Cultured fish cells metabolize octadecapentaenoic acid (all-cis delta3,6,9,12,15–18∶5) to octadecatetraenoic acid (all-cis delta6,9,12,15–18∶4) via its 2-trans intermediate (trans delta2, all-cis delta6,9,12,15–18∶5)

Octadecapentaenoic acid (all-cis Δ3,6,9,12,15-18:5; 18:5n-3) is an unusual fatty acid found in marine dinophytes, haptophytes and prasinophytes. It is not present at higher trophic levels in the marine food web but its metabolism by animals ingesting algae is unknown. Here we studied the metabolism of 18:5n-3 in cell lines derived from turbot (Scophthalmus maximus), gilthead sea bream (Sparus aurata) and Atlantic salmon (Salmo salar). Cells were incubated in the presence of approximately 1 μM [U-14C] 18:5n-3 methyl ester or [U-14C] 18:4n-3 (octadecatetraenoic acid; all-cis Δ6,9,12,15-18:4) methyl ester, both derived from the alga Isochrysis galbana grown in H14CO3, and also with 25 μM unlabelled 18:5n-3 or 18:4n-3. Cells were also incubated with 25 μM trans Δ2, all-cis Δ6,9,12,15-18:5 (2-trans 18:5n-3) produced by alkaline isomerization of 18:5n-3 chemically synthesized from docosahexaenoic acid (all-cis Δ4,7,10,13,16,19-22:6; 22:6n-3). Radio- and mass analyses of total fatty acids extracted from cells incubated with 18:5n-3 were consistent with this fatty acid being rapidly metabolized to 18:4n-3 which was then elongated and further desaturated to eicosatetraenoic acid (all-cis Δ8,11,14,17,19-20:4; 20:4n-3) and eicosapentaenoic acid (all-cis Δ5,8,11,14,17-20:5; 20:5n-3). Similar mass increases of 18:4n-3 and its elongation and further desaturation products occurred in cells incubated with 18:5n-3 or 2-trans 18:5n-3. We conclude that 18:5n-3 is readily converted biochemically to 18:4n-3 via a 2-trans 18:5n-3 intermediate generated by a Δ3,Δ2-enoyl-CoA-isomerase acting on 18:5n-3. Thus, 2-trans 18:5n-3 is implicated as a common intermediate in the β-oxidation of both 18:5n-3 and 18:4n-3

C27-C30 neohop-13(18)-enes and their saturated and aromatic derivatives in sediments: Indicators for diagenesis and water column stratification

A limited suite of C27, C29 and C30 rearranged hopenes identified as neohop-13(18)-enes have been reported in immature Recent and ancient marine/lacustrine sediments and their presence has been explained by dehydration and isomerisation of ubiquitous hopanols or hopenes. Here we investigated the source and fate of neohop-13(18)-enes in a range of Recent and ancient sediments. The analysis of δ13C values of hop-17(21)-ene and neohop-13(18)-ene in Arabian Sea surface sediments, in the Monterey Formation and in immature Cenomanian black shales show that they differ by 2–3‰, suggesting that the C30 neohop-13(18)-ene has a source different from those of the non-rearranged C30 hopenes. A new member of the family of neohop-13(18)-enes, the C28 hopene 28,30-dinorhop-13(18)-ene, was identified based on comparison of its mass spectral data with that of other members of the family of neohopenes. Its occurrence explains the formation of a series of orphan aromatic hopanoids bearing an ethyl group at C-21, known to occur in high concentrations in some organic-rich ancient sediments. Circumstantial evidence for this formation pathway is provided by identical δ13C values for the C28 28,30-dinorhop-13(18)-ene and two aromatic hopanoids in two Cretaceous black shales. Relatively abundant C28 28,30-dinorhopene and related aromatic derivatives were present in ancient sediments where the distribution of other biomarkers (i.e. isorenieratene derivatives) indicated a stratified palaeo water column. Therefore, it is suggested that these compounds are derived from bacteria dwelling at or below the chemocline and may be used as indicators of stratified water bodies in the past. 28,30-Dinorhop-13(18)-ene may also be a precursor of the unusual C28 desmethylhopane 28,30-dinorhopane found in high concentrations in anoxic sediments and a limited suite of crude oils, which is consistent with the proposal that it too ultimately derives from bacteria living at the oxic–anoxic interface

BOCHNER-MARTINELLI COHOMOLOGY CLASSES AND TANGENTIAL CAUCHY-RIEMANN COMPLEXES WITH COEFFICIENTS IN MICROFUNCTIONS

A limited suite of C27, C29 and C30 rearranged hopenes identified as neohop-13(18)-enes have been reported in immature Recent and ancient marine/lacustrine sediments and their presence has been explained by dehydration and isomerisation of ubiquitous hopanols or hopenes. Here we investigated the source and fate of neohop-13(18)-enes in a range of Recent and ancient sediments. The analysis of δ13C values of hop-17(21)-ene and neohop-13(18)-ene in Arabian Sea surface sediments, in the Monterey Formation and in immature Cenomanian black shales show that they differ by 2–3‰, suggesting that the C30 neohop-13(18)-ene has a source different from those of the non-rearranged C30 hopenes. A new member of the family of neohop-13(18)-enes, the C28 hopene 28,30-dinorhop-13(18)-ene, was identified based on comparison of its mass spectral data with that of other members of the family of neohopenes. Its occurrence explains the formation of a series of orphan aromatic hopanoids bearing an ethyl group at C-21, known to occur in high concentrations in some organic-rich ancient sediments. Circumstantial evidence for this formation pathway is provided by identical δ13C values for the C28 28,30-dinorhop-13(18)-ene and two aromatic hopanoids in two Cretaceous black shales. Relatively abundant C28 28,30-dinorhopene and related aromatic derivatives were present in ancient sediments where the distribution of other biomarkers (i.e. isorenieratene derivatives) indicated a stratified palaeo water column. Therefore, it is suggested that these compounds are derived from bacteria dwelling at or below the chemocline and may be used as indicators of stratified water bodies in the past. 28,30-Dinorhop-13(18)-ene may also be a precursor of the unusual C28 desmethylhopane 28,30-dinorhopane found in high concentrations in anoxic sediments and a limited suite of crude oils, which is consistent with the proposal that it too ultimately derives from bacteria living at the oxic–anoxic interface

Assimilation of dietary phytosterols by Pacific oyster Crassostrea gigas spat

Pacific oyster Crassostrea gigas spat were fed either the microalga Chaetoceros muelleri, Isochrysis aff. galbana (strain T-IS0) or Pavlova lutheri to investigate the effect of dietary phytosterols on the sterol composition of spat tissues. After a 6-week feeding period the sterol profile of spat tissues generally reflected that of the diet, but not all sterols were assimilated with the same efficiency. 4-Desmethyl sterols, such as cholesterol and 24-methylcholesterol, were readily incorporated into spat tissues but 4-methyl sterols, such as 4-methyl-24-ethyl-5α-cholest-22E-en-3β-ol and the unusual 3,4-dihydroxy 4-methyl sterol 24-methylpavlovol present in P. lutheri, were poorly assimilated by spat. The absolute content of every sterol decreased in spat that had been starved for 6 weeks, but the relative sterol composition was similar to the initial sterol profile. However, the relative level of cholesterol increased, indicating the important role of this sterol in spat metabolism

The effects of varying CO2 concentration on lipid composition and carbon isotope fractionation in Emiliania huxleyi.

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO2 concentrations (1.1–53.5 μmol L−1). The carbon isotope fractionation of POC (εPOC) varied by ca. 7‰ and was positively correlated with aqueous CO2 concentration [CO2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of εPOC and in the slope of the relationship of εPOC with growth rate and [CO2aq]. Also, a significantly greater offset was obtained between the δ13C of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8‰). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in 13C by 2.3‰ to 4.1‰, phytol was depleted in 13C by 1.9‰, and the major sterol 24-methylcholesta-5,22E-dien-3β-ol was depleted in 13C by 8.5‰. This large spread of δ13C values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have δ13C values differing by just a few ‰. Increases in [CO2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U37k′ used for reconstructions of paleo sea surface temperature

Early steroid sulfurisation in surface sediments of a permanebtly stratified lake (Ace Lake, Antarctica)

Surface sediments (0 25 cm) from Ace Lake (eastern Antarctica), a saline euxinic lake, were analyzed to study the early incorporation of reduced inorganic sulfur species into organic matter. The apolar fractions were shown to consist predominantly of dimeric (poly)sulfide linked C27-C29 steroids. These steroid moieties were identified by GC-MS analysis of the apolar fractions after cleavage of polysulfide linkages using MeLi and MeI and after desulfurisation. The polar fractions contained the oligomeric analogues. The S-bound steroids are most likely formed by sulfur incorporation into steroidal ketones formed from Ä5 sterols by biohydrogenation by anaerobic bacteria. The concentrations of these sulfurised steroids increased with depth in the sediment. The sulfurisation reaction is completed in 1000 3000 years. Despite a wide range of functionalised lipids present in these sediments that are potentially available for sulfurisation, there is a very strong preference for the incorporation of sulfur into steroidal compounds. A predominance of sulfurised C27 steroids contrasted with the distribution of free sterols, which showed a strong predominance of C29 sterols. This indicates that the incorporation of sulfur is biased towards C27 sterols. The results demonstrate that intermolecular sulfurisation of organic matter can occur in surface sediments at low temperatures and in the absence of light

Mid-chain hydroxy long-chain fatty acids in microalgae from the genus Nannochloropsis

Extractable and bound lipids of four species of microalgae from the genus Nannochloropsis have been examined by capillary GC-mass spectrometry. In addition to previously described unusual C28---C34 alkyl diols and n-alcohols, small quantities of compounds identified as C28-C34 monohydroxy fatty acids were detected both in free and bound form. For each carbon chain-length member of the series, a single positional isomer predominated, which was identified as the 18-isomer from characteristic mass spectral fragment ions. The position of the hydroxyl group at 18 rather than at a constant position relative to the carboxylic acid group indicates that the series results from chain-elongation (or perhaps chain-shortening) of a particular hydroxy fatty acid, rather than hydroxylation of a range of fatty acids. Furthermore, two dihydroxy fatty acids, identified as 15,16-dihydroxydotriacontanoic and 16,17-dihydroxytritriacontanoic acids, were also found in the products from acid hydrolysis of the cell residue. Possible biosynthetic correlations between these hydroxy and dihydroxy fatty acids and the long-chains diols also produced by these algae are discussed. Our data suggest that hydroxy acids may be precursors of the diols via reduction of the acid group. However, another pathway must be invoked in order to explain the formation of some diol isomers

Early steroid sulfurisation in surface sediments of a permanebtly stratified lake (Ace Lake, Antarctica)

Surface sediments (0 25 cm) from Ace Lake (eastern Antarctica), a saline euxinic lake, were analyzed to study the early incorporation of reduced inorganic sulfur species into organic matter. The apolar fractions were shown to consist predominantly of dimeric (poly)sulfide linked C27-C29 steroids. These steroid moieties were identified by GC-MS analysis of the apolar fractions after cleavage of polysulfide linkages using MeLi and MeI and after desulfurisation. The polar fractions contained the oligomeric analogues. The S-bound steroids are most likely formed by sulfur incorporation into steroidal ketones formed from Ä5 sterols by biohydrogenation by anaerobic bacteria. The concentrations of these sulfurised steroids increased with depth in the sediment. The sulfurisation reaction is completed in 1000 3000 years. Despite a wide range of functionalised lipids present in these sediments that are potentially available for sulfurisation, there is a very strong preference for the incorporation of sulfur into steroidal compounds. A predominance of sulfurised C27 steroids contrasted with the distribution of free sterols, which showed a strong predominance of C29 sterols. This indicates that the incorporation of sulfur is biased towards C27 sterols. The results demonstrate that intermolecular sulfurisation of organic matter can occur in surface sediments at low temperatures and in the absence of light