Proceedings of a workshop.

Back-of-the-envelope calculations having to do with genetic complexity and intracellular DNA concentration suggest that actively growing cells significantly smaller than those of Mycoplasma genitalium (diameter ~0.3 μm) are unlikely to exist on Earth today. 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.