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Appl Environ Microbiol. 1989 July; 55(7): 1787-1795
Copyright © 1989, American Society for Microbiology. All Rights Reserved.

Protozoan Grazing and Bacterial Production in Stratified Lake Vechten Estimated with Fluorescently Labeled Bacteria and by Thymidine Incorporation

Jaap Bloem^,*, Frank M. Ellenbroek, Marie-José B. Bär-Gilissen and Thomas E. Cappenberg

Vijverhof Laboratory, Limnological Institute, 3631 AC Nieuwersluis, The Netherlands

ABSTRACT

In stratified Lake Vechten, The Netherlands, protozoan grazing was estimated on the basis of uptake of fluorescently labeled bacteria and compared with bacterial production estimated on the basis of thymidine incorporation. By using a grazer-free mixed bacterial population from the lake in continuous culture, an empirical relationship between cell production and thymidine incorporation was established. Thymidine incorporation into total cold-trichloroacetic-acid-insoluble macromolecules yielded a relatively constant empirical conversion factor of ca. 10¹⁸ (range, 0.38 x 10¹⁸ to 1.42 x 10¹⁸) bacteria mol of thymidine^–1 at specific growth rates (µ) ranging from 0.007 to 0.116 h^–1. Although thymidine incorporation has been assumed to measure DNA synthesis thymidine incorporation appeared to underestimate the independently measured bacterial DNA synthesis by at least 1.5- to 13-fold, even if all incorporated label was assumed to be in DNA. However, incorporation into DNA was found to be insignificant as measured by conventional acid-base hydrolysis. Methodological problems of the thymidine technique are discussed. Like the cultures, Lake Vechten bacteria showed considerable thymidine incorporation into total macromolecules, but no significant incorporation into DNA was found by acid-base hydrolysis. This applied not only to the low-oxygen hypo- and metalimnion but also to the aerobic epilimnion. Thus, the established empirical conversion factor for thymidine incorporation into total macromolecules was used to estimate bacterial production. Maximum production rates (141 x 10⁶ bacteria liter^–1 h^–1; µ, 0.012 h^–1) were found in the metalimnion and were 1 order of magnitude higher than in the epi- and hypolimnion. In all three strata, the estimated bacterial production was roughly balanced by the estimated protozoan grazing. Heterotrophic nanoflagellates were the major consumers of the bacterial production and showed maximum numbers (up to 40 x 10⁶ heterotrophic nanoflagellates liter^–1) in the microaerobic metalimnion.

^* Corresponding author.

Present address: Institute for Soil Fertility, P.O. Box 30003. 9750 RA Haren, The Netherlands.

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