Between the living and the inanimate: scientists have found a very strange bacterium

Off the coast of Japan, scientists have discovered unique microorganism, which should not exist according to the ideas of modern biologists.

While sequencing the DNA of tiny marine plankton, a group of researchers led by Rio Harada of Dalhousie University discovered an unexpected branch of genetic material. The fragment was unlike anything scientists had ever encountered. The discovered microorganism balanced on the edge between the life of a cell and a virus. Scientists named it Candidatus Sukunaarchaeum mirabile in honor of a mythological Japanese deity. 

The genome of Sukunaarchaeum consists of only 238 thousand base pairs and is the smallest one discovered for archaea. This is less than half the size of the smallest previously known archaeal genome. By comparison, some viruses have much more genetic material. However, unlike viruses that completely take over the mechanisms of their hosts, Sukunaarchaeum has some replication tools of its own. 

Sukunaarchaeum has genes for for ribosomes, tRNAs and mRNAs, with the help of which cells read their genomes and build proteins. Classical viruses do not have such a mechanism. This unique organism is able to copy its genetic code, but it does not produce nutrients for itself and does not contribute to the survival of the host on which it parasitizes. 

Unlike cells, viruses are not capable of independent reproduction and metabolism Sukunaarchaeum is only half an archaea that uses the strategy of virus existence. Of the 189 protein-coding genes, more than half are designed to read and copy DNA. Almost none perform other biological functions.

For example, Carsonella ruddii, which lives inside sap-feeding insects, has the same tiny genome. But even it retains genes to produce energy and amino acids. 

Even the origin of Sukunaarchaeum remains a mystery to scientists. Phylogenetically, this microorganism belongs to the Archaea domain, which includes extremists and gave rise to the eukaryotes, which include plants, animals, and fungi, among others. However, Sukunaarchaeum does not belong to any of the known types. 

Depending on the computational model used, it may be most closely related to the mysterious Nanobdellati or possibly Halobacteriota. Both groups are known for their small genomes and rapid evolution. 

Scientists discovered Sukunaarchaeum by diving deep into the genome of the marine plankton Citharistes regius. The microorganism was discovered in the waters off the island of Shimoda. The researchers also made sure that Sukunaarchaeum live either inside the tiny plankton or are attached to it from the outside. 

The results of the study are published on the preprint server bioRxiv

Source: ZMEScience