Potential effects of isotopic dilution on apparent respiration in 14C heterotrophy experiments

Abstract

ABSTRACT: Patterns of [U-'4C]glucose uptake and 14C02 production by Vibrio natriegens in model experiments indicated that the percent respiration of a substrate can be underestimated when using radiotracers if effects of isotopic dilution are not considered. Percent respiration of radiolabelled glucose differed in broth cultures of V natriegens that were similar except for the timing of addition to the cultures of tracer [U-14C]glucose. Percent respiration was high (- 45 to 50 %) in cultures to which tracer glucose was added in early log phase but was initially low and increased over time in cultures to which tracer glucose was added in mid-log or stationary growth phases. The observed patterns of [U- '4C]glucose respiration were probably due to changes in the specific activity of the immediate intracellular precursors of 14C02 in accordance with standard models of the phenomenon of isotopic dilution. One implication of these results is that percent respiration in field studies of bacterial heterotrophy may have been underestimated, since in many cases isotopic dilution has not been taken into consideration. Previously published studies of substrate uptake and transformation in the water column have been re-interpreted in view of these findings.