Flagellate predation on a bacterial model community: interplay of size-selective grazing, specific bacterial cell size, and bacterial community composition

Abstract

The influence of grazing by the bacterivorous nanoflagellate Ochromonas sp. strain DS on the taxonomic and morphological structures of a complex bacterial community was studied in one-stage chemostat experiments. A bacterial community, consisting of at least 30 different strains, was fed with a complex carbon source under conditions of low growth rate (0.5 day(-1) when nongrazed) and low substrate concentration (9 mg liter(-1)). Before and after the introduction of the predator, the bacterial community composition was studied by in situ techniques (immunofluorescence microscopy and fluorescent in situ hybridization), as well as by cultivation on agar media. The cell sizes of nonspecifically stained and immunofluorescently labeled bacteria were measured by image analysis. Grazing by the flagellate caused a bidirectional change in the morphological structure of the community. Medium-size bacterial cells, which dominated the nongrazed community, were largely replaced by smaller cells, as well as by cells contained in large multicellular flocs. Cell morphological changes were combined with community taxonomic changes. After introduction of the flagellate, the dominating strains with medium-size cells were largely replaced by single-celled strains with smaller cells on the one hand and, on the other hand, by Pseudomonas sp. strain MWH1, which formed the large, floc-like forms. We assume that size-selective grazing was the major force controlling both the morphological and the taxonomic structures of the model community.

FIG. 1

Photomicrographs of bacteria from reactor I stained with DAPI. (A) Before introduction of the flagellate (phase 1). (B) After introduction of the flagellate (phase 2). (C) After inhibition of the flagellate population (phase 3). (D) After reestablishment of the flagellate population (phase 4). A single cell of the bacterivorous flagellate Ochromonas sp. strain DS is shown in panel D. The images represent different sample volumes filtered onto membrane filters.

FIG. 2

Influence of flagellate grazing on the complex bacterial community cultured in reactor I. The upper panel shows the total bacterial cell numbers and the percentage of single-celled bacteria. The lower panel shows the relative abundances (percentages of total cell numbers) of the three species detected by immunofluorescence microscopy. The four experimental phases are indicated by vertical lines. Flag., flagellate; contr., controlled; P. sp. MWH1, Pseudomonas sp. strain MWH1.

FIG. 3

Plot of the number of single-celled bacteria against the number of cells in flocs. The data set represents only samples from the flagellate-controlled phases of reactor I (circles) as well as from reactor II (triangles). The open circles represent data from a 20-day period (about 10% of the total flagellate-controlled phases), and the solid circles represent data from the other periods.

FIG. 4

Comparison of the relative abundance of the Vibrio sp. determined by the plating approach (open circles) and by immunofluorescence microscopy (solid squares). The plating data represent the percentage of Vibrio sp. colonies (colony types II, III, and XVI) among total CFU, and the microscopically determined data represent the percentage of Vibrio sp. cells among the total number of single-celled bacteria. The plating data are based on 100 to 250 counted colonies (total CFU), and the microscopy data are based on at least 500 counted cells (total single cells).

FIG. 5

Changes in the cell size of the total single-celled bacteria, of C. acidovorans PX54, and of the Vibrio sp. after introduction of the flagellate Ochromonas sp. strain DS into reactor I (center panel). The upper panel gives the total bacterial and flagellate abundances, and the lower panel gives the relative abundances of three investigated bacteria (percentage of total cell numbers).

FIG. 6

Influence of size-specific grazing of the flagellate Ochromonas sp. strain DS (upper graph) on the size distribution of the bacterial community of reactor I. The bars of the graph in the middle represent the size class distribution of the community during phase 1 (mean values of five samples). The hatched parts of the bars indicate the proportion of Vibrio sp. cells. The lower graph represents the distribution after introduction of the predator (five samples from phase 2). The size class >2.0 μm³ is exclusively formed by the Pseudomonas sp. strain MWH1 flocs (the bars represent mean values for the whole microcolony population). This size class contained 4% of the bacterial particles (single-celled bacteria and flocs) but 72% of the bacterial biomass.

FIG. 7

Schematic model of the interplay of size-selective grazing, specific bacterial cell size, and morphological as well as taxonomic structure of the bacterial communities investigated in the chemostat experiments. The upper part of the model depicts the size selectivity of protist grazing. The middle part shows the size distribution of bacterial communities in situations with and without grazing pressure, as well as their possible strain composition (indicated by letters). The lower part depicts the size distribution of single bacterial strains (A to D), as well as grazing-influenced changes in their size distributions (dotted curves). The arrows indicate the direction and strength of induced changes in the size distributions of the strains. Examples for some of the distribution patterns are shown, and the triggers of changes are given. The key result is that differences in the distribution patterns of single bacterial strains produced different grazing mortalities of the respective populations and thus resulted in changes in the morphological and taxonomic structures of the bacterial community. The possible influence of grazing on bacterial competition for substrates or on changes in substrate supply, as well as size-independent grazing defense strategies, is not considered. (a), reference ; (b), reference ; (c), reference ; G, growth rate; ?, unknown.