We conducted a short-term microcosm experiment to study the direct and indirect effects of a bacterivore on bacteria and the dynamics of two species of green algae. We introduced Scenedesmus, Chlorella and Colpidium, a bacterivorous ciliate, succes- sively in a carbon-rich medium. Bacteria were introduced with Scenedesmus, Chlorella and Colpidium. The experiment lasted 40 days, preventing us from detecting whether the populations had reached equilibrium. The bacterivore had a positive effect on both species of algae by limiting the abundance of bacteria. In absence of the bacterivore, bacteria did not exclude the two algal species, despite the high car- bon:nutrient ratio of the medium. Unexpectedly, by the end of the experiment the bacterivore declined in all microcosms. Also, Chlorella growth was impeded by the presence of Scenedesmus. These two observations might be explained by allelopathic interactions. Our experiment suggests that the functioning of such a simple commu- nity is far more complex than assumed in previous theoretical and experimental models. Studying the dynamics of the system, however, allowed us to disentangle species interactions. F.
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