Heterotrophic protists are important grazers on autotrophic components of the plankton and compete with higher trophic levels, especially copepods, for some of these food sources. Primary producers may be limited by different nutrient compounds as well as by light availability, which affect their nutritional quality for higher trophic levels. Food quality factors can be defined e.g. in terms of C:N:P- stoichiometry. Nutritionally limited algae can have significant negative effects on copepods growth and reproduction. Algae with high C:N:P- ratio as food particles result in a higher energy cost for the consumer, as they have to egest the ingested surplus carbon in order to maintain their homeostasis. Knowledge about food quality effects on heterotrophic protists is sparse. In the first step of our project we examined how nutrient limited Rhodomonas sp. affect the heterotrophic dinoflagellate Oxyrrhis marina.