SUMMARYGertia stigmatica is an atypical member of the Kareniaceae that has a peridinin‐containing plastid rather than a tertiary, (acyloxy)fucoxanthin‐containing, haptophyte‐derived plastid, as is found in the canonical genera Karenia, Karlodinium, and Takayama. While the origin of G. stigmatica's plastid is uncertain per its original published description, with alternative hypotheses stating that it could have originated before or after the acquisition of a haptophyte‐derived plastid, the description of the species indicates that it does possess ultrastructural and genetic features that firmly place it within the Kareniaceae. As a photosynthetic, plastid‐containing organism, G. stigmatica, like other algae, possesses two galactolipids, mono‐ and digalactosyldiacylglycerol (MGDG and DGDG, respectively), that form the basis of its plastid‐associated photosynthetic membranes. MGDG and DGDG have been extensively characterized in peridinin‐containing dinoflagellates, where it has been observed that these dinoflagellates can be segregated into two clusters. One cluster is characterized by MGDG and DGDG possessing the polyunsaturated C18/C18 fatty acids (sn‐1/sn‐2 regiochemistry) octadecapentaenoic acid (18:5(n‐3)) and octadecatetraenoic acid (18:4(n‐3)), while the second cluster possesses eicosapentaenoic acid (20:5(n‐3)) in the sn‐1 position while retaining a polyunsaturated C18 fatty acid in the sn‐2 position. By contrast, Karenia brevis and Karenia mikimotoi have been observed to be enriched in species of MGDG and DGDG, such as 18:5(n‐3)/14:0 MGDG and DGDG, uncommon to peridinin‐containing dinoflagellates. Our objective was to characterize the galactolipids of G. stigmatica to compare it to both peridinin‐containing dinoflagellates and other members of the Kareniaceae to search for insight, such as (a) remnant 14:0 fatty acid‐containing galactolipid(s), into the evolution of its plastid. Our results show that G. stigmatica possesses 20:5(n‐3)/18:5(n‐3) MGDG and DGDG as the primary galactolipids, with little evidence of those galactolipid species enriched in K. brevis and K. mikimotoi. The implications of this for the evolution of the G. stigmatica plastid are discussed.

Testudodinium testudo is a peridinin‐containing dinoflagellate recently renamed from Amphidinium testudo. While T. testudo has been shown via phylogenetic analysis of small subunit ribosomal RNA genes to reside in a clade separate from the genus Amphidinium, it does possess morphological features similar to Amphidinium sensu stricto. Previous studies of Amphidinium carterae and Amphidinium corpulentum have found the sterols to be enriched in Δ8(14) sterols, such as 4α‐methyl‐5α‐ergosta‐8(14),24(28)‐dien‐3β‐ol (amphisterol), uncommon to most other dinoflagellate taxa and thus considered possible biomarkers for the genus Amphidinium. Here, we provide an examination of the sterols of T. testudo and show they are dominated not by amphisterol, but rather by a different Δ8(14) sterol, (24R)‐4α‐methyl‐5α‐ergosta‐8(14),22‐dien‐3β‐ol (gymnodinosterol), previously thought to be a major sterol only within the Kareniaceae genera Karenia, Karlodinium, and Takayama. Also found to be present at low levels were 4α‐methyl‐5α‐ergosta‐8,14,22‐trien‐3β‐ol, a sterol previously observed in Karenia brevis to be an intermediate in the production of gymnodinosterol, and cholesterol, a sterol common to many other dinoflagellates. The presence of gymnodinosterol in T. testudo is the first report of this sterol as the sole major sterol in a dinoflagellate outside of the Kareniaceae. The implication of this chemotaxonomic relationship to the Kareniaceae is discussed.

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The dinoflagellates Amphidinium carterae and Amphidinium corpulentum have been previously characterized as having Δ8(14)‐nuclear unsaturated 4α‐methyl‐5α‐cholest‐8(14)‐en‐3β‐ol (C28:1) and 4α‐methyl‐5α‐ergosta‐8(14),24(28)‐dien‐3β‐ol (amphisterol; C29:2) as predominant sterols, where they comprise approximately 80% of the total sterol composition. These two sterols have hence been considered as possible major sterol biomarkers for the genus. Here, we have examined the sterols of four recently identified species of Amphidinium (Amphidinium fijiense, Amphidinium magnum, Amphidinium theodori, and Amphidinium tomasii) that are closely related to Amphidinium operculatum as part of what is termed the Operculatum Clade to show that each species has its sterol composition dominated by the common dinoflagellate sterol cholesterol (cholest‐5‐en‐3β‐ol; C27:1), which is found in many other dinoflagellate genera, rather than Δ8(14) sterols. While the Δ8(14) sterols 4α‐methyl‐5α‐cholest‐8(14)‐en‐3β‐ol and 4α,23,24‐trimethyl‐5α‐cholest‐8(14),22E‐dien‐3β‐ol (C30:2) were present as minor sterols along with another common dinoflagellate sterol, 4α,23,24‐trimethyl‐5α‐cholest‐22E‐en‐3β‐ol (dinosterol; C30:1), in some of these four species, amphisterol was not conclusively observed. From a chemotaxonomic perspective, while this does reinforce the genus Amphidinium's ability to produce Δ8(14) sterols, albeit here as minor sterols, these results demonstrate that caution should be used when considering Δ8(14) sterols, especially amphisterol, as Amphidinium‐specific biomarkers within these species where cholesterol is the predominant sterol.