Phased cell division and facultative mixotrophy of the marine dinoflagellate Fragilidium duplocampanaeforme and its trophic interactions with the dinoflagellates Dinophysis spp. and a ciliate Mesodinium rubrum

• Fragilidium duplocampanaeforme displayed the phased cell division.
• F. duplocampanaeforme did not display any prey preference.
• F. duplocampanaeforme was a facultative mixotroph.
• The food web interactions could vary depending on Dinophysis species included in the food chain.

A mixotrophic dinoflagellate Fragilidium duplocampanaeforme has been recently reported to specifically feed on mixotrophic dinoflagellates Dinophysis spp., which specifically feed on a mixotrophic ciliate Mesodinium rubrum (=Myrionecta rubra), which specifically feeds on a cryptophyte. Using established cultures, in this study we aimed (1) to examine the pattern of phased cell division of a Korean strain of F. duplocampanaeforme, (2) to explore its mixotrophic characteristics, and (3) to address food web interactions among top three protists within the trophic series (i.e., M. rubrum, Dinophysis spp. and F. duplocampanaeforme). During phototrophic growth, F. duplocampanaeforme displayed the phased cell division, with cell division starting 4 h after light was on and finished before the light off. Motile vegetative cell stage of F. duplocampanaeforme, which can ingest the prey, persisted over the dark period. When offered two Dinophysis species (Dinophysis acuminata and Dinophysis caudata) as prey, Korean F. duplocampanaeforme strain did not display any prey preference. The dinoflagellate F. duplocampanaeforme grew well phagotrophically in the dark if offered with plentiful prey (i.e., a facultative mixotroph). The dinoflagellate F. duplocampanaeforme grew well at growth rates of 0.21–0.26 d−1 when supplied with D. acuminata or D. caudata as prey in the dark, but its growth rates were 2.3–3.0 times slower than those when grown mixotrophically in the light. Nonetheless, the growth rates in the dark were similar to those in purely phototrophic growth. Our results from the mixed cultures of three protists demonstrated that the food web dynamics from cryptophyte through ciliate M. rubrum and dinoflagellate Dinophysis spp. to dinoflagellate F. duplocampanaeforme could be affected by predator–prey interaction as well as allelopathic interaction, depending on Dinophysis species included in the trophic series.