Comparative analysis shows that bacterivory, not viral lysis, controls theabundance of heterotrophic prokaryotic plankton

Empirical models derived from literature data were used to compare the fact ors controlling prokaryotic abundance (PN) and prokaryotic heterotrophic pr oduction (PHP) in solar salterns. These empirical relationships were genera ted as multiple linear regressions with PN or PHP as dependent variables, w hile the independent variables were chosen to reflect the likely sources of organic matter, inorganic nutrients and temperature. These variables were then measured in solar salterns and the predictions made by the general rel ationships were compared to actual saltern values of PN and PHP. Saltern po nds of salinity higher than 100 parts per thousand departed significantly f rom the general relationships, while the ponds of salinity lower than 100 p arts per thousand fitted well within the range of values predicted by the g eneral models. The most likely explanation for the discrepancy of the forme r was the absence of bacterivory. This hypothesis was tested with data from other very different aquatic systems: karstic lakes with anaerobic hypolim nia and two marine areas in the Mediterranean and the Southern Ocean. The a noxic regions of karstic lakes departed significantly from the predictions of the general model, while the oxic layers conformed to the predictions. A s in the case of salterns. this difference could be explained by the presen ce of significant predation in the oxic, but not in the anoxic, layers of t hese lakes. Finally, two marine areas with similar predation pressure on pr okaryotes but very different impacts of viral lysis were tested. In all cas es, PN values conformed to the predictions, suggesting that lysis due to vi ruses is not the main factor controlling PN in aquatic systems, which is mo re likely to be determined by the balance between bacterivory and resource supply. The present work also demonstrates the usefulness of empirical comp arative analyses to generate predictions and to draw inferences on the func tioning of microbial communities. (C) 2000 Federation of European Microbiol ogical Societies. Published by Elsevier Science B.V. All rights reserved.