The diversity of marine sterols and the role of algal bio-masses; from facts to hypothesis

Differential distribution of lipids in epidermis, gastrodermis and hosted Symbiodinium in the sea anemone Anemonia viridis

Cnidarian-dinoflagellate symbiosis mainly relies on nutrient recycling, thus providing both partners with a competitive advantage in nutrient-poor waters. Essential processes related to lipid metabolism can be influenced by various factors, including hyperthermal stress. This can affect the lipid content and distribution in both partners, while contributing to symbiosis disruption and bleaching. In order to gain further insight into the role and distribution of lipids in the cnidarian metabolism, we investigated the lipid composition of the sea anemone Anemonia viridis and its photosynthetic dinoflagellate endosymbionts (Symbiodinium). We compared the lipid content and fatty acid profiles of the host cellular layers, non-symbiotic epidermal and symbiont-containing gastrodermal cells, and those of Symbiodinium, in a mass spectrometry-based assessment. Lipids were more concentrated in Symbiodinium cells, and the lipid class distribution was dominated by polar lipids in all tissues. The fatty acid distribution between host cell layers and Symbiodinium cells suggested potential lipid transfers between the partners. The lipid composition and distribution was modified during short-term hyperthermal stress, mainly in Symbiodinium cells and gastrodermis. Exposure to elevated temperature rapidly caused a decrease in polar lipid C18 unsaturated fatty acids and a strong and rapid decrease in the abundance of polar lipid fatty acids relative to sterols. These lipid indicators could therefore be used as sensitive biomarkers to assess the physiology of symbiotic cnidarians, especially the effect of thermal stress at the onset of cnidarian bleaching. Overall, the findings of this study provide some insight on key lipids that may regulate maintenance of the symbiotic interaction.

Sterols of marine microalgae Pyramimonas cf. cordata (Prasinophyta), Attheya ussurensis sp. nov. (Bacillariophyta) and a spring diatom bloom from Lake Baikal

The free sterol compositions of two marine microalgal species Pyramimonas cf. cordata (Prasinophyta), Attheya ussurensis sp. nov. (Bacillariophyta), and diatom bloom samples from Lake Baikal were determined by gas chromatography, gas chromatography-mass spectrometry and (for some sterol constituents) using nuclear magnetic resonance spectra. A variety of sterol profiles were found. The principal sterol in the prasinophyte P. cf. cordata, collected in the Sea of Japan near Vladivostok, was 24(R)-ethylcholesta-5,22E-dien-3beta-ol (poriferasterol), but not 24-ethyl-5,24(28)Z-dien-3beta-ol, as reported earlier in the related species Pyramimonas cordata. The principal sterol in the marine diatom A. ussurensis sp. nov. was identified as 24-ethylcholest-5-en-3beta-ol. The sample of diatom bloom caused by Stephanodiscus meyerii with admixtures of several other diatom species, contained cholesterol and 24-methylcholesta-5,24(28)-dien-3beta-ol as main sterol constituents.

The lipid geochemistry of a Recent sapropel and associated sediments from the Hellenic Outer Ridge, eastern Mediterranean Sea

Five sections (0-7, 29-36, 53-60, 78-85 and 104-111 cm), of a 0-2 m sediment core from the Hellenic Outer Ridge, in the eastern Mediterranean Sea, have been examined for lipids. Three of these sections were from a 73 cm thick S 1 ( ca . 6000—9000 years b.p.) sapropel layer, one from an upper ooze layer and one from a lower marl. The lipids were extracted and the major classes analysed in detail by gas chromatography and computerized gas chromatography—mass spectrometry. In all sections, the n -alkanes were dominated by C 25 —C 31 components, showing a high odd-over-even predominance, with smaller amounts of lower chain-length components. The acyclic ketone fraction consisted mainly of C 37 —C 39 di- and triunsaturated alken-2-ones and alken-3-ones. Alkanols, ranging from C 12 —C 32 with a high even-odd preponderance, were present in all sections, maximizing at n -C 22 or n -C 26 . The sapropel contained abundant phytol (up to 7000 ng g -1 dry sediment), and considerable amounts of 22 :1, 24:1 and 26 :1 n -alkenols; in the non-sapropelic sediment, phytol was only a minor com ponent, and no n -alkenols were detected. In addition to these alcohols, the sapropel also contained C 28 —C 32 1,13-, 1,14- and 1,15- diols and 15-keto-alkan-l-ols, the 30 :0 compound predominating in both series. In all sections, fatty acids were the most abundant lipid class. These were mainly C 12 —C 30 straight-chain compounds, maximizing at 16:0 with a high even—odd predominance; most were saturated, but C 16 , C 18 , C 20 , C 22 and C 24 monoenoic acids and small amounts of C 16 , C 18 , C 20 , C 22 and C 24 polyenoic acids were present. A range of branched and cyclic acids were also identified. The non-sapropelic upper and lower sediments differed from the sapropel in containing higher levels of branched acids (especially C 15 and C 17 iso- and anteiso-compounds) and C 18 monoenoic acids: these differences could be related to differing inputs, especially in terms of microbial communities. The sterol distributions of the sapropel displayed a wide range of structures (C 26 —C 31 ), totalling over sixty different components. These included both 4-methyland 4-desmethylnuclei, a variety of C 8 —C 11 side-chains, and encompassed Δ 5 , Δ 5,22 , Δ 5,24 , Δ 5,24(28) , Δ 22 , Δ 24(28) , Δ 7 and Δ 8(14) unsaturation plus a range of fully saturated stands. Major components were 4α, 23, 24-trimethyl-5α-cholest-22-en-3β-ol (dinosterol), cholest-5-en-3|I-ol (cholesterol), 24-methylcholesta-5,22-dien-3β-ol and 24-ethylcholest-5-en-3β-ol. In contrast, the non-sapropelic sediments contained very low levels of only a few sterols, chiefly cholesterol and dinosterol, probably due to input differences. In addition to sterols, the sapropel also contained small amounts of stanones and sterenes. A significant terrigenous input of lipids is evident throughout the core (especially from the n -alkane data), but the sapropel lipid composition appears to be predominantly of marine origin. Individual ‘biological marker’ lipids suggest inputs from Dinophycean and Haptophycean algae to the sapropel. Potential contributions of lipids from organisms such as foraminifera and pteropods, remains of which were observed in the sediment, are difficult to assess due to a paucity of data on the lipid compositions of such organisms. The lipids of the non-sapropelic sediments showed a much less prominent marine signal, especially in terms of the lower levels of phytol and sterols and the higher relative abundance of terrestrial n -alkanes. Two main models have been proposed to explain the formation of organic-rich sapropel facies; (i) stagnation of the water column and the establishment of anoxic conditions in bottom water and sediments, resulting in enhanced preservation of sedimentary organic matter, and (ii) increased biological production providing an increased input of organic matter to the sediments. The lipid composition strongly suggests that this sapropel received a large marine-derived input of organic matter. Since this was less evident in the overlying and underlying sediments, sapropel deposition appears to have been associated with an increased autochthonous input. The anoxic nature of the sapropel, by restricting degradation to anaerobic processes, will also have contributed to the differences in lipid composition between the sediment types. Little diagenesis of lipids in the sapropel was evident. Small amounts of sterenes and 5β(H)-stands were present, probably formed by dehydration and reduction, respectively, of precursor sterols. Diagenetic dehydration of phytol may have contributed to the presence of minor amounts of certain other isoprenoid lipids.