9508571 Caron Mixotrophy is the combination of heterotrophic and phototrophic nutrition in a single organism. Among the unicellular microalgae, a wide variety of species ingest particulate organic material. Several possible adaptive advantages exist for mixotrophic behavior among phagotrophic algae. Thus far, three general strategies are served by phagotrophic behavior: (1) primarily phagotrophic growth at high bacterial abundances, with phototrophy serving as a mechanism for very slow grouth when bacteria are present at low abundances, (2) primarily phototrophic growth with phagotrophy serving as a mechanism for supplementing growth at low light intensities, and (3) primarily phototrophic growth with phagotrophy serving as a mechanism for acquiring major nutrients such as nitrogen or phosphorus. This research will focus on the ability of phagotrophic microalgae to compete with purely phototrophic protists (algae) and purely heterotrophic protists (protozoa). The framework includes a model that incorporates mixotrophy into a simple microbial food web in order to generate hypotheses concerning the ability of phagotrophic algae to compete with nonphagotrophic algae and protozoa. The model is based on the understanding of the three strategies for mixotrophic behavior. Hypotheses will be tested in the laboratory via a series of competition experiments between algae, protozoa, and phagotrophic algae in continuous cultures. Experiments will examine the effects of light limitation, additions of labile dissolved organic carbon, and inorganic nutrient limitation on the growth of phagotrophic algae competing with nonphagotrophic algae and protozoa. The guiding hypothesis is that the different behaviors exhibited by different species of phagotrophic algae provide them with a competitive advantage that will result in their success under particular circumstances. These experiments will establish those circumstances and provide valuable insights for explaining the distribution of mixotrophic species in na ture.
