A Japanese led team, working with international partners at Dalhousie University and University of Tsukuba, has described a microscopic archaeon called Candidatus Sukunaarchaeum mirabile that blurs the edge between living cells and viruses:
The story began when scientists sequenced DNA from marine plankton and noticed genetic fragments that did not match any known organism. Reconstructing those fragments revealed a circular genome of about 238,000 base pairs.
For comparison, the previous record holder among Archaea, Nanoarchaeum equitans, carries roughly 490,000 base pairs, so this newcomer has kept barely half the DNA of an already-minimalist relative.
Almost everything in this tiny genome is devoted to handling genetic information. The microbe keeps the core instructions for copying DNA and building the ribosomes that read it, yet the usual metabolic pathways for harvesting energy or making amino acids and vitamins are missing. It seems unable to produce most of what it needs and instead leans on its host for supplies.
The authors of the study describe it as a “cellular entity retaining only its replicative core”, a phrase that shows how close it comes to the line between a cell and something simpler.
Even so, Sukunaarchaeum is not a virus. It still builds its own ribosomes and messenger RNA instead of borrowing all of that machinery from its host. At the same time, its tiny genome and single-minded focus on making more copies of itself make its lifestyle look strikingly virus-like.
A report in Science notes that its DNA is “focused almost entirely on replication” and suggests it may sit on an evolutionary path between more conventional cells and fully-viral strategies.