Sterol Composition of the Peridinin-Containing Dinoflagellate Gertia stigmatica, a Member of the Kareniaceae without a Canonical Haptophyte-Derived Plastid.

Gertia stigmatica is a recently described member of the Kareniaceae with a peridinin-containing plastid rather than the aberrant, haptophyte-derived, tertiary plastid found in canonical Kareniaceae genera such as Karenia, Karlodinium, and Takayama. G. stigmatica provides a unique opportunity to compare biochemical traits, such as sterol composition, between these two fundamentally different types of Kareniaceae. To this point, canonical members of the Kareniaceae have been observed to typically produce a set of 4α-methyl-substituted, Δ-nuclear-unsaturated major sterols, such as (24R)-4α-methyl-5α-ergosta-8(14),22-dien-3β-ol (gymnodinosterol) and 27-nor-(24R)-4α-methyl-5α-ergosta-8(14),22-dien-3β-ol (brevesterol), which are very uncommon throughout other members of the class Dinophyceae. Our objective was to compare the sterols of G. stigmatica to canonical Kareniaceae to elucidate whether these same distinctive sterols are found, with our hypothesis being that they would because G. stigmatica is indeed a member of the Kareniaceae. Contrary to our hypothesis, G. stigmatica lacks gymnodinosterol and brevesterol, with its sterols instead dominated by 4-desmethyl sterols, such as cholesterol, 24-methylcholesta-5,22E-dien-3β-ol, and the unusual tri-unsaturated sterols ergosta-5,8(14),22E-trien-3β-ol and cholesta-5,8(14),22E-trien-3β-ol. No sterols were found to possess a 4α-methyl substituent or a single Δ nuclear unsaturation. Thus, G. stigmatica's sterol composition as a member of the Kareniaceae is atypical.