Coexistence of algae and bacteria: a test of the carbon hypothesis

The competition between algae and aquatic bacteria for inorganic nutrients puts some constraints on the coexistence of these 2 essential groups in the water column. Based on competition and stoichiometry theories the carbon hypothesis states that, since bacteria are better competitors for nutrients, they may drive themselves to carbon (C)-limitation, so coexistence would naturally arise between nutrient-limited algae and C-limited bacteria. We tested the carbon hypothesis in 24 closed aquatic microcosms with 4 different artificial media inoculated with an alga (Scenedesmus obliquus) and a natural bacterial community. We monitored algal and bacterial growth, coexistence over time, and the limitation status of S. obliquus and the bacterial community. In all 24 microcosms, both the algal and the bacterial communities showed a net growth phase followed by a plateau, reflecting stable coexistence over time. At the plateau, the algae were limited by nitrogen (N) and/or phosphorus (P), depending on the medium, whereas the bacteria were co-limited by organic C and the nutrients N and P. We argue that these results are consistent with the carbon hypothesis, and we discuss the fact that bacteria were co-limited instead of purely C-limited in light of the resource-ratio theory. Our results provide evidence that, even in the absence of factors preventing competitive exclusion (e.g. spatial heterogeneity, temporal fluctuations, predators), phytoplankton and bacteria can coexist and sustain basic ecosystem functions, namely primary production and nutrient cycling.