used to study biomass increase. The most common incubation substrate for this
purpose is [^3 H]-leucine, an amino acid. After the incubation, bacteria are washed in
TCA and ethanol to extract radiolabeled substrate that may have been taken up by
cells but not yet into macromolecules. Only leucine incorporated in protein, a measure
of growth, will appear in the scintillation counts. Since they measure different
processes, leucine- and thymidine-uptake rates need not be related. Sometimes the
two rates are well correlated (Fig. 5.9), sometimes with outliers. A leucine : TdR
incorporation ratio of 10 implies balanced growth, i.e. bacteria are synthesizing
protein and DNA at approximately the same rates (Ducklow 2000). Both the Antarctic
data (Fig. 5.9) and the subarctic Pacific data from Kirchman (1992; Fig. 5.6 here)
have a typical Leu : TdR ratio around 10, although there is seasonal and regional
variability. Wide variations in the ratio of Leucine : TdR have been found in field
studies, and these need to be interpreted with caution (Sherr et al. 2001).
Fig. 5.9 Correlations from Antarctic waters of thymidine uptake (a measure of cell-
division rates) and leucine uptake (a measure of protein synthesis) in
bacterioplankton. Both single sample results (a) and vertical integrals through the
euphotic zone (b) are compared. Dashed line in (a) is 10 : 1. Bacterial production rates
(mmol C m−2 d−1) for vertical integrals are derived using empirically derived
conversion factors. Symbols represent cruise results from different seasons.
(After Ducklow et al. 2001.)
(^) Bacteria can utilize a wide suite of dissolved organic compounds and, by combining
microautoradiography and fluorescence in situ hybridization (MICRO-FISH) the
relative contribution of different phylogenetic bacterial groups can be determined.