Isotopic labeling of metabolic water with 18 O 2 .

Any organism with aerobic respiration is amenable to labeling with O , and the method described here enables a new approach to investigate questions regarding plant and microbial physiology. In plants, O introduced as metabolic water could be tracked as it moves between living cells and exchanges with external water. For probing soil microbial physiology, the method described here has the advantage over the application of exogenous H O of not increasing the soil moisture, a disturbance that can affect microbial metabolism.