A classification of mixotrophic protists based on their behaviour

1. Mixotrophs are organisms which combine phototrophy and heterotrophy; such nutritional behaviour is widespread among protists. This ability to combine multiple modes of nutrition varies between species and is not related to their taxonomic grouping. A classification of mixotrophic protists, based on their behaviour, is proposed, dividing them into four groups.
2. Group A includes protists whose primary mode of nutrition is heterotrophy and where phototrophy is employed only when prey concentrations limit heterotrophic growth. In groups B, C and D phototrophy is the dominant mode of nutrition. In group B phagotrophy supplements growth when light is limiting, therefore ingestion of prey is inversely proportional to light intensity; in group C phagotrophy provides essential substances for growth and ingestion is proportional to light intensity; and group D includes those who have very low ingestion rates, ingesting prey only, for example, for cell maintenance during prolonged dark periods.
3. This classification is aimed towards predicting the impact of any particular mixotrophic protist on the aquatic food web, and how this impact may vary depending on the environmental conditions. A model representation of the four groups is discussed.