A Compartmental Comparison of Major Lipid Species in a Coral-Symbiodinium Endosymbiosis: Evidence that the Coral Host Regulates Lipogenesis of Its Cytosolic Lipid Bodies

Abstract

The lipid body (LB) formation in the host coral gastrodermal cell cytoplasm is a hallmark of the coral-Symbiodinium endosymbiosis, and such lipid-based entities are not found in endosymbiont-free cnidarian cells. Therefore, the elucidation of lipogenesis regulation in LBs and how it is related to the lipid metabolism of the host and endosymbiont could provide direct insight to understand the symbiosis mechanism. Herein, the lipid composition of host cells of the stony coral Euphyllia glabrescens, as well as that of their cytoplasmic LBs and in hospite Symbiodinium populations, was examined by high performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC/MS), and six major lipid species were identified: wax esters, sterol esters, triacylglycerols, cholesterols, free fatty acids, and phospholipids. Their concentrations differed significantly between host coral cells, LBs, and Symbiodinium, suggesting compartmental regulation. WE were only present in the host coral and were particularly highly concentrated in LBs. Amongst the four species of WE, the monoene R = C18:1/R = C16 was found to be LB-specific and was not present in the host gastrodermal cell cytoplasm. Furthermore, the acyl pool profiles of the individual LB lipid species were more similar, but not equal to, those of the host gastrodermal cells in which they were located, indicating partially autonomous lipid metabolism in these LBs. Nevertheless, given the overall similarity in the host gastrodermal cell and LB lipid profiles, these data suggest that a significant portion of the LB lipids may be of host coral origin. Finally, lipid profiles of the in hospite Symbiodinium populations were significantly distinct from those of the cultured Symbiodinium, potentially suggesting a host regulation effect that may be fundamental to lipid metabolism in endosymbiotic associations involving clade C Symbiodinium.

Fig 1. (A) Microscopic images of tentacle paraffin sections. Both in hospite Symbiodinium (S) and LBs (indicated by blank arrows) are visible in the gastroderm and an isolated symbiotic gastroderm cell (see the inset). (B) Cultured Symbiodinium. Scale bar, 10 μm. (C) The protein profiles and purity of LBs, host gastrodermal cell lysates (“host”), in hospite Symbiodinium (“Sym.”), and cultured Symbiodinium (“Cul.”) by SDS-PAGE and western blot examination.

Arf, ADP-ribosylation factor; Act, actin; Rbc, rubisco.

Fig 2. Concentrations of lipids in coral host gastrodermal cells (“host,” black columns), lipid bodies (“LBs”, dark gray columns), in hospite Symbiodinium (“Sym.”, light gray columns), and cultured Symbiodinium (“Cul.”, white columns).

The full names of the individual lipid species can be found in the text. Values (mean±SD, n = 4) with different superscripts within a lipid species signify significant differences in concentrations between the four fractions (Mann Whitney post-hoc U tests, p<0.05). When a column is not depicted, the respective lipid was not detected.

Fig 3. Principal components analysis of fatty acid moieties of what follows.

(A) total lipids, (B) wax esters (WE), (C) sterol esters (SE), (D) triacylglycerols (TAG), (E) free fatty acids (FFA), and (F) phospholipids (PL) in coral host gastrodermal cells (“host”, solid squares), lipid bodies (“LBs”, solid circles), in hospite Symbiodinium (“Sym.”, solid triangles), and cultured Symbiodinium (“Cul.”, hollow triangles). Values represent mean±SD (n = 4).

Fig 4. Principal components analysis (PCA) of the fatty acid moieties of what follows.

(A) cultured Symbiodinium: sterol ester (SE; solid diamonds), triacylglycerol (TAG; solid circles), free fatty acid (FFA, solid squares), and phospholipid (PL, solid triangles). (B) PCA of fatty acid moieties of host gastrodermal cells (“host”), lipid bodies (“LBs”), and in hospite Symbiodinium (“Sym.”).