Sterols of the ‘dinotom’ Durinskia baltica (Dinophyceae) are of dinoflagellate origin

‘Dinotoms’ are a relatively small group of dinoflagellates with aberrant tertiary plastids of diatom origin, thus differing from the majority of photosynthetic dinoflagellates which possess the carotenoid pigment peridinin and have secondary plastids of red algal origin. As part of our laboratory's continuing efforts to examine such unusual dinoflagellates in the search for clues to the evolution of their lipid compositions, we have examined the sterol composition of the dinotom Durinskia baltica. As such, we here compared its sterols to those of the previously examined dinotom, Kryptoperidinium foliaceum, more broadly to other photosynthetic, peridinin‐containing dinoflagellates, and to the diatom genus Nitzschia, which is the presumed ancestor of the D. baltica dinotom plastid. Sterols are ringed lipids, common to eukaryotes, thought to reinforce phospholipid bilayers. Many peridinin‐containing dinoflagellates have sterol compositions which are enriched by the presence of cholesterol (cholest‐5‐en‐3β‐ol) and 4α‐methyl‐substituted sterols such as dinosterol (4α,23,24‐trimethyl‐5α‐cholest‐22E‐en‐3β‐ol); this has also been found to be true for K. foliaceum despite its aberrant plastid ancestry. Our objective was to determine if this is also true for D. baltica as only the second dinotom to have its sterols characterized in detail, and to determine if there is any indication of prominent sterols which are uncommon to dinoflagellates, possibly originating from the diatom endosymbiont, as has been demonstrated previously with K. foliaceum and D. baltica chloroplast‐associated galactolipids of clear diatom origin. Our results demonstrate that like K. foliaceum, the major sterols of D. baltica are cholesterol, dinosterol, and other 4α‐methyl‐substituted sterols common to dinoflagellates. Although there were a number of minor sterols, none were found with obvious origin from the diatom endosymbiont, indicating that most originated with the dinoflagellate host itself, most likely before acquisition of the diatom tertiary plastid.