Effect of Protistan Grazing on the Frequency of Dividing Cells in Bacterioplankton Assemblages

Abstract

Grazing by phagotrophic flagellates and ciliates is a major source of mortality for bacterioplankton in both marine and freshwater systems. Recent studies have demonstrated a positive relationship between clearance rate and prey size for bacterivorous protists. We tested the idea that, by selectively grazing the larger (more actively growing or dividing) cells in a bacterial assemblage, protists control bacterial standing stock abundances by directly cropping bacterial production. Samples of estuarine water were passed through 0.8-jim-pore-size filters (bacteria only) or 20-pLm-mesh screens (bacteria and bacterivorous protists) and placed in dialysis tubing suspended in 7 liters of unfiltered water. Changes in total bacterial biovolume per milliliter (bacterial biomass), frequency of dividing cells (FDC), and average per cell biovolume were followed over a period of 24 h. In three experiments, the FDC increased more rapidly and attained higher values in water passed through 0.8-,um-pore-size filters (average, 5.1 to 8.9%; maximum, 15.5%) compared with FDC values in water passed through 20-p.m-mesh screens (average, 2.7 to 5.3%; maximum, 6.7%). Increases in bacterial biomass per milliliter lagged behind increases in FDC by about 4 to 6 h. Grazed bacterial assemblages were characterized by lower total biomasses and smaller average cell sizes compared with those of cells in nongrazed assemblages. We conclude that bacterivorous protists control bacterial standing stock abundances partly by preferentially removing dividing cells. Selective grazing of the more actively growing cells may also explain, in part, the ability of slow-growing cells to persist in bacterioplankton assemblages.

Regulation of bacterial biomass and community structure by metazoan and protozoan predation

“…This confirms the conclusion that Daphnia grazing preferentially eliminates the fast-growing part of the bacterial community, which consists of relatively large cells (Jürgens 1994). Thus, despite very different feeding modes of cladocerans and interception-feeding flagellates, both groups seem to have a comparable impact by removing the active, growing, and larger-sized portion of a bacterial community, leaving smaller and more slowly or nongrowing cells (Sherr et al 1992;del Giorgio et al 1996). It can therefore be summarized that grazing by Daphnia as well as by HNFs forces the bacterial community toward the lower end of the size spectrum but that HNF grazing additionally promotes the growth of complex grazing-resistant growth forms.…”

“…Grazing experiments-In order to compare the bacterial grazing impact of Daphnia and HNFs in the experimental containers, we measured the disappearance of fluorescently labelled bacteria (FLB) over an extended incubation period (ϳ48 h) . Bacteria for the preparation of FLB were obtained from a glucose-limited mixed chemostat culture and stained with the dye DTAF (5-[(4,6-Dichlorotriazin-2-yl)amino]fluorescein; Sigma) according to the method of Sherr et al (1987). The mean volume of the FLB was ϳ0.15 m 3 .…”

Rapid Daphnia-mediated changes in microbial community structure: an experimental study

“…Prey size is an important factor determining susceptibility to grazing [11,19,21,39]. Ciliates and Daphnia are ¢lter feeders with the mesh width of the ¢ltration apparatus determining the particle retention e⁄ciency.…”

Microflora of a subalpine lake: bacterial populations, size and DNA distributions, and their dependence on phosphate

“…The size distributions of the bacteria are consistent with significant bacterivore grazing [42,68] although we have had difficulty establishing these rates [ 191 in diluted samples of seawater. The largest organisms were within a deep, potentially rich zone of otherwise labile-carbon-poor water.…”