Bacterioplankton Growth Yield: Seasonal Variations and Coupling to Substrate Lability and beta-Glucosidase Activity.

The seasonal variation in the carbon growth yield of pelagic bacteria in the eutrophic lake Frederiksborg Slotssø was studied. The growth yield was determined in dilution culture experiments, in which a substrate of dissolved organic carbon (DOC) from the lake was incubated with a natural bacterioplankton assemblage. Bacterial growth efficiency varied annually from 8 to 60% with an average (and standard deviation) of 41 +/- 11% (n = 29). Simultaneous measurements of growth yield, substrate lability (DOC(L)), chlorophyll and bacterial production, abundance, and extracellular enzymatic activity revealed new aspects of the regulation of bacterial DOC utilization. Growth yield correlated positively to DOC(L) and negatively to beta-d-glucosidase activity. These results indicated a close coupling between the substrate conditions and the physiological response of the bacteria. The large variations in yield within a few days and the close coupling to substrate availability showed that one single global carbon yield factor cannot be expected to apply in pelagic systems.