the researchers wrote, adding that its existence challenges the distinction between viruses and minimal forms of cellular life. “Its genome is profoundly stripped-down, lacking virtually all recognizable metabolic pathways, and primarily encoding the machinery for its replicative core: DNA replication, transcription, and translation.”
The discovery occurred during genomic analysis of marine plankton (Citharistes regius). Within the genetic data, researchers found a DNA strand that didn’t match any known organism. After deeper analysis, the team concluded that Sukunaarchaeum belongs to the domain Archaea, primitive single-celled organisms that gave rise to more complex eukaryotic life forms.
What makes Sukunaarchaeum even more extraordinary is the minuscule size of its genome. Clocking in at just 238,000 base pairs, it holds the record for the smallest known archaeal genome, barely half the size of the previously smallest one (490,000 base pairs). Some viruses have far more extensive genetic codes, often reaching millions of base pairs.
Named after Sukuna, a diminutive deity from Japanese mythology, this microbe certainly lives up to its namesake in size but not in scientific impact. The researchers suggest its discovery may force scientists to rethink the boundaries of what constitutes life. “The discovery of Sukunaarchaeum pushes the conventional boundaries of cellular life and highlights the vast unexplored biological novelty within microbial interactions.”