Species-specific differences in long-chain n-3 essential fatty acid, sterol, and steroidal ketone production in six heterotrophic protist species

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.