ABSTRACT

Macromolecules such as deoxyribonucleic acid, ribonucleic acid, or membrane lipids, however, will only accumulate radiolabel as a function of growth, and will require several generations before isotopic uptake and growth have equilibrated, that is, before the growth rate of the bacteria and the rate of radiolabel incorporation become equal. In a pulse-labeling experiment, one takes and maintains a relatively large sample at in situ temperature, and periodically withdraws subsamples to measure the increase or decrease in community size as represented by changes in macromolecule synthesis. Three peices of information are obtained in a typical pulse-labeling experiment: the absolute growth rate of the active community, free of any assumptions or qualifications; the relative size of the growing component; and the shape of the growth curve itself. The pulse-labeling technique is particularly useful in establishing cause-and-effect relationships that might exist owing to changes in conditions that may arise through the addition of nutrients and toxicants, or by seasonal events.