Nonequilibrium thermodynamic considerations of the efficiency, control, and regulation of microbial growth

The, submaximal, thermodynamic efficiency of aerobic microbial growth depends on whether no consumption / production of 02, or no carbon product other than biomass, is admitted into the assimilation equation. When the growth substrate has a low degree of reduction, the former efficiency approaches 22 % whilst the latter approaches 54 %. Expectations for maximal growth rate at optimum efficiency and for maximum economic growth rate at optimum efficiency are 24 and 54 %, respectively. A new control analysis method is derived that may be used to deduce properties of intracellular processes from the control properties of the cell as a whole. THERMODYNAMIC EFFICIENCIES For rates to be high, Gibbs (free-) energy must be dissipated, which leads to a reduction in the thermodynamic,efficiency. Using mosaic non equilibrium thermodynamics (MNET) one may calculate which states are optimal with respect to (combinations of) various criteria, such as efficiency, high growth rate, Gibbs-energy rich products, and this in the context of the mechanisms used by the organisms to approach such optimum states [ 11. For aerobic growth on substrates that are "rich in Gibbs energy", experimental thermodynamic efficiencies were low [l, cf., 2, 31, but in correspondence with optimization for maximum growth rate disregarding thermodynamic efficiencies. In the case of substrates that carry little oxidative Gibbs energy per carbon atom, the thermodynamic efficiency appeared to correspond to optimization in view of both efficiency and growth rate [ 1 , see however below]. The thermodynamic efficiency discussed here serves to compare microbial growth to other (bi0)machine.s that transduce Gibbs energy from one form to another. Consequently, it addresses systems in which there is a well-defined input reaction and a well-defined output reaction. Ideally, these are chemically independent of one another. For growth of microbes, one could think of cases where the Gibbs-energy source is photons, and the carbon substrate is unable to yield ATP through substrate level phosphorylation. Then the input reaction is the dissipation of photon Gibbs energy, where the photons have a chemical potential pv, at a rate, Jv. The output reaction is growth with the Gibbs energy difference of the assimilation equation, called AGa, and a rate Ja (in microbiological literature often called p). The thermodynamic efficiency for growth is then defined as: To growth on two carbon substrates of which one is exclusively used for catabolism and one is exclusively used for anabolism, the analogous definition applies. Such cases of growth are rare however. Most often, catabolism and anabolism are intertwined and an unequivocal, mechanistic