Direct and indirect effects of protist predation on population size structure of a bacterial strain with high phenotypic plasticity

Abstract

We studied the impact of grazing and substrate supply on the size structure of a freshwater bacterial strain (Flectobacillus sp.) which showed pronounced morphological plasticity. The cell length varied from 2 to >40 microm and encompassed rods, curved cells, and long filaments. Without grazers and with a sufficient substrate supply, bacteria grew mainly in the form of medium-sized rods (4 to 7 microm), with a smaller proportion (<10%) of filamentous forms. Grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that freely suspended cells of <7 microm were highly vulnerable to grazers, whereas filamentous cells were resistant to grazing and became enriched during predation. A comparison of long-term growth in carbon-limited chemostats with and without grazers revealed that strikingly different bacterial populations developed: treatments with flagellates were composed of >80% filamentous cells. These attained a biomass comparable to that of populations in chemostats without grazers, which were composed of medium-sized rods and c-shaped cells. Carbon starvation resulted in a fast decrease in cell length and a shift towards small rods, which were highly vulnerable to grazing. Dialysis bag experiments in combination with continuous cultivation revealed that filament formation was significantly enhanced even without direct contact of bacteria with bacterivores and was thus probably stimulated by grazer excretory products.

FIG. 1.

Scheme of the experimental setup in the dialysis bag chemostat experiment, showing microbial populations inside and outside the dialysis bag (dashed line). (A) Control treatment with Flectobacillus GC-5 only; (B) control treatment with Ochromonas sp. and Flectobacillus GC-5 inside and outside the dialysis bag; (C) grazing treatment with Ochromonas sp., with Flectobacillus GC-5 as the prey bacterium; (D) grazing treatment with Ochromonas sp., with Pseudomonas putida MM1 as the prey bacterium.

FIG. 2.

Growth of Flectobacillus GC-5 in batch culture on WC medium (plus 10 mg glucose per liter) in the presence and absence of the bacterivorous flagellate Ochromonas sp. (A) Development of bacteria and flagellates. Values are expressed as means of three replicates ± standard deviations. (B and C) Development of different cell length classes (μm) of Flectobacillus GC-5 in the absence (B) or presence (C) of Ochromonas sp. (% of cell numbers).

FIG. 3.

Photomicrographs of DAPI-stained cells of Flectobacillus GC-5 grown in chemostat cultures in the absence (A) or presence (B) of Ochromonas sp. Samples were taken on day 28 of the chemostat run. The scale bar in panel B refers to both photographs.

FIG. 4.

Regrowth of Flectobacillus GC-5, derived from grazer-free (A, C) and grazer-containing (B, D) chemostats, in batch cultures. Growth media were supplemented with 20 (A, B) or 10 (C, D) mg glucose per liter. Chemostat bacteria derived from grazer-free (E) and grazer-containing (F) chemostats were also transferred to starvation conditions in batch culture. The development of flagellates and bacteria, differentiated into edible (<7 μm) and inedible (>7 μm) bacterial cell length classes, and the mean bacterial cell length after transfer into batch culture are shown. Values are means ± standard deviations of three replicates.

FIG. 5.

Growth of Flectobacillus sp. strain GC-5 growing outside dialysis bags in the chemostat dialysis experiment. (A) Control with bacteria only; (B) control with bacteria and flagellates inside and outside the dialysis bags; (C) grazing treatments with flagellates and Flectobacillus sp. strain GC-5 inside the dialysis bags; (D) grazing treatments with flagellates and P. putida MM1 inside the dialysis bags. Cells numbers are given as means of three replicates for each treatment ± standard deviations.

FIG. 6.

Development of cell length of Flectobacillus sp. strain GC-5 growing outside dialysis bags during the last 12 days of the chemostat dialysis experiment. Treatments A to D refer to the same treatments as in Fig. 1 and 5. (A) Only Flectobacillus GC-5 (inside and outside); (B) Flectobacillus GC-5 and Ochromonas sp. (inside and outside); (C) inside, Flectobacillus GC-5 and Ochromonas sp.; outside, Flectobacillus GC-5; (D) inside, P. putida MM1 and Ochromonas sp.; outside, Flectobacillus GC-5. Vertical boxes with error bars in the graphs represent the median and 10th, 25th, 75th, and 90th percentiles of bacterial cell lengths (means of two replicates) of Flectobacillus GC-5.