Comparative lipidomic analysis of phospholipids of hydrocorals and corals from tropical and cold-water regions

To expand our knowledge of lipid and fatty acid (FA) biosynthesis in marine cnidarians, polar lipidomes of hydrocorals were studied for the first time and then compared with those of soft corals from tropical and boreal regions. The structure and content of FAs and molecular species of ethanolamine, choline, serine, and inositol glycerophospholipids (PE, PC, PS, and PI, respectively), and ceramide aminoethylphosphonate (CAEP) in tropical hydrocorals (Millepora platyphylla, M. dichotoma) and the cold-water hydrocoral Allopora steinegeri were determined by chromatography and mass spectrometry. All soft corals and cold-water hydrocorals are characterized by a considerable amount of C20 polyunsaturated FAs (PUFAs) elongated into C22 PUFAs. In the Millepora species, the high level of 22:5n-6 and 22:6n-3 against the background of the extremely low level of C20 PUFAs may be explained by a high activity of rare Δ4 desaturase. In contrast to hydrocorals, soft corals are able to elongate and further desaturate C22 PUFAs into C24 PUFAs. Allopora and soft corals use C20 PUFAs mainly for the synthesis of PE and PC. The molecular species of PS of soft corals concentrate C24 PUFAs, while in Allopora and Millepora the PS molecules are mainly based on 22:4n-6 and 22:5n-6 acyl groups, respectively. Short acyl groups (C14) dominate the CAEP molecules of Allopora. In all the animals compared, most molecular species of PE and PC are ether lipids, but diacyl molecular species dominate PI. Hydrocorals and tropical soft corals contain diacyl and ether PS molecules, respectively, whereas cold-water soft corals contain a mixture of these PS forms. The high similarity of the alkyl/acyl compositions indicates a possible biosynthetic relationship between PS and PI in hydrocorals. The data obtained in our study will provide a resource to further investigate the lipid metabolism in marine invertebrates.


Introduction
Corals occur at depths of up to 6 km from polar to tropical waters of the World Ocean. Hard (or reef-building) corals, which have a hard exoskeleton, and soft corals are widely distributed a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 in the tropical zones [1]. Hydrocorals are also members of tropical coral reef ecosystems [2]. For example, "fire coral" Millepora platyphylla, is an important component of Indo-Pacific reefs [3]. Hard and soft corals belong to the class Anthozoa (the phylum Cnidaria). Hydrocorals resemble hard corals in appearance but belong to the class Hydrozoa [4]. In tropical regions, most shallow-water hard and soft corals species, as well as Millepora, contain endocellular symbiotic dinoflagellates (SDs, microalgae of the family Symbiodiniaceae) referred to as zooxanthellae [5]. SDs are an essential source of photosynthetic organic carbon for their host [6,7]. Many species of hard corals, soft corals, and hydrocorals inhabit the deep-sea and coldwater zones [8,9], but all these cnidarian species do not contain SDs. In warm and cold waters, corals and hydrocorals are an integral part of benthic communities with very high biodiversity [1,10].
In recent decades, a wide range of ecological and biological processes in coral communities have been investigated extensively [11][12][13][14]. Lipids and their fatty acids (FAs) are involved in the majority of biochemical and physiological processes in coral polyps [15] and, therefore, frequently applied as biochemical markers and indicators in coral research [16,17]. Up to 30% of polyps' dry biomass is comprised of lipids [18], which serve as long-term energy stores [19,20]. Total lipid level is used as an overall indicator to estimate the energetic status of coral colonies during the annual cycle [21], spawning [22,23], light regime changes [24], and environmental stresses [25].
Cnidarian total lipids are a large and diverse group of compounds consisting of non-polar storage lipid classes, such as wax esters and triglycerides, and polar structural lipid classes, for example, glycerophospholipids [18,26]. Measuring the levels of different lipid classes provides more detailed information than a determination of total lipids. A balance between storage and structural lipids is important for species-specific thermal resistance of corals [27] and for depth adaptation of cnidarians [28]. A ratio of lipid classes in coral colonies may be a critical factor for surviving a bleaching event [25] and indicates a supply from SDs [29,30]. The lipid class composition of corals and hydrocorals is related to their taxonomic position and geographic region [29].
Both storage and structural lipid classes mentioned above are esters of fatty acids (FAs). There are about 30 major FAs esterified the lipids of corals and hydrocorals [18]. Today, FAs are the most flexible markers among lipid indicators suitable for a study of coral communities. The application of acyl FAs for chemotaxonomy of corals and hydrocorals has been demonstrated [29,31,32]. The unique FA markers of SDs and host tissues of soft corals and hydrocorals have been found [29,33,34]. The composition of acyl FAs is used to identify major food sources and trophic relationships of corals [16,20,35,36], determine thermal sensitivity and stress of SDs [37,38], and confirm the interchange of lipids between coral host and their SDs [34,39,40].
To date, total lipids, lipid classes, and FAs constitute a basis for lipid studies of marine cnidarians. In fact, FAs are obtained by chemical decomposition of a mixture of thousands of lipid molecules, as well as each lipid class is a mixture of hundreds of lipid molecular species [41]. A set of all lipid molecular species present in an organism is defined as lipidome, and the quantitative description of the lipidome is one of the main tasks of lipidomics. Lipidomic analyses started in the early 2000s and have recently been applied to mollusks [42,43], sea anemones [44], hard corals [45], soft corals and their SDs [46,47], but data on lipidomes of marine invertebrates still remain very limited. In addition to common analyses of lipids, the lipidomic approach opens up new opportunities for marine ecological studies [44]. A comparison of lipid molecular species from the different taxonomic groups provides new knowledge of cnidarian lipid biochemistry [46]. Polar structural lipids, such as phosphono-and phospholipids, are a relatively conserved part of lipidome and are influenced by the environment and food sources to a much lesser extent than non-polar storage lipids [17]. In this respect, a polar part of the cnidarian lipidome seems to be most informative for studies of the chemotaxonomic characteristics of corals and hydrocorals.
To date, the polar lipidomes of three tropical soft corals (Xenia sp., Capnella sp., and Sinularia macropodia) and two cold-water soft coral (Gersemia rubiformis, G. fruticosa) [46,[48][49][50], as well as the glycerophosphocholine molecular species profile of the tropical hard coral Seriatopora caliendrum [45], have been described. Information on lipids of hydrocorals is much less than on coral lipids. The polar lipidome of any hydrocoral species was not determined. In the present study, the lipid class and FA compositions of two tropical hydrocorals (Millepora platyphylla and M. dichotoma) and the cold-water hydrocoral Allopora steinegeri were compared and the role of Δ4 desaturase in hydrocoral FA biosynthesis was discussed. The chemical structure and composition of polar lipid molecular species of these hydrocoral species were determined for the first time by high-performance liquid chromatography (HPLC) with highresolution tandem mass spectrometry (MS/MS). To highlight the features of the acyl group distribution, head-group exchange and biosynthesis of phospholipids in cnidarians, the polar lipidomes of hydrocorals were compared with those of soft corals from tropical and cold-water regions. (Alabaster, USA). Neutral lipid standards (stearyl oleate, cholesterol oleate, 1-O-hexadecyl-2,3-dihexadecanoyl-rac-glycerol, glycerol trioleate, oleic acid, cholesterol) and column silica gel (high-purity grade, 70-230 mesh) were from Sigma-Aldrich Co. (St. Louis, USA). A mixture of PUFA methyl esters No. 3 from menhaden oil was obtained from Supelco (Bellefonte, USA). 2-Amino-2-methyl-1-propanol and trifluoroacetic anhydride (Fluka, Germany) were used for the synthesis of 4,4-dimethyloxazoline (DMOX) derivatives of FAs. The precoated silica gel thin-layer chromatography (TLC) plates with a silica sol binder on aluminum foil (PTLC-AF-V) were provided by Sorbfil (Krasnodar, Russian Federation). g of wet weight). The obtained homogenate was filtered through the ash-free prewashed paper filter (5-8 μm), and the residue was repeatedly extracted (6 h, 4˚C) in a chloroform:methanol (2:1, by vol.) mixture (2 × 30 mL) and filtered. The extracts were then mixed and separated into two layers by adding 35 mL of water and 30 mL of chloroform. The lower layer was collected and evaporated under nitrogen at 40˚C with a rotary evaporator IKA RV8 equipped with a chiller IKA RC2control, a water bath IKA HB10 (IKA Werke, Staufen, Germany), and a vacuum pumping unit Vacuubrand PC 201 NT (Wertheim, Germany). The total lipids were dissolved in chloroform and stored at −80˚C.

Chemicals
Lipid class analysis. According to [52], polar lipids (PL) were isolated from the total lipids by low pressure liquid chromatography on a column with silica gel. In brief, the column was sequentially washed with chloroform and acetone, and then the PL fraction was eluted with methanol with 5% water. Lipid classes were separated by one-dimensional TLC. Each sample was placed on two TLC plates (10 cm × 10 cm). For total lipid analysis, one plate was first developed to its full length with hexane:diethyl ether:acetic acid (70:30:1 by vol.) and finally to 25% length with chloroform: MeOH:28% NH 4 OH (65:35:5 by vol.). For PL analysis, the other plate was developed with the latter solvent system. After drying in air stream, plates were sprayed with 10% H 2 SO 4 /MeOH and heated at 240˚C for 10 min. The chromatograms were scanned with an image scanner Epson Perfection 2400 PHOTO (Nagano, Japan) in a grayscale mode. Percentage of lipid content was calculated based on the band intensity using an image analysis program Sorbfil TLC Videodensitometer (Krasnodar, Russia). Units were calibrated with the use of standards for each lipid class as described previously [28].
Fatty acid analysis. Fatty acid methyl esters (FAME) were obtained by treating the lipids with 2% H 2 SO 4 /MeOH at 80˚C for 2 h in a screw-caped vial under argon, extracted with hexane and purified by preparative TLC developed in benzene. 4,4-Dimethyloxazoline (DMOX) derivatives of FAs were prepared according to [53]. A gas chromatography analysis of FAME was conducted on a GC-2010 chromatograph (Shimadzu, Kyoto, Japan) with a flame ionization detector. A SUPELCOWAX 10 (Supelco, Bellefonte, PA) capillary column (30 m × 0.25 mm i.d.) was used at 210˚C. The injector and detector temperatures were 240˚C. Helium was used as the carrier gas at a linear velocity of 30 cm/s. The identification of FAs was confirmed by gas chromatography-mass spectrometry (GC-MS) of their methyl esters and DMOX derivatives using a GCMS-2010 Ultra instrument (Shimadzu, Kyoto, Japan) (electron impact at 70 eV) and a MDN-5s (Supelco, Bellefonte, PA) capillary column (30 m × 0.25 mm ID). The carrier gas was He at 30 cm/s. The GC-MS analysis of FAME was performed at 160˚C with a 2˚C/ min ramp to 240˚C that was held for 20 min. The injector and detector temperatures were 250˚C. GC-MS of DMOX derivatives was performed at 210˚C with a 3˚C/min ramp to 270˚C that was held for 40 min. The injector and detector temperatures were 270˚C. Spectra were compared with the NIST library and FA mass spectra archive [54].
Analysis of lipid molecular species. Separation of lipids was performed on a Prominence liquid chromatograph consisted of two LC-20AD pump units, a high pressure gradient forming module, CTO-20A column oven, SIL-20A auto sampler, CBM-20A communications bus module, DGU-20A 3 degasser, and a Shim-Pack diol column (50 mm × 4.6 mm ID, 5 μm particle size) (Shimadzu, Kyoto, Japan). Lipid samples and authentic standards were eluted with a binary gradient of (A) hexane:2-propanol:AcOH:Et 3 N (82:17:1:0.08, by vol) and (B) 2-propanol:H 2 O:AcOH:Et 3 N (85:14:1:0.08, by vol). The gradient started at 5% mixture B, and its percentage increased to 80% over 25 min. This composition was maintained for 1 min, returned to 5% mixture B over 10 min, and maintained at 5% for 4 min (the total run time was 40 min). The flow rate was 0.2 mL/min. The column was kept at 40˚C. The injection volume was 10 μL. The eluent outlet was connected to a MS analyzer. Lipids were detected by a high resolution tandem (ion trap-time of flight) mass spectrometry on a Shimadzu LCMS-IT-TOF instrument (Kyoto, Japan) operating both in positive and negative modes during each analysis in electrospray ionization (ESI) conditions. The ion source temperature was 200˚C, the range of detection was m/z 100-1200, and the potential in the ion source was − 3.5 and 4.5 kV for negative and positive modes, respectively. The drying gas (N 2 ) pressure was 200 kPa. The nebulizer gas (N 2 ) flow was 1.5 L/min. MS 2 was used consistently, and MS 3 was applied for PC molecular species. For MS/MS analysis, spectra were obtained scanning from m/z 100-1200 at one cycle (MS(+/−) and MS/MS(+/−)) per 1.5 s with an isolation width of 3 Da. According to the recommendations of the manufacturer, the collision energy was 50% of the frame, Ar was the collision gas (50% of the frame), ion accumulation time was 10 ms. The instrument was calibrated using Shimadzu tuning mix. The mass accuracy was between 2 and 7 ppm. PL molecular species were identified as described earlier [48]. Quantification of individual molecular species within each PL class was carried out by calculating the peak areas for the individual extracted ion chromatograms [55].

Statistical analysis
Significance of differences in pairs of mean contents of FAs and lipid molecular species between hydrocorals was tested by one-way analysis of variance (ANOVA). The raw data were used following evaluation of the homogeneity of variances (Leven's test) and the normality of data distribution (Shapiro-Wilk test). All statistical analyses were performed using STATIS-TICA 5.1 (StatSoft, Inc., USA). A statistical probability of p < 0.01 was considered significant. Values are reported as the mean ± standard deviation.

Ethical approval
This article does not contain any studies with human participants or vertebrate animals performed by any of the authors. All the experiments on invertebrate animals were reviewed and approved by the Ethics Committee of the National Scientific Center of Marine Biology of the Far Eastern Branch of the Russian Academy of Sciences and conducted in agreement with the principles expressed in the Declaration of Helsinki.
Two 22:5 isomers were found in hydrocoral lipids. We placed special emphasis on the chemical structure of n-6 and n-3 isomers of 22:5 acid. The positions of double bonds in the isomers were confirmed by mass spectrometry of their 4,4-dimethyloxazoline (DMOX) derivatives (S1 Fig). The substantial amount of 22:5n-6 in the Millepora species distinguishes them from A. steinegeri ( Fig 1A). Moreover, 22:5n-6 was the major polyunsaturated FA (PUFA) obtained by hydrolysis of PL of Millepora (about 30% of total FAs) ( Fig 1B). The 22:5n-3 acid was also detected in Millepora but the average percentage of this isomer was ten-fold lower than that of 22:5n-6 ( Fig 1B). In contrast to Millepora, 22:5n-6 was practically absent in acyl groups of PL molecules of Allopora (Fig 1B). In PL molecules of Allopora, the major C 22 PUFA of n-6 series was 22:4n-6 ( Fig 1B). This acid was also detected in Millepora. There was no difference in the level of 22:4n-6 between Millepora and Allopora lipids (ANOVA test, p > 0.05) (Fig 1).

Molecular species of glycerophospholipids (GPL) of hydrocorals
The chemical structure and content of the molecular species of glycerophospholipids (GPL) from hydrocorals were analyzed by a high-resolution MS/MS with the special attention to the distribution of 22:4 and 22:5 acyl groups. Diacyl GPL molecular species are abbreviated as X/Y with X and Y acyl groups presumably in sn-1 and sn-2 positions, respectively. Alkylacyl GPL molecular species are abbreviated as Xe/Y with X, alkyl group, and Y, acyl group.
The   (Fig 3D). The spectra and fragmentations of the major GLP classes were discussed in detail in our previous publications [46,48,49].
The composition of GPL molecular species of the Millepora hydrocorals is shown in Table 1 In contrast to Millepora, the group of these major compounds in Allopora amounted to less than 30% of total PL.
A very uneven distribution of PUFA acyl groups among phospholipids classes was found (Tables 1-3). In all the hydrocoral species studied, 22:4 acyl groups dominated in PS and PI molecular species, whereas trace amounts of this acyl group were detected in PE and PC molecular species. PE and PC of Allopora mainly contained the molecular species with 20:4 and 20:5 acyl groups, whereas 22:5 acyl groups were abundant in the same GPL classes of the Millepora. In both hydrocoral genera, 22:6 acyl groups concentrated in the molecular species of PC. Different alkyl/alkenyl compositions of GPL were found in the tropical and cold-water hydrocorals (Tables 1-3). GPL of Millepora were characterized by a high level of C 16 and C 18 ether groups. A variety of saturated and polyunsaturated alkyl groups (from C 14 to C 22 ether groups) were represented in the GPL molecular species of Allopora. The level of molecular species with odd-chain alkyl groups was significantly higher in Allopora than in Millepora (ANOVA test, p < 0.01).

Pairwise comparison of the GPL classes of hydrocorals
As is known, the head-group exchange reaction displaces a polar group of GLP molecules but does not affect the chemical structure of their non-polar parts (acyl/alkylglycerol groups). We supposed that a high similarity between the non-polar parts of two GPL classes may be an indicator of possible head-group exchange and/or joint biosynthetic precursor for these GPL classes. Percentages of the molecular species with the same acyl/alkylglycerol groups were calculated and compared for each pair of phospholipid classes (PE, PC, PS, and PI), (S3 Fig). A comparison of the acyl/alkylglycerol group composition of the pairs of GPL classes showed a close similarity between PS and PI in all the hydrocoral species (S3 Fig). For example, in Allopora, 96.6% of total PS molecular species and 81.8% of total PI molecular species had the same acyl/alkylglycerol groups (S3 Fig). No similarity between PS and PC molecular

Comparison of GPL forms of hydrocorals and soft corals
1-O-Alkyl-2-acyl, 1-O-alkenyl-2-acyl (plasmalogen), and 1,2-diacyl forms of GPL were identified in the hydrocorals. The proportion of these lipid forms highly varied depending on the phospholipid class. The proportions of hydrocoral GPL forms were compared with those of five soft coral species described earlier (the tropical species: Sinularia macropodia, Capnella sp., and Xenia sp. [46,48]; the cold-water species: Gersemia fruticosa and G. rubiformis [49,50]) (Fig 4). In all cnidarians, the plasmalogen form (40-95% of total) dominated PE molecular species. The alkylacyl form was abundant in PC molecular species of the tropical cnidarians (80-100% of total), whereas PC of the cold-water cnidarians also contained large proportions of the plasmalogen and diacyl forms. Most of PI molecules were detected in the diacyl form. Clear species-specific differences in the form and composition of PS molecular species were observed between hydrocorals and soft corals (Fig 4). Only the diacyl form was found in PS molecules of hydrocorals. The alkylacyl form dominated PS of tropical soft corals. All three forms were detected in PS of cold-water soft corals. The PS molecular species of these three cnidarian groups also differed in the characteristic acyl groups: 22:4 were found in hydrocorals, C 24 PUFAs in tropical soft corals, and C 24 PUFAs + 20:1 in cold-water soft corals (Tables 1-3, ref. [46,49,50]).
The tropical Millepora species contained noticeable amount of 22:5n-6, which concentrated in structural GPL. We suppose that the high percentage of 22:5n-6 accompanied by the low  (Fig 5).

Distribution of PUFAs among glycerophospholipid classes
A comparison of the hydrocorals studied with the cnidarians described earlier [18] (hard corals, soft corals, and hydrocorals) confirms that their polar lipids are comprised of four major phospholipids (PE, PC, PS, and PI) and one phosphonolipid (CAEP) irrespective of a taxonomic position and geographic region. Total lipids of zooxanthellate cnidarian species are known to include lipids of SDs. Glycolipids dominate the SD total lipids [47], whereas GPL are minor lipid classes, and CAEP is absent in SDs [64]. Hence, five polar lipid classes (PE, PC, PS, PI, and CAEP) mainly characterize animal (polyp) tissue of the cnidarians [32]. The polar lipidome of corals and hydrocorals was highly species-specific. A non-uniform distribution of C 20-24 acyl groups among molecular species of different lipid classes was found. The general pathways of the PUFA distribution among GPL classes of some specimens of hydrocorals and soft corals are shown in Fig 6. In tropical and cold-water alcyonarians (soft corals) PE and PC molecular species are mainly built on C 20 PUFAs (20:4n-6 and 20:5n-3). PS molecular species is based on C 24 acyl groups (24:5n-6 and 24:6n-3), whereas both C 20 and C 24 acyl groups are represented in PI molecular species [46,48,49] (Fig 6). Similar to alcyonarians, the cold-water Allopora hydrocoral builds its PE and PC molecular species on the basis of C 20 PUFAs but synthesizes PI and PC using C 22 PUFAs because of the lack of C 24 PUFAs (Fig 6). In the unusual polar lipidome of the Millepora, all four GPL classes are mainly represented by molecules with C 22 PUFA acyl groups (Fig 6). At the same time, the Millepora kept the difference in acyl composition between GPL classes: 22:5n-6 is directed to PE and PC, while 22:4n-6 is concentrated in PS and PI.
Temperature of gel-liquid transition of lipid bilayers is known to depend on the alkyl and acyl group composition of lipid molecules. An asymmetrical distribution of GPL classes between the internal and external sides of bilayer, as well as clustering of GPL, is characteristic of biological membranes [65]. It is possible that cnidarians regulate a local fluidity and some functions of biological membranes by selecting the appropriate PUFA acyl groups for each GPL class.
Along with phospholipids, CAEP has been characterized from a wide variety of marine invertebrates [66,67]. The key precursor of CAEP is 2-aminoethylphosphonate, which can be incorporated into lipid molecules by a pathway similar to that for PE biosynthesis [68]. A lot of PUFAs provide the diversity of GPL molecular species in alcyonarians and hydrocorals, whereas a few saturated FAs were detected in their CAEP. Compared to tropical Millepora, CAEP of cold-water Allopora contains considerable amounts of molecular species with shortchain acyl groups (14:0 and 15:0). The presence of short-chain acyl groups in lipid molecules is known to reduce the temperature of gel-liquid transition of lipid bilayers. The increase in the content of the structural lipids with short-chain acyl groups may be regarded as an adaptation of the Allopora to low environmental temperatures. It is possible that the higher level of CAEP with odd-chain groups, which are characteristic for bacteria lipids [69], is related with the oddchain intermediates for CAEP synthesis obtained from an advance bacterial community or a bacterial food source of the Allopora. Instead of photosynthetic symbionts, associated bacteria were earlier suggested as a source of some FAs and lipids for the coral species without zooxanthellae [33,70].

Relationships between the polar lipid classes
The comparison between soft corals and hydrocorals showed that most molecular species of PE and PC are comprised of the alkylacyl (plasmanyl) and alkenylacyl (plasmenyl) forms. According to the ether lipid biosynthesis pathways [71], 1-O-alkyl-2-acyl-sn-glycerols are utilized as substrates by choline-and ethanolaminephosphotransferases to form plasmanylcholines and plasmanylethanolamines, respectively, which are the alkyl analogs of phosphatidylcholine and phosphatidylethanolamine. The l'-alkyl desaturase system is responsible for the biosynthesis of ethanolamine plasmalogens from alkyl lipids. Only intact 1-O-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine is known to serve as a substrate for the alkyl desaturase. Choline plasmalogens are probably derived from the ethanolamine plasmalogens.
These pathways of the ether lipid biosynthesis seem to be suitable to explain the PE and PC molecular species profiles observed in the chidarian species studied. The high similarity of the acyl groups among these two GPL classes confirms the same biosynthetic origin of most PE and PC molecular species. The selective action of the l'-alkyl desaturase explains the predominance of plasmalogens in PE molecular species compared to PC molecular species.
Another major pathway for the PE synthesis in eukaryotes is the decarboxylation of PS [65]. This reaction does not influence the non-polar parts of molecules, and, therefore, the possibility of head-group exchange between two GPL classes may be tested by a comparison of the non-polar parts of these classes. In soft corals, C 20 PUFAs dominate in the PE acyl groups, while C 24 PUFAs dominate in the PS acyl groups [46]. Hence, the decarboxylation of PS is not necessary for PE synthesis in soft corals. In hydrocorals, an asymmetrical resemblance between non-polar parts of PE and PS diacyl molecular species were detected. These data point on a possibility of head-group exchange between PE and PS in the hydrocorals.
Christie [54] emphasized that "the basic mechanism for biosynthesis of PS and PI is sometimes termed a branch point in phospholipid synthesis, as PE and PC are produced by a somewhat different route". This statement is confirmed by the sharp difference in the alkyl/acyl composition between the PE/PC and PS/PI groups found in hydrocorals and soft corals. A prokaryotic-like pathway, when PS and PI are formed biosynthetically from phosphatidic acid via the intermediate cytidine diphosphate diacylglycerol (CDP-diacylglycerol) [65], may explain the high similarity of the alkyl/acyl composition between PS and PI molecular species in hydrocorals (S1 Fig). The concentration of the C 24 PUFA acyl groups in PS and PI molecules also shows a biosynthetic relationship between these GPL classes in soft corals [46,48]. However, the pattern of this relationship in soft corals is not enough clear, because their PS is composed of alkylacyl molecules, whereas diacyl molecules dominate PI. The composition of the polar lipidomes indicates that soft corals have an additional way of PI biosynthesis, which does not include PS as an intermediate substrate.
In the present study, we described and compared the general images of the polar lipidome and the lipidomic fingerprints of several cnidarian taxa. We supposed that the basic features of the polar lipidomes of hydrocorals, reef-building corals and soft corals weakly depend on an influence of environmental factors and try to explain these features in lipid biosynthesis terms. Our study has shown that a comparative analysis of the polar lipidomes contributes to better understanding of the fatty acid biosynthetic pathways, the acyl and alkyl groups distribution among polar lipid classes, and the biosynthetic relationships between different phospholipid classes in marine invertebrates. We are sure that the lipidomic approach cans define more exactly the areas of purposeful investigations of marine lipid functions and biosynthesis.