Higher plants isotopic composition

Grocke D. R. (2002) The carbon isotope composition of ancient CO2 based on higher plant organic matter. Philosophical Transactions of the Royal Society of London Series A Mathematical Physical and Engineering Sciences 360, 633 658. 

Figure 10.12. A. Carbon isotopic composition of major groups of higher plants and autotrophic microorganisms compared with oxidized carbon (CO2, HCO3, CC>32 )- The triangles are mean values. (After Holser et al 1988 Schidlowski, 1988). B. Sulfur isotopic composition of bacteriogenic sulfide in modern marine anaerobic sediments (open bars, 1-6) and in the Permian Kupferschiefer (black bar, 7) compared with oxidized sulfate of modem (1-6) and Permian seawater (+11 %o). The black triangles are mean values. (After Holser et al., 1988.)...

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. 

In contrast, sterols, which are derived from a C30 isoprenoid carbon skeleton, are produced in the cytosol (Lichtenthaler 1999). In higher plants, the carbon feedstocks required to support biosynthesis outside the chloroplast are exported mainly as the C3 carbohydrate derivative, dihydroxyacetone phosphate (Schleucher et al. 1998). As a result of these factors, plastidic fatty acids and cytosolic sterols, both derived from acetate, can have different isotopic starting points. Moreover, even when starting points and downstream processes are closely similar, the separation of pathways between compartments can mean that divisions of carbon flows at branch points differ significantly and, therefore, that final isotopic compositions differ sharply. 

Figure 3. Histograms of 5 C of modem plants (above) and the CO3 component of African mammal tooth enamel (below). C3 plants have an average isotope composition of —26%o, whereas C4 plants cluster 14%o higher at This isotope difference is passed onto herbivores C4 consumers...

Sulfur isotope compositions are employed to assess the effect of industrial activities in pristine environments, particularly in regions where the pollutant sulfur has a distinct d S value compared to that of the environmental receptors. For example, in the province of Alberta, Canada, emissions from sour gas processing plants have d S values 201 higher compared to sulfur isotope compositions of unaffected soils. The distinct isotope composition of the anthropogenic sulfur allows it to be followed as it enters the environment via biogeochemical reactions. 

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