Charting dissipation across the microbial world

The energy dissipated by a living organism is commonly identified with heat generation. However, as cells exchange metabolites with their environment they also dissipate energy in the form of chemical entropy. How dissipation is distributed between exchanges of heat and chemical entropy is largely unexplored. Here, we analyze an extensive experimental database recently created 1 to investigate how microbes partition dissipation between thermal and chemical entropy during growth. We find that aerobic respiration exchanges little chemical entropy and dissipation is primarily due to heat production, as commonly assumed. However, we also find several types of anaerobic metabolism that produce as much chemical entropy as heat. Counterintuitively, instances of anaerobic metabolisms such as acetotrophic methanogenesis and sulfur respiration are endothermic. We conclude that, because of their metabolic versatility, microbes are able to exploit all combinations of heat and chemical entropy exchanges that result in a net production of entropy.