Dietary-induced responses in the phagotrophic flagellate Oxyrrhis marina
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Primary producers may be limited by different nutrients as well as by light availability, which in turn affects their quality as food for higher trophic levels. Typically, algae with high C:N and/or C:P ratios are low-quality food for consumers. Heterotrophic protists are important grazers on these autotrophes, but despite their importance as grazers, knowledge of food quality effects on heterotrophic protists is sparse. In the present study, we examined how differently grown Rhodomonas salina (nutrient replete, N-limited and P-limited) affected the phagotrophic flagellate Oxyrrhis marina. The functional response of O. marina (based on ingested biovolume) did not show significant differences between food sources, thus food uptake was independent of food quality. O. marina was weakly homoeostatic which means that its C:N:P ratio still reflected the elemental composition of its food to some extent. Food quality had a significantly negative effect on the numerical response of O. marina. Whereas N-limited R. salina and nutrient replete R. salina resulted in similar growth rates, P-limited algae had a significantly negative effect on the specific growth rate of O. marina. Hence, the lack of elemental phosphorus of O. marina feeding on P-limited algae caused a reduction in growth. Thus, despite their weaker homoeostasis, heterotrophic protists are also affected by high C:P food in a similar way to crustacean zooplankton.
KeywordsFood Quality High Respiration Rate Prey Concentration Heterotrophic Protist Experimental Bottle
This study is a part of the PhD studies conducted by F.M.H. at the Biologische Anstalt Helgoland, Alfred-Wegener-Institut Bremerhaven, Germany, financed by GKSS Geesthacht, Germany, and complies with current German law on animal studies. We thank Daniel Schütz, Jacob Hauschildt and Anneke Purz for laboratory assistance and Martin Löder, Christina Gebühr, Katherina Schoo, Petra Brandt, Arne Malzahn and Nils Gülzow for useful discussions. Special thanks to Prof. Dr. Sebastian Diehl and two anonymous reviewers whose suggestions improved the manuscript.
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