Microbial growth by a net heat up-take:: A calorimetric and thermodynamic study on acetotrophic methanogenesis by Methanosarcina barkeri

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, CH4 and CO2. 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. (C) 2001 John Wiley & Sons, Inc.