| Abstract: | 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, successively 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 carbon: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 community 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. |
