Microbial butanol tolerance: mediated by toxicity or by phase equilibrium?
It is conventionally assumed that the achievable concentration of a bio-alcohol molecule by fermentation is limited by the toxicity of that molecule to the micro-organisms producing it. In the case of ethanol, for example, extremely high titers have been achieved by developing resistant strains of micro-organism. Butanol, conversely, has proven remarkably stubborn to this same sort of progression. Despite widespread efforts, concentration above 2% have not been achieved.
2% also happens to be the limit of solubility of butanol in water. It seems intuitive that intra-cellular phase separation would result in highly toxic localized concentrations of butanol, and generally disrupt material transport through cell membranes. This would suggest that butanol titers are fundamentally limited by phase equilibrium and that conventional research to improve butanol tolerance will prove fruitless.
This hypothesis could be tested experimentally in one of two ways. Firstly, ethanol-producing organisms could be cultivated in a 2% butanol solution. If toxicity is severe then they would be highly inhibited in this environment but if phase equilibrium is the limit then they will cope well. Alternatively, higher temperatures improve solubility. Hence, if butanol tolerance increases with temperature it can be inferred that solubility is limiting.