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The ocean’s biological pump strongly influences atmospheric CO2 and regulates Earth’s climate. Determining the contribution of different phytoplankton groups to the biological pump over geological timescales remains an important, yet elusive, goal in biogeochemistry and organic geochemistry. Towards that end, source-specific lipid biomarkers can be used, but this approach requires the quantification of the biomarker-to-carbon ratio in different phytoplankton species, and under differing environmental conditions. We investigated responses of brassicasterol, dinosterol and C37 alkenones to three temperatures (15, 20 and 25 °C) and three N:P supply ratios (10:1, 24:1 and 63:1 mol mol−1) in three diatoms, three dinoflagellates and one coccolithophore, in laboratory experiments. Brassicasterol was produced by one diatom species, three dinoflagellates and the coccolithophore Emiliania huxleyi, while dinosterol and C37 alkenones were produced by dinoflagellates and E. huxleyi, respectively. Overall, carbon-normalized contents of lipid biomarkers varied by about a factor of three over the wide ranges of temperature and N:P supply ratios, in all species. Within the factor of three, brassicasterol was highest under the balanced N:P condition in diatoms, but under N and P deficiency in dinoflagellates. Brassicasterol in E. huxleyi was highest at lower temperatures. Dinosterol in dinoflagellates and C37 alkenones in E. huxleyi varied with temperature and N:P supply ratios, but not systematically. Compared to those in our experiments, smaller ranges are expected of N:P ratios and temperature and hence carbon-normalized biomarker contents at individual locations over time. Thus, our results imply that lipid biomarkers can be used to estimate taxon-specific carbon fluxes through time.

The effect of iron (Fe) fertilization on biological productivity and community structure in the equatorial Pacific, a region with high nutrient and low chlorophyll concentrations, has been debated for many years. Here we present lipid biomarker and geochemical proxy records in a sediment core from the central equatorial Pacific spanning from 1100 to 600 ka to investigate the response of marine phytoplankton communities to eolian Fe supply and to assess links to environmental fluctuations during the Pleistocene. We found a strong link between change in eolian Fe input and phytoplankton community structure. Our data demonstrate that an increase in eolian Fe supply across the 800 ka boundary resulted in a shift in the major primary producer from coccolithophores (alkenones) to diatoms (epi-brassicasterol) and dinoflagellates (dinosterol) at the study site. This strong link between changes in eolian Fe supply and the community structure of plankton during 800–600 ka may have been associated with a change in atmospheric and ocean conditions due to southward migration of the Intertropical Convergence Zone (ITCZ). The lipid biomarker results show that such a high supply of eolian Fe might have led to a change in the dominant phytoplankton group. However, the opal record, which is often used as a primary productivity indicator, was not consistent with the biomarker results. Therefore, the results indicate that multi-proxy approaches should be used in paleoenvironmental studies because different proxies can reveal different aspects of environmental conditions, thus contributing to a better understanding of paleoceanographic conditions.

The burial of sedimentary organic matter (SOM) in large river-influenced estuarine-coastal regions is affected by hydrodynamic sorting, diagenesis and human activities. Typically, the inner shelf region of the East China Sea is a major carbon sink for the Yangtze River-derived fine-grained sediments. Most of the previous work concentrated on studies of surface sediments or used a single-proxy in this region. In this study, two cores from the Zhejiang-Fujian Coast were analyzed using bulk (TOC, TN and δ 13 C TOC ) and molecular biomarker ( n -alkane, brassicasterol, dinosterol and glycerol dialkyl glycerol tetraether lipids) techniques to clarify the sources, spatiotemporal distribution and fate of SOM in the Yangtze River Estuary and adjacent shelf. Results indicated that the effects of diagenesis and diffusion on different sedimentary biomarkers resulted in overestimation of the relative contribution of terrestrial organic matter (%OM terr ), compared with those based on δ 13 C TOC . The amount of terrestrial plant organic matter (OM plant ) and %OM terr in sediments decreased offshore. In contrast, the amount of marine organic matter (OM marine ) increased offshore, but closer to the Yangtze River mouth, the amount of soil organic matter (OM soil ) increased. Moreover, the amounts of TOC, OM plant and OM marine biomarkers increased, but OM soil and %OM terr decreased over time in recent decades. Our study suggests that spatial OM distribution patterns in marine shelf sediments were controlled primarily by hydrodynamic sorting and nutrient concentration, and temporally diverse patterns were controlled predominantly by anthropogenic influence (e.g. dam construction and soil conservation, reclamation and agricultural plantations, anthropogenic nutrient input, dust storms, eutrophication, etc.) and climate events (e.g. interdecadal climatic jump and heavy rain events) in the geological period.

Understanding early organic matter alteration and preservation in marine carbonate-evaporite systems could improve understanding of carbon cycling and hydrocarbon source rock prediction in such environments. It is possible that organic-rich microbial mats are important contributors to preserved hydrocarbons, and to explore this we examined changes in lipid composition in such a mat from a mesohaline intertidal lagoon, eastern Qatar. The mat reaches > 5 cm thickness over a carbonate mud substrate, rich in seagrass, gastropods and other small bioclasts. Clear lamination, with distinct downward colour changes from green to pink to brown, reflects different microbial mat communities. The layers contain spheroids of probable dolomite, the precipitation of which was plausibly bacterially-mediated. Lipids [n-alkanes, fatty acids (FAs), hopanoids, isoprenoid hydrocarbons, dialkyl glycerol diethers (DAGEs), and isoprenoid (0 to 4) and branched (Ia to IIIa) GDGTs] reflect the diverse mat-building phototrophic, heterotrophic and chemoorganotrophic microorganisms, as well as some likely allochthonous material (i.e. steroids, n-alkanols, high molecular weight n-alkanes). The lipids clearly document the change in microbial community, with phytene being the predominant hydrocarbon in the phototrophic surface layer. Oxygen and pH drop significantly 0.2 cm below the mat surface, coincident with the predominance of Deltaproteobacteria and increased concentrations of archaeal and bacterial glycerol dialkyl glycerol tetraether lipids andC15/C16 and C16/C17 dialkyl glycerol ether lipids in the deeper layers. Archaeol, likely of methanogen origin, is most abundant in the deepest layer. Allochthonous inputs occur throughout the mat, including abundant steroids, especially dinosterol and dinostanol, that are possibly related to periodic algal (dinoflagellate) blooms in the Arabian Gulf. Both n-alkanes and n-alkanols appear to be derived from seagrass. Organic matter contents decrease in the deepest mat; this suggests an overall low preservation potential of OM in intertidal mesohaline-hypersaline mats of the Arabian Gulf, which in turn suggests that these are not the major source of hydrocarbons in microbially-dominated carbonate-evaporite systems.

A variety of lipid biomarkers were identified in sediments from El Junco Lake, Galapagos and their sources investigated for potential use in paleoclimate applications. A series of unusual sterols was also found, including 4α-methylgorgostanol, reported in only four species of dinoflagellates to date. We also tentatively assigned 22,23-methylene-4α-methyl-24-ethylcholest-5-en-3β-ol, the mass spectrum of which matched a sterol found in resting cysts of the dinoflagellate Peridinium umbonatum. In addition, we identified the novel sterol 4α,22,23,24-tetramethyl-5α-cholest-22E-en-3β-ol. Based on the unique sterol distribution, we hypothesize that a dinoflagellate from the genus Peridinium was the primary source of dinosterol and the novel sterols throughout the sediment record. The source specificity and abundance throughout the 3.7 m of recovered sediment make dinosterol an excellent target for hydrogen isotope analysis for use as a paleohydrological proxy in future studies. The abundant C30 and C32 1,ω20-diols and keto-ols, C29 9,10-diol and C29 1,ω9,ω10-triol likely derive from the ferns Azolla microphylla and Cyathea weatherbyana, while sources of the C30 1,ω16-diol and keto-ol, C32 1,ω18-diol and keto-ol, and the C30–C32 n-alken-1-ols are likely limited to aquatic microalgae. Due to their source specificity, these diol, triol, keto-ol, and n-alkenol biomarkers present further tools for studying past environmental and climatic change.

Three methods are presented on how to purify acetylated sterols, acetylated triterpenols and individual alkenones for hydrogen isotope analysis from marine and lacustrine sediments using reverse-phase high performance liquid chromatography (RP-HPLC). The main advantages over previous HPLC methods are reduced operator time, increased automation and the ability to simultaneously purify multiple target compounds from a sample. These gains are achieved primarily by acetylating compounds prior to purification rather than after, and also by using a fraction collector with semi-preparatory rather than analytic configuration. The effectiveness of the method is demonstrated for (i) dinosterol and taraxerol in sediment from the brackish pond Poza del Diablo, Galapagos, (ii) for di- and tri-unsaturated C37 and C38 alkenones in cultured Emiliania huxleyi, (iii) for brassicasterol, and di-, tri- and tetra-unsaturated C37 alkenones in sediment from Manito Lake, Saskatchewan, Canada, and (iv) for brassicasterol, dinosterol and di-, tri- and tetra-unsaturated C37 alkenones in sediment from the Great Salt Lake, Utah. The purification process yields 80–90% recoveries and results in no measurable hydrogen isotope alteration.

We present two new methods for purifying dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) from sediments for the purpose of hydrogen isotope analysis via gas chromatography–isotope ratio mass spectrometry (GC–IRMS). The first method uses reversed phase-high performance liquid chromatography (RP-HPLC) to purify dinosterol from structurally similar 4α-methyl sterols that co-elute on GC analysis. Dinosterol purified from sedimentary sterol/alcohol fractions using this RP-HPLC method demonstrated an average yield of 80%. A very large isotope effect was observed during RP-HPLC purification, with a 560‰ range in δD value between the first and last 5% of a cholesterol standard, which is four times that during normal phase-HPLC (NP-HPLC) purification. However, we show that dinosterol recombined from 3–4 min of eluent during RP-HPLC purification yields highly reproducible and unbiased isotope values. Due to a larger isotope effect and lower sterol recovery during RP-HPLC, NP-HPLC purification is recommended for samples that do not contain 4α-methyl sterols that co-elute with dinosterol during GC. However, for samples that contain a variety of 4α-methyl sterols, RP-HPLC is more likely to yield baseline resolution of dinosterol. In the second method presented, RP-HPLC purification is preceded by NP-HPLC purification. Using this two step procedure, baseline resolution between dinosterol and all other compounds present was achieved for all samples with an average yield of 60% and, in many cases, dinosterol was purified from all other sedimentary lipids. For samples that contain a variety of 4α-methyl sterols and sitostanol concentration > 2× that of dinosterol, the two step purification method is recommended, as neither NP-HPLC or RP-HPLC alone is likely to yield baseline resolution of dinosterol.

Abundances and isotopic compositions of sterols and of total organic carbon in surface sediments were measured at 18 stations in the Ross Sea, Antarctica. Ten sterols, 5α-cholestan-3β-ol (cholestanol), cholest-5-en-3β-ol (cholesterol), cholest-5,22E-dien-3β-ol, 24-methyl-5α-cholest-22E-en-3β-ol (brassicastanol), 24-methyl-cholest-5,22E-dien-3β-ol (brassicasterol), 24-ethyl-5α-cholestan-3β-ol (sitostanol), 24-ethyl-cholest-5-en-3β-ol (sitosterol), 24-ethyl-cholest-5,22E-dien-3β-ol (stigmasterol), 4α,23,24-trimethyl-5α-cholestan-3β-ol (dinostanol), and 4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol (dinosterol), are most widely distributed. Polytopic vector analysis of the variations in abundance resolved four sources for these compounds: an assemblage of phytoplankton characteristic of the Ross Sea Polynya, diatoms and associated consumers, zooplankton, and processes associated with heterotrophic dinoflagellates. Concentrations of stanols were strongly correlated with those of dinosterol and dinostanol. Concentrations of total organic carbon (TOC) ranged from 0.1% to 1.2% and were lowest on crests and banks and higher in basins. The mole fraction of organic carbon occurring as sterols ranged from 3 to 1100 ppm. Values were lowest at stations with anomalously old TOC (estimated from regional variations in the radiocarbon age of acid-insoluble organic carbon), thus pointing to weathering and redistribution of surface sediments as important factors in the differential degradation of sterols and TOC. The difference in first order rate constants for the degradation of these materials is ca. 0.002 yr−1. Stigmasterol and the C27 sterols were significantly enriched in 13C relative to other sterols. The abundance of 13C in TOC at four western stations was 4‰ higher than elsewhere. Abundances of 13C in all sterols at these stations is also 4‰ higher than elsewhere, indicating enrichment of 13C in the entire biological community. Independent observations of P CO 2 in surface waters, together with known relationships between isotopic fractionation and the concentration of dissolved CO2, show that the isotopic zonation in organic carbon is due entirely to dynamic drawdown of CO2 in western surface waters. At those locations, late melting of ice produces salinity gradients that inhibit mixing of CO2 from deeper waters.

The sources of organic matter in the sediments from the Ord River and estuary were assessed using stable carbon isotope values (δ13C), lipid biomarkers and distributions of chlorophylls and carotenoids as part of a larger multidisciplinary study examining the biogeochemistry of this tropical, macrotidal and large scale estuary. Two field trips were undertaken to obtain samples from both the dry (September, 2002) and wet (February, 2003) seasons. Both biomarker and stable isotope data indicate predominant contributions of organic material from the surrounding vegetation. However, the character of the terrestrial organic matter changes from the freshwater to the marine sections of the river. The sterol profiles in the freshwater sediments are relatively simple and dominated by the C29 sterols 24-ethylcholest-5-en-3β-ol (sitosterol) and 24-ethylcholesta-5,22E-dien-3β-ol (stigmasterol) with small quantities of 24-methylcholest-5-en-3β-ol (campesterol), all of which are common sterols in higher plants. In the dry season survey, triterpenoid alcohols were generally absent from the freshwater sediments, but in the wet season there was a small contribution from compounds such as α- and β-amyrin and lupeol, indicating additional higher plant input, possibly from further inland in the catchment. Within Cambridge Gulf and the Ord estuary, the waterways are fringed with mangroves which contribute substantial quantities of organic matter as shown by distinctive distributions of triterpenoid alcohols dominated by taraxerol. However, this is not the major triterpenoid in the mangrove trees studied, but seems to be considerably more stable to biodegradation and so dominates the triterpenoids found. Isotope, biomarker (fatty acids, phytol and sterols) and pigment data suggest that only a small part of the sedimentary organic matter is derived from microalgae. Contributions from diatoms (24-methylcholesta-5,22E-dien-3β-ol; fucoxanthin), green algae (Δ7-sterols), cyanobacteria (C32 hopanol; zeaxanthin) and dinoflagellates (dinosterol; peridinin) could be recognized. In this environment the high turbidity of the water column severely limits phytoplankton production, although productivity of microphytobenthos on the banks of the river can potentially be high.

We report high resolution molecular abundance data for alkenones, dinosterol, long chain diols and n -alkanols from ODP Site 658 (20°45′N, 18°35′W; 2263 m water depth). These data are compared with previously reported data for TOC, chlorins, opal and long chain n -alkanes. The contents and mass accumulation rates (MARs) of chlorins are used as total phytoplankton productivity proxies, while the contents and MARs of opal, dinosterol, alkenones and diols are used as productivity proxies for diatoms, dinoflagellates, haptophytes and eustigmatophytes/diatoms, respectively. Five high productivity intervals (HPIs) are identified: two at the glacial/interglacial transition boundaries of 132–122 and 15–8 ka, two within the warmer periods of MIS 3 (50–30 ka) and the late Holocene (4–0 ka), and one within the glacial period of MIS 4 (67–60 ka). Productivity was lowest during both the penultimate glacial maximum of MIS 6 (150–140 ka) and the last glacial maximum (22–18 ka), as revealed by both the total phytoplankton and individual phytoplankton productivity indicators. Phytoplankton community structure changed significantly over the last 160 kyr. The Termination I&II HPIs were characterized by higher opal content, possibly reflecting increased diatom contributions to total productivity. Eustigmatophyte/diatom and haptophyte contributions were higher during MIS 5, but decreased toward the Holocene. Dinosterol revealed an opposite trend, with the highest contribution in the Holocene. We ascribe such changes in phytoplankton communities to the effects brought about by the varying amounts and nature of nutrient inputs contributed regionally by eolian dusts, river influxes, ocean currents and upwelling water masses.

A study of lipid biomarkers in predominantly laminated, organic carbon-rich sediments in Lake Albano (central Italy) has been used to reconstruct changes in the aquatic system variations and changes in the lake catchment area during the Holocene. Biomarkers of aquatic organic matter such as dinosterol, isoarborinol, tetrahymanol, diplopterol, and diploptene reflect two major changes in the mixing behaviour and the biological assemblages of Lake Albano at around 6.5 and 3.8 ky BP, respectively. Biomarkers for higher land plants such as friedelin, long-chain n -alkanes, β-amyrenone and α-amyrenone display different distribution patterns, indicating changes in the lake's catchment at 6.5 and 5.0 ky BP, resulting from soil erosion and changes in the local vegetation. Analogously, the recognised changes in the water column structure and biological components are coherent with previous reconstructions based on independent parameters. The new data highlight the importance of multiproxy studies to better characterize palaeoenvironmental changes that are in turn the first step towards elucidating their trigger mechanisms (i.e. climate vs. human-induced).

Microalgae are major sources of lipids in lacustrine and marine environments. This paper provides a review of some recent advances in our knowledge of the wide variety of lipid types that have been isolated from microalgae with an emphasis on those likely to be useful biomarkers for identifying sources of organic matter in sediments. Extensive data are now available on the fatty acids in all of the major classes of microalgae and some useful characteristic features have been observed in the abundance of particular polyunsaturated fatty acids. Despite several decades of study, it is now apparent that some of the biosynthetic steps leading to the formation of these unsaturated fatty acids are still not known with certainty as shown by the occurrence of C28 polyunsaturated fatty acids in some dinoflagellates and the likely involvement of chain-shortening reactions. Considerable data have also been obtained on the sterols in microalgae, but some classes of organisms are still not well documented (e.g. cryptomonads, eustigmatophytes, xanthophytes and raphidophytes). Diatoms show a great variety of sterol compositions and no sterol appears to be either unique or representative. However, 24-methylenecholesterol in sediments is probably derived in most cases from diatoms. High contents of C25 highly branched isoprenoid (HBI) alkenes have been identified in the diatom Haslea ostrearia and both C25 and C30 HBI alkenes have been found in diatom strains thought to be Rhizosolenia setigera. Genetic and environmental factors appear to be important controls on the relative abundances of the various homologues identified. Microalgae are also suspected to be a source of long-chain saturated fatty acids having an even carbon number predominance and of long-chain alkanes with no odd over even carbon number predominance, although the available data are not conclusive. An exciting development in recent years is the identification of highly aliphatic biopolymers (algaenans) in some species of marine and freshwater green algae and eustigmatophytes. This material persists in sediments and may be a source of the alkyl chains in ancient kerogens and crude oil constituents. Algaenans do not occur in all algal species and may be absent from some classes, such as diatoms. This implies that the organic matter preserved in sediments is strongly influenced by a subset of the microalgal contributors of organic matter. Although reasonable sources have been identified for many of the lipids in sediments, there are still many gaps in our knowledge and further studies are clearly required.

Lipid class compounds were studied in the deep-sea surface sediments from the central Pacific across a latitudinal transect at 175°E (48°N-15°S) and a box core from the western tropical Pacific to discuss their utility as paleoclimatological indicators. Higher concentrations of C25C35 n-alkanes were observed in the higher latitudes in the north Pacific. Their latitudinal distributions are similar to the spatial distributions of the observed soil dust concentrations observed in the central-north Pacific atmosphere, suggesting that the sedimentary C25C35 n-alkanes are atmospherically transported from terrestrial higher plants, and can be used as an indicator of terrestrial input to ocean environments. Downcore distributions of total C25C35 n-alkanes suggest that terrestrial input doubled over the western tropical Pacific at the LGM and decreased by a factor of two at the last deglaciation period in comparison with the Holocene. The latitudinal distributions of C17C20 n-alkanes and pristane are higher in the high latitudes and equatorial region, probably reflecting the higher primary productivity in surface waters of these oceans. The downcore profiles of these compounds, as well as total organic carbon, long-chain alkenones and dinosterol suggest that biological production decreased significantly during the deglaciation period and slightly increased during the LGM compared with that of the Holocene. The downcore UK37 record suggests that the SST was almost constant over the last 20 000 y, which agrees well with the CLIMAP results and foraminifera δ18O, although it disagrees with results from coral Sr Ca ratios and δ18O records.

The molecular record of 20th Century climatic variability in laminated sediments of the Santa Barbara basin has been compiled. Distributions and abundance profiles of alkenones and selected sterols reveal annual oscillations in sea surface temperatures and phytoplankton productivity. Positive excursions in alkenone temperature profiles coincide with all major 20th Century El Nino events recording oceanic warming. Downcore abundance profiles for dinosterol, brassicasterol, cholesterol and alkenones reflect productivity trends in phytoplankton and illustrate the comparative resilience of their molecular records. Series of subsurface concentration maxima for dinosterol, a dinoflagellate-derived sterol, appear synchronous with regional historical reports of dinoflagellate blooms.

Lipid analyses of several aquatic plants growing in a freshwater lake allowed us to show that Utricularia neglecta L. (Lentibulariaceae), a world-wide distributed submerged aquatic plant, is an important potential source for sedimentary 4-methyl sterols in lacustrine environments. This plant contains, besides the 4-desmethyl sterols, high proportions of 4-methyl sterols, 4α-methyl-24-ethyl-5α-cholestan-3β-ol (citrostanol) being predominant. Other 4-methyl sterols include 4α-methyl-5α-cholestan-3β-ol, 4α,24-dimethyl-5α-cholestan-3β-ol, 4α,23,24-trimethyl-5α-cholest-22-en-3β-ol (dinosterol) and 4α,23,24-trimethylcholesta-5,22-dien-3β-ol. Steroidal ketones comprising both 4-desmethyl and 4-methyl components are also present in significant amounts. The similarity of the distribution of the 4-methyl sterols in the surface sediment with those in U. neglecta L. suggests that this plant is an important source for sedimentary 4-methyl sterols.

A study of the neutral lipids of the marine microalgae Nannochloropsis oculata Droop (2 strains), Nannochloropsis salina Hibberd and an unnamed species, all from the little-studied yellow-green algal class Eustigmatophyceae (Chromophyta), has shown that these microalgae contain unusual C30C32 1,15-alkyl diols and a monounsaturated C32 1,15-diol. Alkane diols are abundant in many marine sediments, but this is the first report of their occurrence in cultured microalgae. This results argues against the suggestion, based on the identification of the same diols in a natural bloom of the cyanobacterium Aphanizomenon flos-aquae from the Baltic Sea, that these compounds are specific cyanobacterial markers. C32 homologues were more abundant than C30 homologues in the eustigmatophytes, as in the field sample of A. flos-aquae, whereas in most sediments the reverse is found. This suggests that the biological origin for these compounds in sedements may well be microalgae other than those studied here. The diols do not occur as free lipids in the eustigmatophytes, at least under the culture conditions used, but they could be liberated from more polar lipids by acid hydrolysis. In contrast, the major form of the alkyl diols in sediments appears to be as free lipids and not esters. Acid hydrolysis also liberated C30C32 mono and diunsaturated straight-chain alcohols. The carbon number distributions of the alcohols and diols are identical, and the relative proportions of homologues are similar, suggesting that both compound classes are formed by the same biosynthetic pathway. The sterol compositions are also unusual with cholesterol comprising about 75% of the total sterols. C28 sterols were not detected, but small amounts of the C29 sterols 24-ethylcholesta-5,24 (28)E-dien-3β-ol and 24-ethylcholesta-5,24(28)Z-dien-3β-ol were found in all algae. N. salina also contained 24-ethylcholesterol.

Lipid analyses of four species of marine unicellular prymnesiophyte algae of the genus Pavlova have shown that each contains significant amounts of 4α-methyl sterols, the major one being the C 30 sterol 4α-methyl-24-ethyl-5α-cholest-22E-en-3gb-ol. This sterol has a similar structure to dinosterol, which occurs in dinoflagellates, except that the side-chain contains a 24-ethyl group rather than 23,24-dimethyl substitution. Other 4-methyl sterols include 4α,24-dimethyl-5α-cholest-22E-en-3β-ol, 4α,24-dimethyl-5α-cholestan-3β-ol and 4α-methyl-24-ethyl-5α-cholestan-3β-ol. The major 4-dimethyl sterol in each of the species is 24-ethylcholesta-5,22E-dien-3β-ol. Two species also contain significant amounts of the 5α(H)-stanol 24-ethyl-5α-cholest-22E-en-3β-ol. Unusual hydroxylated compounds found in each species were tentatively identified as dihydroxylated 4-methylsteranes. Species of Pavlova are common in marine and brackish water environments and so might be important sources of 4-methyl and 4-desmethyl stenols and stanols in some sediments.

A review of literature on the occurrence of 4-desmethyl sterols in unicellular algae indicates that few sterols are sufficiently restricted in distribution to be considered unambiguous markers for specific algal groups. Almost all of the 4-desmethyl sterols found in higher plants occur in marine algae, sometimes as major constituents, so sterol distributions do not always allow to distinguish between marine or terrigenous organic matter. Some of the problems associated with the use of sterols as markers for specific sources are highlighted by comparison of the sterol distribution in selected marine sediments and seawater samples. Although each sediment represents a very different depositional environment the sterol distributions are surprisingly similar. The sterol distributions in a saline Antarctic lake and samples of particulate matter from oligotrophic waters off the east Australian coast show that marine phytoplankton biosynthesize a wide range of sterols, including large amounts of 24-ethylcholest-5-en-3β-ol which is often used as a marker for terrigenous organic matter. Similar distributions occur in marine sediments from the upwelling area off Peru and in temperate intertidal sediments despite large differences in algal productivity between the two areas. In deeper sediments, most of the sterols are not derived from phytoplankton but from higher plants. These data indicate that inferences drawn from sterol distributions regarding sources of organic matter must be made with caution and should be supported using other lipid data. It further follows that in ancient sediments and crude oils a high proportion of C 29 steranes need not indicate that most of the organic matter was derived from vascular plants.

The fate of 4α,23,24-trimethylcholest-22-en-3β-ol (dinosterol) during passage through the gut of the copepod Calanus helgolandicus and fecal pellet sedimentation has been investigated. An artificial diet of starch granules containing 14C-labelled dinosterol was fed to copepods and compared to 14C-cholest-5-en-3β-ol as a common sterol present in copepod tissues. After feeding, the distribution of radioactivity and the detailed lipid composition of animals and their fecal pellets was determined. Cholest-5-en-3β-ol was readily assimilated by the copepod, with over 62% of the total radioactivity residing in animal tissues. In contrast, 4α,23,24-trimethylcholest-22-en-3β-ol was not assimilated and could be quantitatively recovered in fecal pellets. Additional experiments using copepod fecal pellets containing 14C-labeled dinosterol found no structural alteration during 19 days of simulated sedimentation, with all lipid radioactivity present as dinosterol. Small amounts (6.3%) of radioactivity were present as non-lipid material remaining within fecal pellets. Results from these experiments suggest that dinosterol (and by analogy other ring saturated sterols and stanols) is resistant to crustacean and microbial alterations over the relatively short (20 day) periods typical of sedimentation times for large particles in open ocean areas.

The extractable lipid composition of four layers of a microbial mat from Hao, French Polynesia, shows differences that reflect both the imprint of their microbial populations and the selective diagenetic transformations of specific microbial compounds. The uppermost layers, principally composed of cyanobacteria and other bacteria, contain lipids typical of such microbes, namely n-heptadecane, heptadecene, hexadecanoic acid, and various sterols. With increasing depth the selective degradation of lower n-alkane homologues occurs; n-alkenes also show enhanced degradation. The predominant sterols in the bottom horizon of the mat are C2, and C30 components, including dinosterol and other 4-methylsterols presumably derived from dinofiagellates. In addition, there is an increase in the proportion of stanols with depth, perhaps arising from preferential degradation of δ5-stenols.