Pulse labelling: a method for measuring microbial growth rates in the ocean

Pulse labelling provides a means of generating the growth curve of a microbial population in natural samples. By suitable choice of radioisotopes, a variety of cellular components may be labelled, although adenine to label nucleic acids and 32P to label phospholipids proved most successful. The labelling of phospholipids also enabled us to measure the growth of algae. When cycloheximide was used as a eukaryote inhibitor to prevent grazing it served to reduce the incorporation of adenine into the bacterial component of the population. Sample dilution as means of controlling predation effects actually increased the sample growth rate until the bacterial component reached the same level as in the undiluted sample, at which time the growth rate fell to that found in the undiluted control. This finding suggests that the bacterial population may be maintained at the carrying capacity of the menstruum, and dilution actually induces an artifact rather than eliminate predation effects.