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This article presents a simple, unstructured mathematical model describing microbial growth in continuous culture limited by a gaseous substrate. The model predicts constant gas conversion rates and a decreasing biomass concentration with increasing dilution rate. It has been found that the parameters influencing growth are primarily the gas transfer rate and the dilution rate. Furthermore, it is shown that, for correct simulation of growth, the influence of gaseous substrate consumption on the effective gas flow through the system has to be taken into account. Continuous cultures of Methanobacterium thermoautotrophicum were performed at three different gassing rates. In addition to the measurement of the rates of biomass production, product formation, and substrate consumption, microbial heat dissipation was assessed using a reaction calorimeter. For the on‐line measurement of the concentration of the growth‐limiting substrate, H2, a specially developed probe has been used. Experimental data from continuous cultures were in good agreement with the model simulations. An increase in gassing rate enhanced gaseous substrate consumption and methane production rates. However, the biomass yield as well as the specific conversion rates remained constant, irrespective of the gassing rate. It was found that growth performance in continuous culture limited by a gaseous substrate is substantially different from “classic” continuous culture in which the limiting substrate is provided by the liquid feed. In this report, the differences between both continuous culture systems are discussed.

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Growth of Methanobacterium thermoautotrophicum, an anaerobic archaebacterium using methanogenesis as the catabolic pathway, is characterized by large heat production rates, up to 13 W g −1 , and low biomass yields, in the order of 0.02 C-mol mol −1 H 2 consumed. These values, indicating a possibly "inefficient" growth mechanism, warrant a thermodynamic analysis to obtain a better understanding of the growth process. The growth-associated heat production (⌬ r H X 0,min ) and the growth-associated Gibbs energy dissipation per mol biomass formed (⌬ r G X min ) were −3730 kJ C-mol −1 and −802 kJ C-mol −1 , respectively. The Gibbs energy change found in this study is indeed unusually high as compared to aerobic methylotrophes, but not untypical for methanogens grown on CO 2 . It explains the low biomass yield. Based on the information available on the energetic metabolism and on an ATP balance, the biomass yield can be predicted to be approximately in the range of the experimentally determined value. The fact that the exothermicity exceeds vastly even the Gibbs energy change can be explained by a dramatic entropy decrease of the catabolic reaction. Microbial growth characterized by entropy reduction and correspondingly by unusually large heat production may be called entropy-retarded growth.

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