Galactolipid composition of the star‐shaped dinoflagellate Asterodinium gracile (Kareniaceae): presence of hexadecatetraenoic acid (16:4(n‐3))‐containing monogalactosyldiacylglycerol as the predominant galactolipid and chemotaxonomic closeness to Karenia mikimotoi as the only other known Kareniacean producer of this fatty acid

Abstract

SUMMARYAsterodinium gracile is a morphologically distinct, star‐shaped member of the Kareniaceae with, like canonical Kareniaceae, a tertiary plastid of haptophyte origin. However, A. gracile's complement of carotenoid photosynthetic pigments has been shown to be chemotaxonomically atypical in that it possesses much less fucoxanthin when compared to that of other, canonical Kareniaceae in the genera Karenia, Karlodinium, and Takayama, also with a tertiary plastid of haptophyte origin. To date, Karenia mikimotoi, Karenia papilionacea, and Karenia selliformis are the only canonical Kareniaceae that have been shown to have a chemotaxonomically atypical carotenoid pigment composition in that they possess a gyroxanthin diester‐like carotenoid not observed in other species of Karenia, Karlodinium, or Takayama (recognizing that Karenia, in general, produces fucoxanthin derivatives not observed in Karlodinium or Takayama). As a photosynthetic organism, K. mikimotoi has been shown to resemble Karenia brevis such that both species possess the chloroplast‐associated galactolipids mono‐ and digalactosyldiacylglycerol (MGDG and DGDG, respectively) enriched with octadecapentaenoic acid (18:5(n‐3)) in the sn‐1 position, and hexadecenoic acid (16:0) and tetradecanoic acid (14:0) at the sn‐2 position. However, K. mikimotoi is chemotaxonomically atypical beyond its carotenoid composition in that it possesses MGDG and DGDG with hexadecatetraenoic acid (16:4(n‐3)), which has not been observed in any other members of the Kareniaceae, in the sn‐2 position as major galactolipids. The goal of this study was to characterize the galactolipids of A. gracile with the hypothesis that they would also be atypical when compared to other canonical Kareniaceae because of A. gracile's atypical carotenoid pigment composition. To this end, we report that like K. brevis and K. mikimotoi, A. gracile produces MGDG and DGDG enriched in 18:5(n‐3) at the sn‐1 position and C14 fatty acids, such as 14:0, at the sn‐2 position, and like K. mikimotoi, it produces 18:5(n‐3)/16:4(n‐3) MGDG, yet here as its most abundant galactolipid.