Does microbial life always feed on negative entropy? Thermodynamic analysis of microbial growth

Abstract

Schrödinger stated in his landmark book, What is Life?, that life feeds on negative entropy. In this contribution, the validity of this statement is discussed through a careful thermodynamic analysis of microbial growth processes. In principle, both feeding on negative entropy, i.e. yielding products of higher entropy than the substrates, and generating heat can be used by microorganisms to rid themselves of internal entropy production resulting from maintenance and growth processes. Literature data are reviewed in order to compare these two mechanisms. It is shown that entropy-neutral, entropy-driven, and entropy-retarded growth exist. The analysis of some particularly interesting microorganisms shows that enthalpy-retarded microbial growth may also exist, which would signify a net uptake of heat during growth. However, the existence of endothermic life has never been demonstrated in a calorimeter. The internal entropy production in live cells also reflects itself in the Gibbs energy dissipation accompanying growth, which is related quantitatively to the biomass yield. An empirical correlation of the Gibbs energy dissipation in terms of the physico-chemical nature of the growth substrate has been proposed in the literature and can be used to predict the biomass yield approximately. The ratio of enthalpy change and Gibbs energy change can also be predicted since it is shown to be approximately equal to the same ratio of the relevant catabolic process alone.

Biothermodynamics of live cells: a tool for biotechnology and biochemical engineering

“…Simplification (i) is possible because the system does not include the whole culture vessel, but only the biomass contained in the broth. Allowing for these constraints, an entropy balance reads [2,43] …”

“…Solving this for P n i and substituting into Eq. (10) for a cell working at steady state, (dG=dt D 0) yields [43] r G X r X V D P W T P S prod (12) with…”

“…Over many years, energy dissipation and biomass yields have been measured for a variety of microbial growth systems in the laboratory of the author [2,3,6,43]. In these studies, the dissipation of energy in the form of heat was directly measured in a reaction calorimeter and compared with predictions from an energy balance.…”

“…However, the Gibbs energy of combustion of dry biomass cannot be measured experimentally. There 436 U. von Stockar are many more or less speculative estimations for c G 0 X in the literature, which are mostly based on model estimates for the entropy of dry biomass (for a review, see von Stockar and Liu [43]). The only value of the absolute entropy in dry biomass with an experimental basis was determined by lowtemperature calorimetry by Battley et al [46].…”

“…In several experimental research projects [3,6,43] the biomass yields were measured for aerobic batch and chemostat cultures under very many different growth conditions at varying specific growth rates, and r G 0 X values were calculated according to Eq. (20a).…”