Scientists have discovered a previously unknown microorganism, which has the smallest genome ever found, that is possibly able to replicate and express its own genetic information.
The microorganism belongs to the archaea, a group of single-celled organisms that are fundamentally different to bacteria.
The microbe is named Candidatus Sukunaarchaeum mirabile, or Sukunaarchaeum for short, in honour of a small Japanese god. It has an unusually small genome but despite this, Sukunaarchaeum still contains the genes needed to copy its DNA and use its genetic information to make proteins.
What makes the discovery particularly interesting is what Sukunaarchaeum does not have - it has lost almost all the genes needed to produce its own nutrients and energy. This suggests that it depends heavily on other organisms for the resources it needs to survive.
The research was carried out by an international team including researchers from the University of Nottingham and the University of Tsukuba in Japan. The findings are published in Current Biology.
Professor Thorsten Allers, from the School of Life Sciences at the University of Nottingham and co-author of the study, said: "The key to defining life is whether something can replicate itself, and whether it can do this autonomously. This exciting discovery provides new clues about how simple a living cell can become while remaining capable of reproducing and maintaining its own genetic information."
The research team discovered Sukunaarchaeum while analysing the genetic material of individual marine microorganisms (plankton). The genome of Sukunaarchaeum is less than half the size of the smallest archaeal genome previously known and contains just 189 protein-coding genes.
The organism is also genetically very different from all previously known groups of archaea. Related genetic sequences found in marine samples suggest that Sukunaarchaeum is part of a much larger group of organisms that has so far gone largely unnoticed.
Most organisms with very small genomes have given up some of their ability to function independently. For example, mitochondria and chloroplasts inside our cells, which were originally free-living bacteria, rely on their host cells for some of the machinery needed to copy and use their genetic information.
Sukunaarchaeum, however, appears to have taken a different evolutionary path.
"Although it has lost almost all of the genes needed to make nutrients and energy, it has kept much of the machinery required to copy its genome and produce proteins from its genetic information," says Professor Allers.
"This suggests that Sukunaarchaeum may be close to the minimum level of genetic information needed for an organism to remain an independent cell."
The researchers also found that around a quarter of its genome consists of genes for unusually large membrane proteins whose functions are not yet known. Similarly large membrane proteins are found in some parasitic archaea, raising the possibility that Sukunaarchaeum may live as a parasite inside or alongside another organism.
Whilst the scientists have identified the genome of Sukunaarchaeum, they have not yet directly observed the organism or identified its host.
The team plans to investigate where Sukunaarchaeum lives, what it depends on, and how it interacts with other organisms.
The findings could help scientists better understand the limits of life, and how much genetic information an organism needs to survive and reproduce.
