Indications fpr low net productivity of pelagic bacterioplankton

In contrast to the current view on the trophic role of bacteria in pelagic environments, the impression of a rather unproductive bacterioplankton arose from an example of the deep mesotrophic Lake Constance. Based on measurements of bacterial DNA, thymidine incorporation rates and grazing rates, turnover times of bacterial biomass exceeding 10 days were estimated during the growth period. Similarly, low productivity of bacterioplankton was indicated by low RNA/DNA ratios (< 1). Additional indications for a rather inactive bacterioplankton were provided by the observation of long lag phases in lake water cultures. Low bacterial productivity has also to be expected from energetic considerations. There is increasing evidence for low bacterial growth efficiencies under natural conditions. The presented material points to the possible need for reconsideration of current estimates of bacterial in situ growth