Thermodynamics of Growth, Non-Equilibrium Thermodynamics of Bacterial Growth: The Phenomenological and the Mosaic Approach

Microbial growth is analyzed in terms of mosaic and phenomenological non-equilibrium thermodynamics. It turns out that already existing parameters devised to measure bacterial growth, such as YATP, µ, and Qsubstrate, have as thermodynamic equivalents flow ratio, output flow and input flow. With this characterisation it becomes possible to apply much of the already existing knowledge of phenomenological non-equilibrium thermodynamics to bacterial growth. One of the conclusions is that the frequent observation that YATP is only 50% of its theoretical maximum does not mean that the microbe corresponds to a thermodynamic system that has been optimized for maximal output power, as has been suggested. Rather, at least in some cases, it corresponds to a system that has been optimized towards maximum growth rate. When the degree of reduction of the (single) carbon source is significantly smaller than that of the biomass produced, the efficiency of biomass synthesis has been kept as high (i.e., about 24%) as is consistent with maximization of the growth rate at optimal efficiency. Mosaic thermodynamics allows an analysis of processes which in microbial metabolism may be responsible for any particular growth behaviour. Equations are derived that predict the effect of uncoupling through leaks, futile cycling, or 'slip' on microbial growth. It turns out that uncoupling is expected to affect both the growth rate-independent and the growth rate-dependent 'maintenance coefficient'. The effect on the latter is different when catabolic substrate limits growth than when anabolic substrate limits growth. In the latter case, the growth rate-dependent maintenance coefficient is negative. It is concluded that mosaic non-equilibrium thermodynamics will be a powerful theoretical tool especially in future experimental analyses of the metabolic basis for microbial growth characteristics and growth regulation.