Heterotrophic flagellates (HNF) and small ciliates have been recognized as important grazers of picoplanktonic organisms (heterotrophic bacteria, autotrophic picoplankton). By repackaging their picoplanktonic prey into particles accessible for crustacean grazers, phagotrophic protists are often implicated as a ‘trophic link’ to higher trophic levels. However, the role of protozoa as intermediary grazers might not be restricted to channelling energy; they might also upgrade the biochemical composition of food deficient in essential lipids. In experimental tritrophic food chains (autotrophic picoplankton—protozoa—Daphnia) we investigate the ability of HNFs and ciliates as intermediary grazers to upgrade a poor quality food source for Daphnia by producing essential lipids such as PUFAs and/or sterols (Martin-Creuzburg et al. 2005; Bec et al. 2006).
Reports on trophic interactions between ciliates and Daphnia are controversial. Experimental results provide evidence that daphnids are important ciliate predators. However, ciliates are obviously less nutritious for daphnids than many algae. The dietary sterol content is a crucial parameter in determining food quality for Daphnia. Ciliates, however, might be incapable of synthesizing sterols de novo. We therefore investigate the role of sterols in determining the food quality of ciliates for Daphnia. In growth experiments with D. magna, the lipid content of the ciliate Colpidium campylum was altered by feeding on fluorescently labeled albumin beads supplemented with different sterols. Sterol limitation was thereby identified as the major constraint of ciliate food quality for Daphnia. Furthermore, by supplementation of sterols unsuitable for supporting Daphnia growth, we provide evidence that ciliates as intermediary grazers biochemically upgrade unsuitable dietary sterols to sterols appropriate to meet the physiological demands of Daphnia (Martin-Creuzburg et al. 2006).