Conceptual models of mixotrophy in planktonic protists and some ecological and evolutionary implications

Mixotrophy is a common phenomenon among planktonic algae and protozoa. Mixotrophic protists that combine phagotrophy and phototrophy are often abundant in the euphotic zone in fresh, estuarine and oceanic waters. Mixotrophy can be important in waters ranging from eutrophic to oligotrophic and from polar to tropical. Mixotrophic protists differ both qualitatively and quantitatively in their dependence on feeding, light and uptake of dissolved inorganic nutrients. Conceptual models for six physiological types of mixotrophs are presented. Functional relationships of phototrophy and phagotrophy to the availability of dissolved inorganic nutrients, light and particulate food are predicted for each model. The hypothetical costs of phototrophy and phagotrophy in mixotrophic protists as well as the possible ecological and evolutionary implications of these costs are described. The probable effects of mixotrophy on gross growth efficiency, productivity of the microbial food web and coupling of the microbial food web to metazoan zooplankton are discussed.