SUMMARY ‘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.
VULCANODINIUM rugosum E. Nézan & N. Chomérat is an armored marine dinoflagellate initially discovered off the French Mediterranean coast (Nézan & Chomérat, 2011). Vulcanodinium rugosum is considered to be the producer of the pinnatoxin found, for example, in the filter-feeding fan mussel species Pinna attenuata (Rhodes et al., 2011). As such, V. rugosum has been identified as a causative organism of many cases of shellfish poisoning in regions such as Japan, Australia, New Zealand, and China (Garrett et al., 2014). Pinnatoxins are macrocyclic compounds that inhibit neural and muscular nicotinic acetylcholine receptors, which can lead to an onset of neurological symptoms and, in cases of high dosages and long-term ingestion, death (Geiger et al., 2013). To the best of our knowledge, literature suggests the Vulcanodinium genus is the sole producer of pinnatoxins, where the toxins are regularly documented within bivalve populations and are found in low amounts in both sediment and water samples taken from several marine locations around the globe (Lamas et al., 2019).Vulcanodinium rugosum is the only named species within the genus Vulcanodinium. The closest relatives, as defined by large ribosomal subunit (LSU) rRNA and internal transcribed spacer (ITS) sequence data, are members of the Peridiniales (Nézan & Chomérat, 2011), though the hypotheca and cingular plates of Vulcanodinium suggest possible relation to the order Gonyaulacales (Zeng et al., 2012). Specifically, V. rugosum is related to the peridinioid species Ensiculifera aff.
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