G. Eglinton

Leaf Epicuticular Waxes

The external surface of the higher plants comprises a cuticular layer covered by a waxy deposit. This deposit is believed to play a major part in such phenomena as the water balance of plants and the behavior of agricultural sprays. The wax contains a wide range of organic compounds. These complex mixtures are amenable to modern microchromatographic and microspectrometric analytical procedures. The few surveys which have been made of the species distribution of certain classes of constituents indicate that such distribution may be of limited taxonomic value; however, the wax composition of a species may differ for different parts of the same plant and may vary with season, locale, and the age of the plant. This fascinating subject, in which the disciplines of botany, biochemistry, chemistry, and physics overlap and interact, is still in a very active state. Much remains to be learned about the composition and fine structure of the wax deposits, and, for this, experimental study of wax crystallization and permeation through artificial membranes will be required. Enzymic studies, radiolabeling, and electron microscopy will be needed to reveal the mode of biogenesis of the wax constituents and their site of formation and subsequent pathway through the cuticle to the leaf surface.

The molecularly-uncharacterized component of nonliving organic matter in natural environments.

Molecularly-uncharacterized organic matter comprises most reduced carbon in soils, sediments and natural waters. The origins, reactions and fates of these ubiquitous materials are relatively obscure, in large part because the rich vein of geochemical information that typically derives from detailed structural and stereochemical analysis is yet to be tapped. This discussion highlights current knowledge about the origins and characteristics of molecularly uncharacterized organic matter in the environment and outlines possible means by which this structurally uncharted frontier might best be explored.

Lipids of aquatic organisms as potential contributors to lacustrine sediments—II☆

Abstract The relationship between the lipid composition of organisms in the water column of an eutrophic lake and the lipid composition of underlying sediments, previously examined for n-alkanols and steroids, is now reported for hydrocarbons, ketones and carboxylic acids. The n-C7 alkane and alkenoic acids from two primary sources are rapidly metabolized in the water column and surficial sediment. Bacterial biomarkers, including hopenes and i ai - branched fatty acids, were detected in the photosynthetic bacterial layer occurring just above the sediment-water interface. Within the sediment the apparent conversion of free n-alkanes, alkan-2-ones and ω-hydroxy acids to the corresponding bound form is noted; microbiological oxidation of n-alkanes to alkan-2-ones is supported by the detection of the intermediate alkan-2-ols with a distribution similar to that of the ketones. The geochemistry of sediment deposited c. 1900, prior to biological study of the site, was interpreted from stable biomarkers and the diagenetic changes recognised in the study of contemporary deposition. A qualitative difference in algal input to the older sediment is inferred from the low Δ7-sterol content and presence of 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane. However, there was still significant dinoflagellate input, as indicated by the presence of 4α-methylsterols. A difference in higher-plant input to the older sediment, indicated from the n-alkane, alkene and triterpenoid ketone distributions, is consistent with the recent development of tree cover.

Compound-specific δ 13C analyses of leaf lipids from plants with differing carbon dioxide metabolisms

Abstract The stable carbon isotope (δ 13C) values of (1) total leaf tissue (2), total surface lipid extracts and (3) individual n-alkanes isolated from leaves were determined for plants utilizing the C3, C4, and Crassulacean Acid (CAM) pathways of carbon fixation. The C4 and CAM species had leaf tissue δ 13C values of −10 to −16‰; the total surface lipid extract δ 13C values were on average 8‰ more depleted. The C3 plants had leaf tissue values of −25 to −30‰; total surface lipid extracts were on average 4‰ more depleted in 13C. The average n-alkane values for all plants were around 4‰ more depleted in 13C than the total surface lipid extracts and were between 1.6 and 13.8‰ depleted in 13C relative to total tissue. This depletion relative to the total tissue was on average greater for CAM (11.0‰) and C4 plants (9.9‰) than for C3 plants (5.9‰). Variations ranging from 0.1 to 6‰ were observed between n-alkalene homologues for the plants studied, emphasizing the need for caution when interpreting small variations in the isotopic compositions of individual compounds isolated from sedimentary environments. n-Aldehydes from C3 plants had carbon-isotopic compositions similar to the n-alkanes from the same plants, suggesting that other biosynthetically related n-alkyl compounds (e.g. n-acids and n-alcohols) will have δ 13C values similar to the n-alkanes. Phytol isolated from C3 plants was, on average, 1.5‰ enriched in 13C relative to the average n-alkane δ 13C values.

Long-chain alkenes and alkenones in the marine coccolithophorid Emiliania huxleyi

Abstract Very long-chain n -alkenes and n -alkenones have been identified in the ubiquitous marine alga Emiliania huxleyi . The alkenes range from C 31 to C 38 and are almost exclusively odd-chain. Dienes, trienes and tetraenes were identified but no monoenes were found. The ketones ranged from C 37 to C 39 and consisted of both alken-2-ones and alken-3-ones, with trienes more abundant than dienes. Examination of three different forms of the alga, i.e. motile, sessile and coccolith, indicated that these ketones are formed throughout the growth cycle with only minor variations in the relative proportions of the individual compounds. Although these novel compounds have not been reported previously in organisms, they are widespread in marine sediments and may be useful biological markers for E. huxleyi input to sediments.

The biogeochemistry of Ellesmere Lake, U.K.-I: source correlation of leaf wax inputs to the sedimentary lipid record

Abstract Analyses of the free lipid distributions from Ellesmere Lake sediments taken from two depths, and comparison with leaf-wax lipid distributions from six deciduous tree species around the lake were made in order to assess lipid sources. Close correlations of aliphatic lipid distributions in the carbon number range C 20 C 50 were seen. C 28 and C 29 sterols were observed in both sediment and leaf waxes, but C 27 sterols were only present in the sediments and were thus assigned a nonleaf-wax source. Unsaturated steroidal ketones were observed both in the sediment and in certain leaf waxes and were assigned specific sources. Alkan-2-ones were not present in the leaf waxes, but were present in the sediment samples, with an almost identical distribution to the sediment n -alkanes indicating a microbially mediated source. C 20 C 30 n -aldehydes which have not previously been widely reported in free lipid distributions of freshwater lacustrine sediments, were prominent in the surface sediment and in the leaf-waxes examined.

Long chain (n-C37–C39) alkenones in the Prymnesiophyceae. Distribution of alkenones and other lipids and their taxonomic significance

Long chain (C37–C39) n-alkenones, esters of polyunsaturated n-C36 acids and C27–C29 sterols have been examined in thirteen species from nine genera of algae from the class Prymnesiophyceae and appear to have chemotaxonomic value. The alkenones and esters have been shown to occur in Chrysotila lamellosa and three species of Isochrysis and their presence in Emiliania huxleyi has been confirmed. They were absent from five other members of the order Isochrysidales, and from those representatives of the orders Coccosphaerales, Prymnesiales and Pavlovales examined. This discrimination was reflected in the distribution of the sterols; all five of the above-named species having high concentrations of 24-methylcholesta-5,22E-dien-3β-ol relative to cholest-5-en-3β-ol (cholesterol). In contrast, the former sterol is a minor component in, or is absent from, members of the Prymnesiales and Pavlovales. The sterol distributions suggest that some species at present included in the Isochrysidales (e.g. Ochrosphaera) have ...

North Pacific seasonality and the glaciation of North America 2.7 million years ago

In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitchs orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.

Hydrocarbon constituents of the wax coatings of plant leaves: A taxonomic survey

Abstract A gas-chromatographic study has been made of the hydrocarbon content of the leaf waxes of the sub-family Sempervivoideae (Crassulaceae), through the study of a compact grouping of closely related genera endemic to the Canary Islands. Within the limits of the investigation, it has been shown that a single species possesses a fairly constant hydrocarbon distribution pattern, thereby justifying the use of this criterion for taxonomic purposes. The variations from one species to another reveal a rough parallelism with the botanical classification of the sections within a genus. Isoalkanes are sometimes present to the extent of more than 50 per cent of the total alkane content.

Novel unsaturated straight-chain C37C39 methyl and ethyl ketones in marine sediments and a coccolithophore Emiliania huxleyi

Abstract Novel, very long (C37C39) straight-chain ketones have been identified in a range of marine sediments varying in age from very Recent to Miocene. The ketones are separable by thin-layer chromatography into methyl ketones and ethyl ketones, and within each group only diunsaturated and triunsaturated ketones are present. Relative to other lipid classes present, concentrations are high in Plio-Pleistocene and Miocene Japan Trench sediments (38–720 ng/g), and in very Recent Walvis Bay (1200–2100 ng/g) and northern North Sea sediments (650 ng/g). Identical compounds have been identified in the marine coccolithophore Emiliania huxleyi and with the exception that triunsaturated ketones are more abundant in this alga, the distributions are quite similar to those of the sediments. Studies involving feeding of E. huxleyi to the copepod Calanus helgolandicus indicate that the copepod assimilates only a small proportion of the ketones, most of which are excreted in the faecal pellets. This is not simply due to excretion of whole algal cells, but due to the apparent inability of the copepod to digest these compounds. This resistance to biological degradation may explain the high abundances observed in some sediments.