Mono- and digalactosyldiacylglycerol composition of dinoflagellates. VII. Evidence against galactolipid production and plastid presence in the heterotrophic, basal dinoflagellate, Oxyrrhis marina

The heterotrophic genus Oxyrrhis contains two species, O. marina and O. maritima, which occupy positions basal to the dinoflagellate lineage. Oxyrrhis is also related to apicomplexan parasites, which have recently been shown to have a non-photosynthetic, relic plastid referred to as the apicoplast. A recent study by Slamovits & Keeling (2008) demonstrated the presence of plastid-targeted proteins within O. marina. We hypothesized that if O. marina does indeed have plastids, then mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), which are the two most prominent plastidial membrane lipids, would be present. Therefore, we examined three isolates of O. marina to determine if they can produce MGDG and DGDG. We observed that O. marina, when fed the chlorophyte Dunaliella tertiolecta, possessed forms of MGDG and DGDG containing a C-18:3 fatty acid at the sn-1 position and most containing a C16:3-4 fatty acid at the sn-2 position; these were derived solely from the prey itself. Examination of published expressed sequence tag (EST) and transcriptome databases of O. marina for the genes encoding MGDG and DGDG synthases, two enzymes integral to the incorporation of galactose in the final forms of these lipids, failed to reveal their presence. Taken together, these results indicate that O. marina does not produce a non-photosynthetic, relictual plastid. However, the presence of plastid-targeted proteins may indicate that O. marina maintains, however briefly, plastids acquired from its prey in a form of kleptoplasty that has been observed previously in dinoflagellates.