Galactolipid composition of Gertia stigmatica (Dinophyceae), an atypical member of the Kareniaceae with a peridinin‐containing, non‐haptophyte‐derived plastid

Gertia 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.