On the Strategy of "Eating Your Competitor": A Mathematical Analysis of Algal Mixotrophy

Recent experimental evidence indicates the importance in some pelagic systems of mixotrophic protists that combine photosynthetic ability with the ability to ingest bacteria. If both bacteria and phytoplankton are mineral nutrient limited, this should provide the mixotrophs with the double benefit of combining removal of their competitor with ingestion of the limiting nutrient in pelleted form. It is the objective of this study to expand the classical theories of competition and predation to explore the effect on the microbial food web of one trophic group possessing both strategies. In a chemostat scenario, we analyzed the two—species situation of a mixotroph preying on mineral—nutrient—limited bacteria, and also the situations when the mixotroph in addition has to compete with specialized photoautotrophic and phagotrophic protists, each superior to the mixotroph in their specialized nutritional modes. In the mixotroph—bacteria relationship, somewhat paradoxically, high predatory abilities will reduce the quantitative importance of predation in the mixotroph's nutrition. The reason is a strong reduction in prey abundance, allowing the mixotroph to survive as a photoautotroph despite its low competitive ability. In the three—species case with mixotrophs, bacteria, and specialized phagotrophs, it is shown that the mixotroph can compensate for a "price" paid in reduced affinity for bacterial prey by a sufficiently high affinity for mineral nutrients. In the other three—species case where the mixotroph has to compete with a specialized photoautotroph, the situation is more complex; there is an optimum value for the mixotroph's predatory ability at which mixotroph biomass is maximized. In the general situation with all four species (bacteria, mixotrophs, and specialized auto— and phagotrophs) potentially present, different mixotrophic strategies will alter the equilibrium composition of the consortium, with the mixotroph being most successful with a high affinity for nutrients and an intermediate affinity for bacteria. In the simple form used here, the model predicts no equilibrium with all four species simultaneously present. The theory is in principle directly applicable to laboratory experimentation.