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Applied and Environmental Microbiology, January 2006, p. 78-86, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.78-86.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Direct and Indirect Effects of Protist Predation on Population Size Structure of a Bacterial Strain with High Phenotypic Plasticity

Gianluca Corno1,2 and Klaus Jürgens1,3*

Max Planck Institute for Limnology, 24306 Plön, Germany,1 CNR, Institute of Ecosystems Study, 28922 Verbania Pallanza, Italy,2 Baltic Sea Research Institute Warnemünde, 18119 Rostock, Germany3

Received 27 July 2005/ Accepted 17 September 2005

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 µm 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 µm), with a smaller proportion (<10%) of filamentous forms. Grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that freely suspended cells of <7 µm 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.

* Corresponding author. Mailing address: Baltic Sea Research Institute Warnemünde, Seestr. 15, D-18119 Rostock, Germany. Phone: 49-381-5197 250. Fax: 49-381-5197 211. E-mail: klaus.juergens{at}io-warnemuende.de

Applied and Environmental Microbiology, January 2006, p. 78-86, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.78-86.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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