Interactions between algae and the microbial loop in experimental microcosms

We conducted a short-term microcosm experiment to study the direct and indi rect effects of a bacterivore on bacteria and the dynamics of two species o f green algae. We introduced Scenedesmus, Chlorella and Colpidium, a bacter ivorous ciliate, successively in a carbon-rich medium. Bacteria were introd uced with Scenedesmus, Chlorella and Colpidium. The experiment lasted 40 da ys, preventing us from detecting whether the populations had reached equili brium. The bacterivore had a positive effect on both species of algae by li miting the abundance of bacteria. In absence of the bacterivore, bacteria d id not exclude the two algal species, despite the high carbon:nutrient rati o of the medium. Unexpectedly, by the end of the experiment the bacterivore , declined in all microcosms. Also, Chlorella growth was impeded by the pre sence of Scenedesmus. These two observations might be explained by allelopa thic interactions. Our experiment suggests that the functioning of such a s imple community is far more complex than assumed in previous theoretical an d experimental models. Studying the dynamics of the system, however, allowe d us to disentangle species interactions.