Read more: Extreme survival: The toughest life forms on Earth
The question of how small life can get rubs up against the question of how we define life
The question of how small life can get rubs up against the question of how we define life. Viruses are generally seen as the smallest replicating beings, with some a mere 20 nanometres or so across. But as intracellular parasites, they are so utterly dependent on other life forms that they are often regarded as not being truly alive.
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The smallest known bacteria were discovered in 2006 in hot, arsenic-filled mine effluent in northern California. Called ARMAN (Archaeal Richmond Mine acidophilic nano-organisms), they measure only 200 nanometres, one-third the diameter of E. coli and about the size of a large virus (see illustration). Their tiny genome of around 1 million base pairs, compared with the 3 billion base pairs in the human genome, was sequenced this year ().
Some would argue that the accolade should go to nanobacteria. These small, egg-shaped structures, just 50 to 100 nanometres across, are found in human blood and saliva. They look like cells and even seem to divide like cells, but no genetic material, essential for life, has ever been isolated from them. In 2008, research suggested they are nothing more than particles of calcium carbonate ().
Because genetic material is the essence of life, another way of looking at the question is to identify the smallest possible genome. The top candidate is Mycoplasma genitalium, a parasitic bacterium that causes human urinary tract infections. Its genome has only 580,000 base pairs and has very little genetic redundancy: of the 482 genes that code for proteins, 382 have been shown to be essential for life (). That makes it likely to be very close to the smallest genome that is possible. In 2008, it became the first genome to be artificially sequenced, by Craig Venter’s team as part of the Minimal Genome Project.
Organisms can have smaller genomes, but only at the expense of their identity as a separate life form. Carsonella ruddii, for example, is a symbiotic bacterium that lives inside sap-sucking insects called psyllids, providing them with essential amino acids. It has only 182 genes and is so dependent on the insect’s cells that it may be in the process of evolving into an organelle – an essential subunit of the cell (). This echoes the way early cells incorporated the bacteria that became mitochondria and chloroplasts to, respectively, release energy and carry out photosynthesis.
