In addition, the limitations on genome size that accompany small cell size guarantee that cells of that size will have to depend on larger, more complex organisms for supplies of most of the small organic molecules required for their growth. Hence, a biosphere consisting entirely of ultra-small organisms is highly implausible. Introduction Cells differ enormously in size.
Viruses can be almost two orders of magnitude smaller than that.
In light of the recent report of McKay et al. My perspective on this problem is that of a biophysical chemist, not that of a systematic microbiologist. As it happens, I have spent much of my career working with Escherichia coli , a bacterium whose linear dimensions are of the order of micrometers, and it is useful to start by reviewing its capabilities and chemical composition. Even though this environment is nutritionally rich, E. coli has remarkable metabolic capabilities. It thrives in media consisting of mineral salts and glucose, a diet far too sparse to support human life, for example.