Lipids isotopic composition

We can now appreciate that this explanation is incorrect, because the energy food for an animal is all of its diet and not just carbohydrates and lipids. Therefore we should not expect any selective offset due to the presence of lipids in the flesh of herbivores. Indeed, in general, the average 5 Cof total consumable herbivore tissues (flesh, lipids, etc.) is very close to that of the diet, and we might not expect any difference in the isotopic composition of the collagen or carbonate of a consumer of pure Cj plants as opposed to a consumer of the flesh of Cs-eating herbivores. We must seek elsewhere for the cause of the trophic level effect on A,p.co-... 

Figure 10.2. Schematic diagram showing how restricted conversion of fatty acids to amino acids influences the fractionation between collagen and CO3 of bone apatite LI = lipid component, PR = protein, T = total isotopic composition AP = COj component of apatite, a) Herbivorous diet (Cj plants only) b) Carnivorous diet, assuming rj = 1 (no barrier to fatty acid conversion to AAs) c) Carnivorous diet, assuming ri < 1 note that carbonate-collagen fractionation is smaller.

A DIFF relates a specified body component isotopic composition to the various compositions of a specified and complete set of dietary components. The complete diet must be accounted for in the DIFF, but it can be partitioned in any way that seems sensible for example, into individual amino acids or into protein, carbohydrate and lipid, etc. 

Eleven controlled diet and environment experiments have been designed in a way that can be used to investigate the effects of protein nutrition and heat and/or water stress on diet-tissue A N. Laboratory rats were raised on purified, pelletized diets in which the isotopic composition of proteins, lipids and carbohydrates were well characterized and their proportions accurately and precisely measured (Ambrose and Norr 1993). Four experiments involved manipulation of temperature and/or water availability. Of these four experiments, one used a diet with high (70%) protein concentrations and heat/water stress (36°C) and three used normal (20%) protein concentrations. Seven experiments were conducted at normal temperature (21°C) with water ad libitum. Of these seven experiments, two used diets formulated with veiy low protein (5%), three with normal protein and two with high protein concentrations. 

Amino acid synthesis in heterotrophs primarily relies on dietary protein rather than on lipids or carbohydrates (Schwarcz, 2000), and thus the isotopic composition of the resulting new collagen is related more to dietary protein rather than to bulk diet. Trophic... 

Canuel, E.A., Freeman, K.H., and Wakeham, S.G (1997) Isotopic compositions of lipid biomarker compounds in estuarine plants and surface sediments. Limnol. Oceanogr. 42, 1570-1583. 

Schouten, S., Klien, W.C.M., Breteler, K., Blokker, P., Schogt, N., Irene, W., Rupstra, I.C., Grice, K., Bass, M., and Damste J.S.S. (1998) Biosynthetic effects on the stable carbon isotopic compositions of algal lipids Implications for deciphering the carbon isotopic biomarker record. Geochim. Cosmochim. Acta 62, 1397-1406. 

Shi, W., Sun, M.Y., Molina, M., and Hodson, R.E. (2001) Variability in the distribution of lipid biomarkers and their molecular isotopic composition in Altamaha estuarine sediments implications for the relative contribution or organic matter from various sources. Org. Geochem. 32, 453-468. 

Lipids form a minor part of the carbon content of herbivore diets As a result, despite the fact that they are used primarily for energy metabolism, their abundance is not sufficient to greatly influence the isotopic composition of blood bicarbonate For this reason, the isotopic composition of lipids will not significantly affect the isotopic ratio of carbon in bone apatite ... 

In carnivores9 bone collagen reflects the protein of prey animals directly, while the isotopic composition of apatite is largely controlled by lipids in the diet ... 

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

Lockheart M.J., van Bergen P.F., Evershed R.P. (1997) Variations in the stable carbon isotope compositions of individual lipids from the leaves of modern angiosperms implications for the study of higher land plant-derived sedimentary organic matter. Org. Geochem. 26, 137-53. 

Pancost R.D., Sinninghe Damste J.S. (2003) Carbon isotopic compositions of prokaryotic lipids as tracers of carbon cycling in diverse settings. Chem. Geol. 195, 29-58. 

Useful lessons. Generalities can be drawn from the foregoing discussion. First, isotopic compositions of intermediates (e.g. B) can differ substantially from those of final products (e.g. I). Second, the division of carbon flows at branch points can strongly affect isotopic compositions downstream. As a result, it is practically impossible to predict the isotopic compositions of final, biosynthetic products on the basis of observed isotopic compositions of intermediates. In plain words, if you want to know about the isotopic composition of lipids, it is very risky to rely entirely on analyses of acetate, even though most lipids are produced from acetate. 

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