Control of phytoplankton–bacteria interactions by stoichiometric constraints

In aquatic ecosystems, phytoplanktonic organisms are the major primary producers and bacteria the major decomposers. The interactions between phytoplankton and bacteria may be dependent on nutrient resources. Anthropogenic inputs, by modifying nutrient status and stoichiometry of lakes, might induce changes in these interactions, and thus, could have many consequences on some ecological processes such as primary production or importance of microbial recycling activity. To test this hypothesis, we grew an axenic strain of a green alga, Scenedesmus obliquus , in a range of stoichiometric situations, in absence and in presence of a natural bacterial community. Here, we show that different phytoplankton limiting factors can generate between algae and bacteria either competition for nutrients in phosphorus‐limited conditions, commensalism in nitrogen‐limited conditions, or mutualism in eutrophic nutrient‐unlimited conditions. Causes of these different interaction types are discussed, in particular the hypothesis that in very eutrophic systems with high primary production, mutualism between algae and bacteria could be due to CO 2 supply by heterotrophic respiration to inorganic carbon limited algae. Some probable consequences for aquatic ecosystems functioning are proposed.