Control of phytoplankton  bacteria interactions by stoichiometric constraints Michael Danger, Jose ´phine Leflaive, Catherine Oumarou, Loı ¨c Ten-Hage and Ge ´rard Lacroix M. Danger (danger@biologie.ens.fr), C. Oumarou and G. Lacroix, Laboratoire Bioemco, Bioge ´ochimie et Ecologie des Milieux Continentaux, UMR 7618 (CNRS, INRA, ENS, Universite´Paris 6), Ecole Normale Supe ´rieure, 46, rue d’Ulm, FR-75230 Paris cedex 05, France.  J. Leflaive and L. Ten-Hage, Laboratoire d’Ecologie des Hydrosyste `mes, UMR 5177, Universite´Paul Sabatier, 118 route de Narbonne, FR-31062 Toulouse cedex 04, France. 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. Phytoplankton and heterotrophic bacteria, as primary producers and decomposers, form the basis of aquatic ecosystems (Loreau 2001), largely controlling pelagic energy flow and nutrient cycling. Much attention has been directed towards the relationships between these two functional groups (Rhee 1972, Cole 1982, Bratbak and Thingstad 1985, Danger et al. 2007). In an ecosystem where phytoplankton is the main C source, bacteria largely depend on their exudates. This relation- ship has been considered as commensalism (Bratbak and Thingstad 1985, Gurung et al. 1999), i.e. bacteria benefit from algae which in turn are neither harmed nor helped by bacteria. However, phytoplankton, as carbon providers, and decomposers, as actors of organic matter degradation, can also be considered as mutualists (Harte and Kinzig 1993, Daufresne and Loreau 2001). As algal exudates are generally dominated by carbohydrates, they are often lacking essential nutrients such as nitrogen or phosphorus (Myklestad 1974). Consequently, bac- teria must take up free mineral nutrients from their environment to equilibrate their chemical composition (Kirchman 1994). This process is called ‘‘immobiliza- tion’’. Thus, if algae are limited by nutrients, as it is often the case in natural conditions (Elser et al. 1990), algae and bacteria may compete for them. There is no clear evidence of a bacterial competitive advantage for nitrogen uptake, but many studies have shown that bacteria are superior competitors for phosphorus than algae (Rhee 1972, Currie and Kalff 1984, Jansson 1993, Joint et al. 2002). Moreover, the more algae are stressed by a lack of nutrients or a high light:nutrient ratio, the more they release N- or P-poor compounds, such as dissolved organic carbon (DOC) (Baines and Pace 1991, Obernosterer and Herndl 1995, Sterner et al. 1997, Danger et al. 2007). In comparison with algae, the increase in exudates permits bacteria to reach greater Oikos 116: 1079  1086, 2007 doi: 10.1111/j.2007.0030-1299.15424.x, Copyright # Oikos 2007, ISSN 0030-1299 Subject Editor: Dag Hessen, Accepted 16 February 2007 1079