Comparative Lipid Profiling of the Cnidarian <em>Aiptasia pallida</em> and Its Dinoflagellate Symbiont

<div><p>Corals and other cnidarians house photosynthetic dinoflagellate symbionts within membrane-bound compartments inside gastrodermal cells. Nutritional interchanges between the partners produce carbohydrates and lipids for metabolism, growth, energy stores, and cellular structures. Although lipids play a central role in the both the energetics and the structural/morphological features of the symbiosis, previous research has primarily focused on the fatty acid and neutral lipid composition of the host and symbiont. In this study we conducted a mass spectrometry-based survey of the lipidomic changes associated with symbiosis in the sea anemone <i>Aiptasia pallida</i>, an important model system for coral symbiosis. Lipid extracts from <i>A. pallida</i> in and out of symbiosis with its symbiont <i>Symbiodinium</i> were prepared and analyzed using negative-ion electrospray ionization quadrupole time-of-flight mass spectrometry. Through this analysis we have identified, by exact mass and collision-induced dissociation mass spectrometry (MS/MS), several classes of glycerophospholipids in <i>A. pallida</i>. Several molecular species of di-acyl phosphatidylinositol and phosphatidylserine as well as 1-alkyl, 2-acyl phosphatidylethanolamine (PE) and phosphatidycholine were identified. The 1-alkyl, 2-acyl PEs are acid sensitive suggestive that they are plasmalogen PEs possessing a double bond at the 1-position of the alkyl linked chain. In addition, we identified several molecular species of phosphonosphingolipids called ceramide aminoethylphosphonates in anemone lipid extracts by the release of a characteristic negative product ion at <i>m/z</i> 124.014 during MS/MS analysis. Sulfoquinovosyldiacylglycerol (SQDG), an anionic lipid often found in photosynthetic organisms, was identified as a prominent component of <i>Symbiodinium</i> lipid extracts. A comparison of anemone lipid profiles revealed a subset of lipids that show dramatic differences in abundance when anemones are in the symbiotic state as compared to the non-symbiotic state. The data generated in this analysis will serve as a resource to further investigate the role of lipids in symbiosis between <i>Symbiodinium</i> and <i>A. pallida</i>.</p> </div

SQDG species identified in FLAp1AB lipid extracts.

*<p>Total number of carbons:total unsaturations.</p

Plasmalogen phosphatidylethanolamine species identified in <i>A. pallida</i> lipid extracts.

*<p>Total number of carbons:total unsaturations.</p>#<p>This is likely the ester linked acyl chain. The second chain, attached via an ether linkage would not be observed in the MS/MS spectra.</p

Phosphatidylinositol species identified in <i>A. pallida</i> lipid extracts.

*<p>Total number of carbons:total unsaturations.</p>#<p>The position of the acyl chain cannot be determined from this data.</p

Alkyl-phosphatidylcholines identified in <i>A. pallida</i>.

<p>Panel A shows the EIC of the [M+H]⁺ ion at <i>m/z</i> 770.6. Panel B shows the positive-ion mass spectrum from <i>m/z</i> 700 to 860 for the lipids eluting between 28.9 and 30.1 Minutes. Panel C shows the positive-ion MS/MS spectrum. The aPC predicted from this spectrum and the exact mass is shown with selected product ions indicated. Panel D shows the negative-ion MS/MS spectrum with the major product ions are indicated on the aPC structure shown below.</p

Phosphonosphingolipid species identified in <i>A. pallida</i> lipid extracts.

*<p>In the ceramide portion.</p

The symbiosis between the sea anemone <i>Aiptasia pallida</i> and the dinoflagellate <i>Symbiodinium</i>.

<p>Panel A: Symbiotic polyps of <i>Aiptasia pallida</i> demonstrate a golden brown color conferred on them by photosynthetic pigments from <i>Symbiodinium</i>. Panel B: A single tentacle from a symbiotic polyp contains thousands of individual <i>Symbiodinium</i> cells visible as small golden brown spheres within the gastrodermal tissue of the tentacle (epidermal tissue does not contain symbionts and is visible as the thin transparent layer at the periphery of the tentacle). Panel C: Four tentacles from an aposymbiotic polyp demonstrate the absence of <i>Symbiodinium</i> in the gastrodermis. Panel D: A schematic diagram of the intracellular arrangement of membranes of the host (blue) and symbiont (brown). The symbiont lives enclosed in the symbiosome, a membrane-bound compartment that arises from the host phagosome but that likely is modified upon establishment. HN = host nucleus, SN = symbiont nucleus, Chl = symbiont chloroplasts, HPM = plasma membrane of the host cell, SPM = plasma membrane of the symbiont cell, SSM = putative symbiosome membrane contributed by the symbiont shedding of external membrane, HSM = symbiosome membrane that arises from the phagosome membrane produced when the host takes up the symbiont into the host cell via phagocytosis.</p

Phosphatidylserine species identified in <i>A. pallida</i> lipid extracts.

*<p>Total number of carbons:total unsaturations.</p>#<p>The position of the acyl chain cannot be determined from this data.</p

Phosphatidylinositol identified in aposymbiotic <i>A. pallida</i> total lipid extracts.

<p>The total lipid extracts from aposymbiotic <i>A. pallida</i> were analyzed using normal phase liquid chromatography mass spectrometry (LC-MS) detecting negative ions. The extracted ion current (EIC) for <i>m/z</i> 913.5, the mass of 40∶4 PI is shown in Panel A. Panel B shows the mass spectrum from <i>m/z</i> 880 to 950 for the lipids eluting between 23.6 and 24.0 minutes. The MS/MS spectrum of <i>m/z</i> 913.5 is shown in Panel C. The structure of the predominant PI molecular species and the predicted product ions is shown below panel C.</p

Alkyl-phosphatidylcholine species identified in <i>A. pallida</i> lipid extracts.

*<p>Total number of carbons:total unsaturations.</p>#<p>The alkyl linked acyl chain is indicated by an “a”.</p