Researchers have discovered a novel archaeon in symbiotic relationship in a marine microbial system that displays an extreme genome reduction in having a highly stripped-down genome of only 238 kbp and has an extreme functional bias towards genetic information processing. Its genome primarily encodes the machinery for DNA replication, transcription, and translation. It lacks nearly all metabolic pathways hence displays total metabolic dependence on the host. Provisionally named Candidatus Sukunaarchaeum mirabile, it is essentially a cellular entity retaining only its replicative core and has evolved to approach viral way of existence. With Sukunaarchaeum mirabile appearing as a link between cellular entities and viruses, this discovery forces one to wonder about minimal requirements of cellular life.
Dinoflagellates are group of eukaryotic single-celled algae bearing two dissimilar flagella. They are mostly marine plankton and are known to maintain symbiotic microbial communities.
In a recent study, single-cell genome amplification of bacteria associated with the dinoflagellate Citharistes regius revealed presence of a highly unusual circular sequence of 238 kbp with a low GC (guanine-cytosine) content of 28.9%. It was found that the sequence represented the complete genome of a prokaryote. Further analysis revealed that the organism bearing this genome is an archaeon. Until now, the smallest known archaeal complete genome is the 490 kbp genome of Nanoarchaeum equitans. The archaeon genome discovered in this study is less than half this size, yet it is found to be highly complete. Further investigation confirmed that it indeed represents a complete archaeon genome and has been named Candidatus Sukunaarchaeum mirabile.
The newly discovered archaeon Ca. Sukunaarchaeum mirabile displays an extreme genome reduction in having a highly stripped-down genome of only 238 kbp (for a comparison, genome size of typical archaea is about 0.5 to 5.8 Mbp while genome size of viruses range between 2 kb to over 1 Mbp). Further, it is also found to have an extreme functional bias towards genetic information processing. It primarily encodes the machinery for DNA replication, transcription, and translation. It lacks nearly all metabolic pathways hence displays total metabolic dependence on the host.
Ca. Sukunaarchaeum mirabile resembles viruses in having a minimal genome dedicated to genetic self-perpetuation and absolute host dependence necessitated by metabolic reduction. However, unlike viruses, Sukunaarchaeum mirabile possesses its own core transcriptional and translational apparatus and ribosomes. It does not lack core replication machinery genes and is not dependent on host for this. This is the key distinction between cellular entities and viruses. Sukunaarchaeum mirabile is fundamentally a cellular entity retaining only its replicative core that has evolved to approach viral way of existence.
With Sukunaarchaeum mirabile appearing as a link between cellular entities and viruses, this discovery forces one to wonder about the minimal requirements of cellular life.
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References:
- Harada R., et al 2025. A cellular entity retaining only its replicative core: Hidden archaeal lineage with an ultra-reduced genome. Preprint at bioRxiv. Submitted on 02 May 2025. DOI: https://doi.org/10.1101/2025.05.02.651781
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Related articles:
- Eukaryotes: Story of Its Archaeal Ancestry (31 December 2022)
- Thiomargarita magnifica: The Biggest Bacterium that Challenges the Idea of Prokaryote (25 June 2022)
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