International audienceThe kinetics and efficiency of sterol production and bioconversion of phytosterols in two heterotrophic protists Oxyrrhis marina and Gyrodinium dominans were examined by feeding them two different algal species (Rhodomonas salina and Dunaliella tertiolecta) differing in sterol profiles. R. salina contains predominantly brassicasterol (≅99%) and <2% cholesterol. The major sterols in D. tertiolecta are ergosterol (45-49%), 7-dehydroporiferasterol (29-31%) and fungisterol (21-26%). O. marina fed R. salina metabolized dietary brassicasterol to produce 22-dehydrocholesterol and cholesterol. O. marina fed D. tertiolecta metabolized dietary sterols to produce cholesterol, 22-dehydrocholesterol, brassicasterol and stigmasterol. G. dominans fed either R. salina or D. tertiolecta metabolized dietary sterols to make cholesterol, brassicasterol and a series of unknown sterols. When protists were fed R. salina, which contains cholesterol, the levels of cholesterol were increased to a magnitude of nearly 5- to 30-fold at the phytoplankton-heterotrophic protist interface, equivalent to a production of 172.5 ± 16.2 and 987.7 ± 377.7 ng cholesterol per mg R. salina carbon consumed by O. marina and G. dominans, respectively. When protists were fed D. tertiolecta, which contains no cholesterol, a net production of cholesterol by the protists ranged from 123.2 ± 30.6 to 871.8 ± 130.8 ng per mg algal C consumed. Cholesterol is not only the dominant sterol, but a critical precursor for many physiologically functional biochemicals in higher animal. As intermediates, these heterotrophic protists increase the amount of cholesterol at the phytoplankton-zooplankton interface available to higher trophic levels relative to zooplankton feeding on algae directly

We investigated the capability and species-specific differences in long-chain n-3 essential fatty acid (LCn-3EFA), sterol, and steroidal ketone production of 6 heterotrophic protists: 3 thecate dinoflagellates (Cryptoperidiniopsis brodyi, Pfiesteria piscicida, and Luciella masanensis), 1 athecate dinoflagellate (Amphidinium longum), 1 herbivorous ciliate (Strombidinopsis sp.), and 1 bacterivorous ciliate (Uronema sp.) by feeding them algae (Rhodomonas salina or Dunaliella tertiolecta) or bacteria. The 3 thecate species did not convert algal sterols to other usual and common sterols. Instead, they produced sterols and steroidal ketones, such as dinosterol, dinostanol, dinosterone, and dinostanone, usually found in autotrophic dinoflagellates when fed R. salina or D. tertiolecta, both of which do not contain them. The A. longum, Strombidinopsis sp., and Uronema sp. did not bioconvert dietary sterols to other sterols or produce sterols and steroidal ketones. Pfiesteria piscicida and L. masanensis grown on the LCn-3EFA-deficient alga D. tertiolecta and Uronema sp. were capable of producing the long-chain n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for organisms at higher trophic levels. The bacterial prey of Uronema sp. lacked EPA, DHA, and LCn-3EFA precursors. Although the nutritional values of the sterols and steroidal ketones produced by the 3 thecate dinoflagellates are not known, the contribution of EPA and DHA by 2 of them and the bacterivorous ciliate are noteworthy. To further understand the intermediate roles of heterotrophic protists and their essential nutrient contribution in planktonic food webs, it is necessary to examine more species, particularly those newly discovered and isolated.KEY WORDS: Heterotrophic dinoflagellates and ciliates · Sterols and steroidal ketones · Algae · Long-chain n-3 fatty acids · Eicosapentaenoic acid · Docosahexaenoic acid

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