Metabolic water is water produced as cellular fuels (lipids, carbohydrates, proteins) are oxidized during aerobic respiration, the central energy-producing process in animals, plants, and most microorganisms. Metabolic water is formed when oxygen is reduced as the terminal electron acceptor in the electron transport chain, and the rate of metabolic water production can be estimated from the rate of oxygen consumption or carbon dioxide production. Although importance of metabolic water is known for animals such as desert rodents, for which it can be the exclusive water source, the role of metabolic water in the ecophysiology of microbes has been largely ignored. Our object was to determine if metabolic water is of quantitative significance to the water balance of soil microbes. If so, this would imply that organic matter oxidation functions as a source of water in addition to its recognized role as a source of energy and carbon skeletons. We used published rates of soil heterotrophic respiration and microbial biomass to estimate the volume of metabolic water produced by microorganisms in diverse terrestrial ecosystems. We used published relationships between mass-specific oxygen consumption and body mass of bacteria to estimate the fractional contribution of metabolic water to cellular water volume. Lastly, we measured oxygen consumption, soil moisture, and isotopic composition of soil water labeled with 18-oxygen enriched oxygen gas during laboratory incubations of soils with and without added carbon substrate to document the quantitative relationship between microbial carbon metabolism and metabolic water production.
Soil respiration and microbial biomass data from boreal, Mediterranean, and tropical forest indicate that respiratory metabolism produces a water volume equal to 4% to 70% of microbial biomass water each day. From published data on metabolic rates and body size of bacteria we estimate that aerobic respiration produces an amount of water equivalent to 10 – 30% of organism mass daily. Our direct measurements of oxygen consumption during soil incubations indicate that daily microbial respiration produces a mass of water that is 5% (no added glucose) to 15% (added glucose) of microbial cell water mass. Moreover, using an isotopic tracer technique, we have detected the presence of the oxygen from metabolic water in microbial biomass. Together these studies suggest that metabolic water may contribute significantly to the water balance of soil microorganisms and that this may be particularly important during periods of soil drying.