Abstract
Summary The chemical composition of primary producers, which occupy the base of food webs, is highly variable over space and time. Determining the causes of these variations is crucial to the understanding of ecosystem functioning. By differentially stocking and recycling nutrients, heterotrophic organisms such as bacteria may change the nature of the factor that limits algal growth, thus influencing algal chemical composition. We tested this hypothesis by growing a green alga, Scenedesmus obliquus (Turpin) Kützing, along a phosphorus gradient in the presence or absence of a bacterial community. We first confirmed that the limiting factor affects the chemical composition of phytoplankton. We then observed that the bacteria switched the nutrient that limited algal growth, from nitrogen to phosphorus in high N : P media. This generated considerable variation in algal stoichiometry. We propose that the bacterial compartment may have a greater influence on the structure and functioning of aquatic ecosystems than previously believed.
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