Biochemical and bioinformatics approach to the study of lipids and their biosynthetic pathways in Chromera velia and Vitrella brassicaformis

Abstract

Vitrella brassicaformis and Chromera velia are marine microalgae that photosynthetic plastids of red algal origin, are closely related to non-photosynthetic apicomplexan parasites and are classified as the phylum Chromerida and familes Vitrellaceae and Chromeraceae, respectively. To dates, only one study has been performed on the galactolipid of Chromera velia and few other studies have been based on the ultrastructural and phylogenetic studies in Chromerida. We used a combination of mass spectrometry techniques and RNA-seq data to elucidate the sterol and galactolipid composition of the Chromerida, and to identify their biosynthetic genes, respectively. The identified genes were further utilized to perform the gene phylogenies of the Chromerida with red algae, dinoflagellates and apicomplexans.

Sterols are amphipathic, ringed lipids that are synthesized mostly by eukaryotes and few prokaryotes via mevalonate and/or non-mevalonate pathways. Some of the major functions of sterols include regulating membrane dynamics, hormonal activity, and acting a secondary messengers in developmental signaling. C velia produces two primary sterols 24-ethylcholesta-5,22-dien-3β-ol, and 24-ethylcholest-5-en-3β-ol, not common to dinoflagellates, but commonly found in other groups of algae and plants. Three sterol biosynthetic genes- sterol-24C-methyltransferase (smt1), farnesyl diphosphate farnesyl transferase (fdft1), and isopentenyl diphosphate isomerase (idi1) were identified after expressed sequence tag (EST) analysis in C. velia. V. brassicaformis produces two sterols 24-ethylcholest-5-en-3β-ol and an unknown C26:2 sterol along with several biosynthetic genes related to the non-mevalonate pathway for sterol biosynthesis.

The galactolipids, MGDG and DGDG, are the major plastid structural lipids in all photosynthetic organisms that play a role in the organization of thylakoid membrane providing stability to the photosystem I and II complexes in chloroplast. V. brassicaformis produces a diversity of galactolipids that primarily contained C20/C14, C20/C16, and C20/C18 (sn-1/sn-2 regiochemistry) fatty acid components attached to the glycerol moieties of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacyl glycerol(DGDG). These forms of MGDG and DGDG are different and structurally more diverse than the previously identified C20/C20 forms that compose nearly the entirety of C. velia's MGDG and DGDG.

The sterol and galactolipid diversity between these chromerids indicates possible differences in their biosynthesis. I also also present hypotheses for how other aspects of the galactolipid biosynthesis pathways, namely fatty acid elongation and desaturation, in the Chromerida must differ prior to the addition of galactose to MGDG and DGDG.