In studying the functional and evolutionary significance of compartmentation in biology, it is instructive to consider its thermodynamic context as a conceptual centrepiece of entropy and phase transitions. Here we focus specifically on compartmentation at the intracellular level of microbial organellar cytology. Via a colloid-statistical argument, supplemented with order of magnitude estimates for the relevant physical quantities, we find that the DNA-containing nucleoid of prokaryotes presents a plausible nucleation site for phase-transitional behaviour, provided the genome exceeds some threshold size of the order of 10 Mbp. Large genome size seems capable in this respect of seeding compartmentation effects such as the nuclear envelope of Planctomycetes bacteria, which is widely regarded as a possible precursor to the nuclear envelope of eukaryotes.
