Marine and terrigenous lipids in coastal sediments from the Peru upwelling region at 15°S: Sterols and triterpene alcohols

The distributions of free 4-desmethyl sterols in sediments from the Peru coastal zone at 15°S have been determined. Major free sterols in the surface sediments include cholesterol, which is mainly derived from zooplankton, and two C₂₈ sterols: 24-methylcholesta-5,24(28)-dien-3β-ol and 24-methylcholesta-5,22E-dien-3β-ol both of which are derived fro diatoms. Their concentrations decrease by almost an order of magnitude in the top 20 cm of sediment depth, indicating that free sterols are rapidly degraded in this sedimentary environment. Lipids from higher plants were also detected: long chain fatty acids and alcohols and various triterpenoid alcohols, including taraxerol, lupeol and α- and β-amyrin. The concentrations of most terrigenous lipids varied by less than a factor of 3 over the same depth, and these changes were not correlated with changes in the concentrations of total organic carbon. Below 3 cm, lipids from higher plants predominated in the extractable lipid distributions due to the more rapid degradation of marine lipids. We postulate that there are significant marine sources of the higher plant sterols 24-ethylcholesterol, 24-ethylcholesta-5,22E-dien-3β-ol and 24-methylcholesterol in these sediments. A high proportion of many of the terrigenous lipids in these sediments are probably transported into the coastal zone by rivers, rather than from the atmosphere, and then redistributed by bottom currents.     

青海湖沉积物中的甾醇及其演化

Ｃ_２７，Ｃ_２８和Ｃ_２９甾醇（△５.２２，△２２，△５和△５.２４（２８）和５α－甾烷醇）在青海湖三孔近代沉积物（ＱＨ，ＱＥ和ＱＯ）中被检出。其中以Ｃ_２９甾醇占优势。在ＱＥ和ＱＧ孔沉积物中还检出了低浓度的三个Ｃ_３０甾醇（４α，２３，２４－三甲基－５α－胆甾－２２Ｅ－烯醇（甲藻甾醇），４α－甲基－２４－乙基－５α－胆甾－２２Ｅ－烯醇和４α－甲基－２４－乙基－５α－胆甾烷醇］。在ＱＨ和ＱＧ孔上部层段沉积物中，Ｃ_２７，Ｃ_２９.饱和的和不饱和的甾醇的碳数分布的平行性，甾烷醇/△５－甾醇及△２２－/△５.２２－甾醇的比值随沉积物埋深而增加提供了甾酸氢化作用的间接证据。青海湖沉积物中的甾醇主要来自湖区丰富的高等植物和湖中的藻类等。在ＱＧ孔８０～１００ｃｍ和１２０～１４０ｃｍ层段沉积物中，５ａ－胆甾烷醇浓度迅速增加可能反映特殊的生物（介形类）对该二层段沉积物中高浓度的胆甾烷醇的贡献。     

Sterols in interstitial waters of marine sediments

In order to attempt to elucidate the nature of biogeochemical processes occurring at the water-sediment interface, sterols have been analysed in near bottom sea and interstitial waters collected in the eastern and western intertropical Atlantic ocean. Free and esterified sterol concentrations range from 0.2 to 82 μg l^?1 and are much higher than those found in overlying sea water, which range from 0.2 to 1.7 μg l^?1 for the dissolved fraction and from 0.01 to 0.07 μg l^?1 for the particulate fraction. Cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are the dominant sterols in sea and interstitial waters. The variability encountered for the relative importance of minor sterols such as 24-methylcholesta-5,24(28)-dien-3β-ol and stanols, 5α-cholest-22(E)-en-3β-ol, 5α-cholestan-3β-ol and 24-ethyl-5α-cholestan-3β-ol in interstitial water and their variation with depth is discussed in terms of diversity of inputs and bacterial activity. For sediments cored off the Mauritanian coast, a productive area characterized by an intense upwelling, the chemical signatures observed in interstitial water through stanol/stenol ratios occur at levels of very high heterotrophic aerobic bacterial biomass estimations. The study of the sterol composition of interstitial water could constitute a valuable tool in appreciating the intensity of chemical and biological processes occurring in the first few metres of recent marine sediments.     

Occurrence and biomarker potential of 23-methyl steroids in diatoms and sediments

23-Methyl sterols have been reported to be synthesized by a few marine algae, but unambiguous identification of 23-methyl steroids in sediments and petroleum is lacking. We report the presence of 23-methylcholesta-5,22E-dien-3β-ol in 14 out of 106 diatom cultures, thereby showing that diatoms, together with dinoflagellates, may be an important environmental source for such steroids. Synthesis of authentic 23-methylcholestanes showed that their mass spectra are identical to those of 24-methylcholestanes, but that they elute earlier on apolar stationary phases during gas chromatography (GC) analysis. Co-injection of the authentic standards with sediment extracts revealed the presence of these compounds in the Skole unit of the Oligocene Menelite Formation, the Miocene Monterey Formation and the Messinian Vena del Gesso Formation. In addition, we tentatively identified 23,24-dimethyl-27-norcholestanes in some of these sediments. Molecular clock calculations suggest that diatoms are a possible source for 23-methyl steroids in sediments and petroleum from the late Jurassic onwards.     

Sources and Distribution of Particulate Organic Matter of a Tropical Estuarine-Lagoon System from NE Brazil as Indicated by Lipid Biomarkers

In the present work, we evaluated the origin of organic matter in the tropical estuarine-lagoon system of Mundaú–Manguaba, NE Brazil, by considering the bulk (organic carbon and chlorophyll-a) and lipidic (n-alcohols and sterols) composition of suspended particles. Water samples were collected in August 2006 from 24 stations covering the salinity gradient from the rivers down to the sea outlet. Chlorophyll-a (Chl-a) varied from 22.7 to 134.1 μg L⁻¹ in the lagoons, indicating eutrophic to hypertrophic conditions at the time of sampling. The high correlation between Chl-a and phytol together with the molar C:N ratio indicated the presence of fresh and recently produced autochthonous particulate organic matter throughout the system, except for the river samples. The elevated concentrations of short-chain n-alcohols and phytosterols, mainly 24-methylcholesta-5,24(28)-dien-3β-ol, also corroborated the predominance of autochthonous organic matter in the lagoons but were generated by distinct sources: cianobacteria in the freshwater Manguaba lagoon and diatoms in the brackish Mundaú lagoon compartments. Input of terrestrial organic matter was only detected in the rivers themselves or at the upper river–lagoon interfaces. Coprostanol indicated contamination by sewage in Mundaú lagoon and in some rivers, but at lower levels when compared to other Brazilian coastal lagoons and estuaries.     

δC and δD identification of sources of lipid biomarkers in sediments of Lake Haruna (Japan)

Organic materials in lacustrine sediments are from multiple terrestrial and aquatic sources. In this study, carbon (δ¹³C) and hydrogen isotopic compositions (δD) of phytol, various sterols, and major n-fatty acids in sediments at Lake Haruna, Japan, were determined in their solvent-extractable (free) and saponification-released forms (bound). The δ¹³C-δD distributions of these lipid molecules in sediments are compared with those of terrestrial C3 and C4 plants, aquatic C3 plants, and plankton to evaluate their relative contributions. δ¹³C-δD of free phytol in sediments is very close to that of phytol in plankton samples, whereas δ¹³C-δD of bound phytol in sediments is on a mixing line between terrestrial C3 plant and plankton material. Unlike phytol, no significant δ¹³C-δD difference between free and bound forms was found in sterols and n-fatty acids. δ¹³C-δD values of algal sterols such as 24-methylcholesta-5,22-dien-3β-ol in sediments are close to those of plankton, whereas δ¹³C-δD of multiple-source sterols such as 24-ethylcholest-5-en-3β-ol and of major n-fatty acids such as n-hexadecanoic acid in sediments are between those of terrestrial C3 plants and plankton samples. Thus, δ¹³C-δD distributions clearly indicate the specific source contributions of biomarkers preserved in a lacustrine environment. Free phytol and algal sterols can be attributed to phytoplankton, and bound phytol, multiple source sterols, and major n-fatty acids are contributed by both terrestrial C3 plants and phytoplankton.     

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

Sources of neutral lipids in a temperate intertidal sediment

The compositions of sterol, alcohol and ketone fractions from an oxic intertidal surface sediment from Corner Inlet. Victoria, Australia, have been examined by capillary gas chromatographymass spectrometry and related to the lipids of diatoms cultured from the sediment and to lipids of the sea-grass Zostera muelleri. Of the more than thirty sterols in the sediment most appear to derive from diatoms, including the major sterol 24-methylcholesta-5.22E-dien-3β-ol. Small amounts of 24-ethylcholesterol probably derive from Zostera, with a minor diatom contribution. 5α-stanols, both fully saturated and with a side-chain double bond, represented ca. 14% of the total sterols: a significant proportion of these are probably derived from marine invertebrates. These organisms also contribute C₂₆ sterols. most of the cholesterol and possibly small amounts of Δ-sterols. The data suggest that in situ biohydrogenation of stenols was not a major process in sterol transformation in the surface oxic sediment. Alcohols ranged from C₁₂ to C₃₀ and showed an unusual abundance of unsaturated components with Δ⁹-16:1 being the major constituent. The long-chain alcohols probably originate largely from Zostera but the shorter-chain. C₁₂-C₂₀. alcohols are at least partly derived from wax esters of unknown origin. Diatoms do not contribute significant amounts of alcohols. Ketones were not major constituents of the sediment and consisted mainly of 6.10.14-trimethylpentadecan-2-one and a series of n-alkan-2-ones ranging from C₁₆ to C₃₃. The latter showed a similar distribution to that of the n-alkanes but the correspondence was not sufficient to substantiate a product-precursor relationship. Very long-chain C₃- C₃₉ unsaturated methyl and ethyl ketones, which may originate from the marine unicellular alga Emiliania huxleyi. were present in low concentration.     

A biogeochemical study of sediments from the eutrophic Lake Lugano and the oligotrophic Lake Brienz,Switzerland

The biomarker composition and stable isotope (C, O) ratio values of organic matter (OM) and carbonate from sediment cores from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both in Switzerland) are compared, in order to obtain information about OM sources and transformation processes. Eutrophic conditions at Lake Lugano are reflected in elevated total organic carbon (TOC) content and hydrogen index (HI) values, as well as higher lipid concentrations. Parallel down core trends in δ¹³C values of TOC and calcite in the Lake Lugano sediments reflect bioproductivity cycles. Variations in δ¹⁸O values of calcite are consistent with changes in mean summer temperature over the time interval covered by the core. In contrast, such a correlation does not exist for Lake Brienz and there the stable isotope composition of calcite reflects its allochthonous origin. In the sediments of both lakes, fatty acid (FA) distributions and the composition of n-alkanols and n-alkanes indicate highly variable proportions of autochthonous OM sources (algae, zooplankton, bacteria) and OM from land plants. Enhanced in situ microbial synthesis during sediment deposition in Lake Lugano is suggested by the higher TOC-normalised concentrations of branched chain FAs (C₁₅–C₁₇), hopanoic acids and triterpenoid alcohols (i.e. tetrahymanol, diplopterol). Variations in the concentrations of cholesterol are related to contributions from zooplankton and/or green algae, while sitosterol concentrations reflect the input of vascular plants. Periods of increased input of OM from diatoms are evidenced by high 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol) and/or highly branched isoprenoid (HBI) alkenes concentrations. High relative concentrations of diplopterol in Lake Lugano sediments are consistent with the predominance of cyanobacteria commonly observed in eutrophic lakes. The presence of archeol and hydroxyarcheol in very low concentrations in the Lugano sediments argues for the activity of methanogens and/or anaerobic methanotrophs.Differences in OM degradation processes are reflected in higher chlorin index values in the Brienz sediments but higher saturated vs. unsaturated n-FAs in the core from Lugano. Higher concentrations of branched chain FAs and 16:1ω7 n-FA, as well as enhanced 18:1ω7/18:1ω9 n-FA, are consistent with enhanced bacterial biomass in the Lugano water column or sediments. The preservation of phytol seems to be enhanced in sediments with a high relative contribution of land plant OM. Major factors affecting OM accumulation in the lakes are differences in OM sources (i.e. terrestrial OM vs. autochthonous production), extent of bacterial activity and most likely oxygen availability in the water column.     

Vertical transport of steroid alcohols and ketones measured in a sediment trap experiment in the equatorial Atlantic Ocean

The vertical flux and free steroid alcohol (sterol) and ketone composition of particulate material was determined using sediment traps deployed at 389, 988, 3755 and 5068 m at a station in the equatorial North Atlantic, PARFLUX E. Cholest-5-en-3β-ol (cholesterol) was found to be the dominant sterol in all the traps. This compound had a maximum flux at 988 m, accounting for more than 90% of the sterols at this depth. Inputs from mesopelagic Zooplankton populations living in or migrating to depths between the 389 and 988 m traps appear to be responsible for this distribution. The deeper two traps exhibited an increased flux of phytosterols relative to cholesterol, probably due to (a) the incorporation of labile phytoplankton remains in fecal pellets and rapid transport into the deep sea and (b) differential dissolution of heterogeneous large particles. A maximum of 5–22% of the sterols produced in the euphotic zone were present in the 389 m trap. This value drops to less than 1% for the 5068 m trap, 200 m above the sediment surface.In general steroid ketone fluxes gradually decreased with depth. Δ4-Stenones were found in greater abundance than their saturated counterparts. Cholest-4-en-3-one was the major steroid ketone detected in all the traps. A five-fold increase with depth in the cholest-4-en-3-one to cholesterol ratio is most likely due to microbial oxidation of sterols to steroid ketones, or higher Δ4-stenone inputs relative to sterols from organisms.     

Sterols as source indicators of organic materials in sediments

The sterol contents of plankton and sediment samples have been determined. Cholesterol, 22-dehydrocholesterol, brassicasterol, campesterol, 24-methylenecholesterol, stigmasterol, and β-sitosterol are the major sterols in these samples. Systematic variations have been observed in the compositions of the sterols from bay and Gulf of Mexico plankton and from sediments collected in the mouth of the Aransas River, Capano Bay, Aransas Bay, Corpus Christi Bay and the Gulf near Port Aransas. The concentration of cholesterol increases as the concentration of β-sitosterol decreases in the sample sequence of river inlet sediments-bay sediments-Gulf sediments-bay and Gulf plankton. Calculations based on either the cholesterol or the β-sitosterol contents of the sterol fractions of the fresh water sediments and the Gulf plankton and sediments suggest that 34% of the sterols in the Gulf sediments are derived from terrigenous sources.     

Stenols and stanols in the oxic and anoxic waters of the Black Sea

Water samples collected from a slope station and two deep stations in the western basin of the Black Sea were analyzed for stenols and stanols by glass capillary gas chromatography. These results were used in conjuction with hydrographic, particulate organic carbon, and chlorophyll a data to better understand sterol sources and their transport and transformation mechanisms in anoxic basins.The total free sterol concentrations found in the surface waters were 450–500 ng/l dropping rapidly to values well below 100 ng/l at depths below the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. In the upper 200 m of the water column a strong association of sterols with particulate matter is suggested. Structural elucidation by a gas chromatograph-mass spectrometer-computer system revealed the presence of at least sixteen different stenols and stanols in the surface waters of the Black Sea. Cholesterol, 24-methylenecholesterol and 24-methylcholesta-5,22-dien-3β-ol were the major sterols in the surface waters. Cholesterol and 24-ethylcholesterol both exhibited a subsurface maximum at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. In the anoxic deep waters (200–2000 m) only cholesterol and 24-ethylcholesterol were found. Two stenols were found that have not been reported in seawater: a C₂₆ stenol with a saturated C₇H₁₅ side chain (presumably 24-norcholesterol) and 24-ketocholesterol. At least six 5α-stanols could be identified in the surface samples, each of them comprising about 10–20% of the concentration of the corresponding Δ5-stenol. From these comparatively high surface values the stanol concentrations drop rapidly to values near zero at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. Except for very low concentrations of 5α-cholestanol (< 4ng/l) no other stanols could be detected in the anoxic zone.From this data it appears that no detectable stenol → stanol conversion is occurring at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface or in the deep anoxic waters of the Black Sea.     

Biogeochemistry of particulate organic matter from lakes of different trophic levels in Switzerland

Biomarker compositions of particulate organic matter (POM) from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both Switzerland) are compared, in order to obtain information about organic matter (OM) production and transformation processes in relation to water column stratification. Eutrophic conditions in Lake Lugano are reflected by enhanced alkalinity, elevated total organic carbon (TOC) and chlorin contents compared with Lake Brienz. Lower δ¹³C values of dissolved inorganic carbon (DIC) in Lake Lugano reflect enhanced OM respiration in the water column.Differences in OM dynamics between both lakes, as well as seasonal variations, are evidenced by TOC-normalised concentration profiles of total fatty acids (FAs) and total neutrals. In Lake Brienz, the results reflect the relative contributions of primary productivity and refractory, allochthonous OM to POM, governed by particle load and interflows due to density stratification. The depth trends at Lake Lugano are a result of high primary productivity, water column stratification and associated particle load in the upper layers, as well as microbially induced degradation close to the chemocline and greater preservation under anoxic conditions. Minor differences exist with regard to the OM composition. In both lakes, FA distributions and the composition of n-alkanols indicate a predominant autochthonous OM source (algae, zooplankton, bacteria). Inputs of OM from diatoms are reflected in highly-branched isoprenoid (HBI) alkenes, 16:1 n-FAs and 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol). Differences in relative proportions of n-C₁₆ vs. n-C₁₈ FAs and alkanols, respectively, as well as in the percentages of C₂₇, C₂₈ and C₂₉ sterols relative to the sum of sterols are related to differences in the abundances of chrysophytes, diatoms and green algae within the euphotic zone of both lakes as well as in bacterial activity and soil in-wash. High relative proportions of cholesterol in the autumn samples, most pronounced at Lake Lugano, were attributed to an increased input from zooplankton grazing in the water column.Differences in OM degradation processes are reflected in slightly higher chlorin index values and higher relative proportions of saturated vs. unsaturated n-FAs in Lake Lugano. Higher contents of branched chain FAs, 16:1ω7 n-FA, and enhanced 18:1ω7/18:1ω9 n-FA ratios suggest enhanced bacterial biomass in the water column of Lake Lugano close to the chemocline. Increasing proportions of saturated n-FAs and n-alkanols with increasing water depth, most distinct in the autumn for both lakes, argue for intensified bacterial activity and degradation of OM during autumn. High relative contents of sterols and low n-alkanol concentrations in POM close to the chemocline at Lake Lugano during spring are interpreted to reflect higher primary productivity in the photic zone, OM export to the deeper parts and enhanced degradation rates of more labile constituents (i.e. C₁₃–C₂₀ n-alkanols), as compared to Lake Brienz.     

A study of oxic/anoxic effects on degradation of sterols at the simulated sediment–water interface of coastal sediments

Laboratory incubation experiments were conducted to determine the behavior of sterols at the sediment–water interface in oxic and anoxic sediments. Both plankton and 4- -cholesterol were used as tracers. Cholesterol was rapidly degraded at the sediment–water interface: 55% of -cholesterol was lost from sediments under anoxic conditions and 78% under oxic conditions over three and one-half months. About 3% of initially-added free radiolabel was incorporated into a bound pool that was released only by saponification of solvent-extracted sediment. Less than 1% of initially-added radiolabel remained in pore waters after three and one-half months. Rate constants for degradation of cholesterol in oxic and anoxic surficial sediments were estimated by tracking variations in radioactivity and sterol concentration as a function of time. We discuss our results in terms of factors affecting sterol degradation in coastal marine sediments, including molecular structure, sediment matrix effect, and redox conditions.     

Sterols of a contemporary lacustrine sediment

C₂₆, C₂₇, C₂₈ and C₂₉ sterols, including Δ⁵-, Δ²²- and Δ^5,22-sterols and both 5α- and 5β-stanols, have been identified in the contemporary lacustrine sediment of Rostherne mere (Cheshire, England). Amounts of total sterols decrease from ca. 400 ppm extracted sediment dry weight for the 0–7 cm sediment section to ca. 40 ppm for the corresponding 18–30 cm section. 5α-stanols are of far greater abundance than the 5α-stanol isomers. The carbon number distributions of unsaturated and saturated sterols and the increase in stanol: Δ⁵-sterol ratio with sediment depth provide indirect evidence for the operation of a sterol hydrogenation process. C₂₉ sterols preponderate at lower sediment depths, suggesting a predominantly higher plant input, whereas C²⁷ sterols are more abundant in the surface sediment.     

Molecular biomarker evidence of origins and transport of organic matter in sediments of the Pearl River estuary and adjacent South China Sea

Surface sediments from the subtropical Pearl River estuary and adjacent South China Sea were investigated by molecular organic geochemical methods to determine the composition, distribution and origin of extractable lipids (n-alkanes, n-alkanols and sterols). The absolute and organic C normalized concentrations of total alkane, n-alkanol, and sterol ranged from 0.16 to 2.67 μg g⁻¹ and 0.9 to 12.3 μg g⁻¹ OC, 24.4 to 427.3 ng g⁻¹ and 63.2 to 1966.7 ng g⁻¹ OC, and 9.0 to 493.5 ng g⁻¹ and 58.4 to 1042.4 ng g⁻¹ OC, respectively. The spatial distributions of these biomarkers indicated that terrestrial-derived molecular biomarkers such as long-chain n-alkanes, n-alkanols and plant-derived sterols were higher at the river mouth and along the coastline, suggesting that a higher proportion of terrestrial particulate organic matter was deposited there. Relatively lower amounts of marine-derived biomarkers such as short-chain n-alkanes, algal sterols at the river mouth reflected the lower primary productivity due to high turbidity. The spatial patterns of these biomarkers were partially related to the estuarine processes and conditions, evidencing an increased terrestrial signal from the Pearl River mouth to the inner estuary, and enhanced marine conditions further offshore.     

Modeling diagnosis of suspended sediment transport in tidal estuarine system

A three-dimensional, time-dependent hydrodynamic and suspended sediment transport model was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in northern Taiwan. The model was validated with observed time-series salinity in 2001, and with salinity and suspended sediment distributions in 2002. The predicted results quantitatively agreed with the measured data. A local turbidity maximum was found in the bottom water of the Kuan-Du station. The validated model then was conducted with no salinity gradient, no sediment supply from the sediment bed, wind stress, and different freshwater discharges from upstream boundaries to comprehend the influences on suspended sediment dynamics in the Danshuei River estuarine system. The results reveal that concentrations of the turbidity maximum simulated without salinity gradient are higher than those of the turbidity maximum simulated with salinity gradient at the Kuan-Du station. Without bottom resuspension process, the estuarine turbidity maximum zone at the Kuan-Du station vanishes. This suggests that bottom sediment resuspension is a very important sediment source to the formation of estuarine turbidity maximum. The wind stress with northeast and southwest directions may contribute to decrease the suspended sediment concentration. When the freshwater discharges increase at the upstream boundaries, the limits of salt intrusion pushes downriver toward river mouth. Suspended sediment concentrations increase at the upriver reaches in the Danshuei River to Tahan Stream, while decrease at Kuan-Du station.     

Extractable and bound lipid components in a freshwater sediment

Lipids directly extracted by organic solvents from sediment of a small eutrophic lake (Crose Mere) comprise free hydrocarbons, alcohols, sterols and monocarboxylic acids together with an esterified component yielding additional amounts of alcohols, sterols and monocarboxylic acids on saponification. The carbon-number distributions of the free lipid compound classes indicate a major contribution from allochthonous sources whereas the distributions within compound classes released on saponification are attributable to autochthonous sources. The more advanced state of decomposition of terrestrial biota relative to aquatic biota and the composition of the source organisms are postulated to be factors leading to the differences in distribution of corresponding free and esterified lipid classes. Acid hydrolysis of solvent-extracted sediment liberates additional lipid constituents belonging to the above compound classes. Of these bound lipids, the n-alkanes and branched/cyclic alkanoic acids show a more significant bacterial contribution than do the corresponding components obtained by solvent extraction alone.     

Application of allochthonous organic carbon and phosphorus forms in the interpretation of past environmental conditions

Organic matter in sediments, for instance, carbon, nitrogen and phosphorus, can be used to reconstruct the paleoecological and pollution history of lakes and their catchment basins. In this paper, the contents of allochthonous organic carbon (allochthonous OC) and autochthonous organic carbon (autochthonous OC) in sediment cores taken from Wuliangsuhai Lake and Daihai Lake in northern China are quantified by using a binary model, and phosphorus forms in the sediment cores from the two lakes are extracted by sequential extraction techniques. The results indicate that the palaeoenvironment and paleoclimate of Daihai Lake and its catchment basin in the recent 250 years can be well reconstructed based on the content of allochthonous OC. The climate was relatively humid and warm in the period of 1865–2005, while relatively dry and cold in the period of 1765–1865. The sedimentary information of allochthonous OC in the 22–42-cm portion of the sediment cores in Daihai Lake corresponds to the final cold fluctuation of the Little Ice Age that occurred since the Middle Holocene. The difference of phosphorus forms in the sediment cores between the two lakes indicates that phosphorus input to the lakes and the correlation between phosphorus forms and distribution and the changes of environment are influenced by the eutrophication mechanisms and environmental conditions of the two lakes.     

Use of microbial enrichments for the study of the anaerobic degradation of cholesterol

To demonstrate the potential of model microbial assemblages in studies of the biogeochemistry of complex organic molecules, anaerobic microbial populations capable of degrading cholesterol (cholest-5-en-3β-ol) have been enriched from marine sediment sources. The bacterial enrichments actively mineralized the C-4 nucleus of the cholesterol ring system to carbon dioxide when nitrate was present as the terminal electron acceptor. Nitrite was found as an intermediate in the reduction of nitrate, indicating the presence of denitrifying bacteria in the enrichments. When sulfate was supplied as the sole electron acceptor, active dissimilation of cholesterol was not observed. In enrichments containing 5 mM nitrate, 95–98% of the added cholesterol was recovered as carbon dioxide (2–5%), transformation products (20–30%), or as the unmodified sterol (70–80%). Cholesterol transformation products thus far identified include 5α- and 5β-cholestan-3β-ol, cholest-4-en-3-one, 5α-androstan-3, 17-dione and androst-4-en-3, 17 dione.