I heard an assertion nanobacteria exist along with some speculation about them and so on. It was news to me and got me wondering if I had missed a discovery announcement, so I ran some numbers:
Consider a large nanobacteria, 30 nm by 100 nm. It has an approximate volume of 7e-23 m3.
How much DNA could this volume contain? Pop up a DNA molecule in Chime and get an estimate of dimensions: 20 bp run for 6.4 nm, so 0.32 nm / bp. The double helix has a radius of 1.3 nm.
Length of DNA with a volume equal to the cell = 7e-23 m3 / [ (1.3 nm)2 x Ï€ ] = 1.3e-5 m
Bps = length / bp length = 1.3e-5 m / 0.32 nm = 42 kb of DNA.
A low average bacterial gene length is 300 bp, so a nanobacteria could hold a maximum of 42 kb / 300 bp = 139 genes. The minimum number of genes for an independent living organism is 300 – 500, so a nanobacteria canâ€™t have enough genes. It canâ€™t exist. But letâ€™s run the other numbers, on proteins.
A typical protein is 3 nm, lets call it a cube with a volume of (3 nm)3 = 2.7e-26m3.
A nanobacteria can hold 7e-23m3 / 2.7e-26m3 = 2600 proteins total. Low but not a hard limit on existence.
Now letâ€™s put this together and describe a nanobacteria. 30% of the volume is water and small molecules. Of the remainder, letâ€™s say 2/3 is DNA and 1/3 is protein. The nanobacteria contains enough DNA for 65 genes, and 600 proteins. Which means nanobacteria canâ€™t exist, they are too small. There are quite a few unlikely bits in biology (pretty much every rule in biology has exceptions), so Iâ€™ll qualify it and say it is quite unlikely that nanobacteria exist. And to my estimates, add the evidence from projects to mass sequence DNA extracted from the environmental samplesâ€“if nanobacteria exist, their DNA would turn up in these projects, and no new phylum of sequences has emerged.
An E. coli by contrast is 1 Âµ x 3 Âµ and can hold 600 Mb of DNA and 20 million proteins. Itâ€™s genome is actually 4.6 Mb, and the extra space is taken up by a cell wall and protein.