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.

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To answer the intriguing question whether or not endothermic microbial growth exists, and in particular, to verify Heijnen and van Dijken's prediction (1992), acetotrophic methanogen, Methanosarcina barkeri, has been cultivated in a highly sensitive bench-scale calorimeter (an improved Bio-RC1 reaction calorimeter) in a pH auxostat fashion. A growth yield of 0.043 C-mol C-mol(-1) has been obtained and a cell density as high as 3 g L(-1) was attained. Heat uptake during growth has indeed been quantitatively measured with calorimetry, resulting in a heat yield of +145 kJ C-mol(-1). Thermodynamics of the growth of acetotrophic methanogens was analyzed in detail. The changes in Gibbs energy, enthalpy, and entropy during growth of M. barkeri were compared with some typical aerobic and anaerobic growth processes of different microorganisms on various substrates. In the growth of M. barkeri on acetate, the retarding effect of the positive enthalpy change on the driving force of growth is overcompensated by the large positive entropy change, resulting from converting one organic molecule (acetic acid) to two gaseous products, CH(4) and CO(2). Both the enthalpy and the entropy increases are due partially to the transition of these two products into the gaseous phase. The thermodynamic role of this phase transition for the growth process is analyzed. Microbial growth characterized by enthalpy increase and correspondingly by a large increase in entropy may be called enthalpy-retarded growth.

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