Scientists have discovered a new organism off the Japanese coast that challenges traditional definitions of life, straddling the line between cellular organisms and viruses.
Ryo Harada and his team at Dalhousie University have identified an unusual organism named Sukunaarchaeum mirabile, found in DNA taken from a plankton species off the coast of Japan. This discovery is prompting scientists to reconsider what qualifies as a living organism, given that Sukunaarchaeum exhibits characteristics of both cellular life forms and viruses.
Traditionally, organisms have been defined by their ability to grow, produce energy, and reproduce independently. However, this simple definition does not accommodate viruses, which can only replicate within a host organism and remain inactive outside it. Sukunaarchaeum, while undeniably cellular, borrows several viral traits, complicating the clear-cut distinctions long relied upon in biology.
This discovery shakes up existing definitions by showing that Sukunaarchaeum, despite having the cellular components necessary for making ribosomes and messenger RNA, depends heavily on its host for nearly all other functions. Its genome is remarkably small, encapsulating only 238,000 base pairs, which is substantially smaller than the previous record holder, Nanoarchaeum equitans, which has about 490,000 base pairs.
The researchers describe Sukunaarchaeum as a “cellular entity retaining only its replicative core,” lacking almost all recognizable metabolic pathways. Its genome, characterized by elements typically found in viruses rather than in fully autonomous microbes, includes instructions only for basic replication, transcription, and translation activities.
Sukunaarchaeum resides within the domain Archaea, one of the three principal domains of life, and doesn’t belong to the virus group. Detailed phylogenetic analysis positions it on a distinct and distant branch within the Archaea domain, suggesting the possibility of a new phylum specifically for this organism.
The organism was discovered unexpectedly while sequencing DNA from the dinoflagellate Citharistes regius. Researchers identified a string of foreign DNA that bore no resemblance to any previously catalogued species. Sukunaarchaeum seems to rely heavily on its host, similar to some other plankton that depend on bacterial or algal partners for essential functions.
These findings suggest that traditional views of life as a binary concept may need revision. Some scientists argue that life should be understood as a spectrum, which Sukunaarchaeum pushes into the spotlight. The discovery brings to light the possibility that many more such lineages might exist, previously dismissed in environmental sequence data as mere contaminants or viral anomalies.
The definition of “alive” carries significant implications, influencing funding, public health, and even planetary protection measures for space exploration. If other organisms like Sukunaarchaeum are found, biosecurity measures that only target free-living microbes could overlook significant classes of symbiotic parasites.
Researchers view this organism as potentially valuable in synthetic biology, offering insights into the minimal genetic load necessary for cellular function. Harada and his team speculate that the extreme streamlining of Sukunaarchaeum’s genome could result from a host environment that provides guaranteed nutrients, making some biological pathways obsolete over time.
Some paleobiologists suggest that such extreme genome reduction might provide a glimpse into early evolutionary history, when cells likely shared genes and resources more liberally. If this is true, today’s viruses and streamlined symbionts might come from an ancient lifestyle rather than being biological outliers.
Ongoing research aims to determine whether similar organisms exist in other marine ecosystems or symbiotic relationships, possibly through re-analyzing existing metagenomic databases for overlooked sequences resembling Sukunaarchaeum. Another objective is identifying the specific host organism that supports Sukunaarchaeum’s survival, which remains undiscovered.