I mentioned a manuscript in a recent post written together with ECEHH colleague Will Stahl-Timmins on ‘The Secret Lives of Bacteria’. In this post, I wanted to highlight one of the figures from this paper highlighting the relationship between mass, generation time and genome size for a range of microbes and animals. It is clear that 1) these measures span an enormously large range (hence the logarithmic scales, so for the human example 104.8 g= 63 kg and a generation time of 105.3 h= 25 years) and 2) there seems to be a quite good positive correlation between the three parameters:
The data on genome sizes are easy to find on GenBank. However, there are still a lot of organisms without sequenced genomes; the largest animal that has ever lived, the Blue Whale, would have been a nice one to add. It is possible to construct a more extensive figure using the weight of genomes instead. The weight for haploid genomes is measured in picograms and is termed the C-value. This has been measured for a lot of organisms and databases exist for animals and plants. There is a conversion for DNA weight to number of nucleotides, for a Bottlenose Dolphin (there are no data for the Blue Whale) the estimated (haploid) genome size would be (0.978 x 109) x 3.3 pg= 3227 Mb (similar to that of humans).
Generation time equals doubling time in bacteria. For some other organisms this is a tricky one. Chlamydomonas for example can undergo two or three rounds of meiosis before division, resulting in four or eight daughter cells. Division and growth of course also depend on the quality of the environment.
Data for mass were easy to find for big animals, harder for insects (dry weight is often used, which makes sense as the weight of a mosquito will be very different before and after it has taken sips of your blood for instance) and difficult for bacteria. Cell lenght and width can be taken from EM images and using approximations for spheres and cilinders converted to volumes and mass when assuming cells mainly consist of H20. However, this is quite imprecise, as cells are not always spherical or cylindrical. Pelagibacter ubique is so small that 30% of its cell volume is taken up by DNA alone, changing density significantly. Moreover, cell shape and size can be quite variable (e.g. changing across developmental stages in Dictyostelium).
The differences between familiar vertebrates and bacteria (and a highly diverse range of organisms ‘inbetween’) are vast. The log scale might actually obscure these differences rather than emphasize them, but it is the only way to make such a plot.