Optimal energy dissipation in growing microorganisms and rectification columns

This paper proposes a new point of view in analyzing the optimal Gibbs energy dissipation in growing microorganisms. Small Gibbs energy dissipation in growth would be of biological advantage because less resource is consumed and the biomass yield on these resources could be maximized. It is however not so clear why microorganisms still dissipate considerable amounts of Gibbs energy while growing. Distillation columns are used as a simple qualitative model system in order to gain a qualitative understanding of the question. In both growing microorganisms and continuously operated distillation columns, small energy dissipation values result in small process driving forces and therefore in tentatively slow operation. In microorganisms this entails relatively higher maintenance energy requirements, increasing thereby the resource cost for growth and decreasing the biomass yield. In distillation columns, it results in higher capital and maintenance cost, thereby increasing overall process costs. A simple model is proposed to calculate the biomass yield as a function of Gibbs energy dissipation. It shows that this function goes through a maximum because of maintenance requirements at small dissipations. Experimental data confirm that growing microorganisms dissipate an amount of Gibbs energy that is associated with this maximum.