Stable isotope ecology of Miocene large mammals from Sandelzhausen,southern Germany

The carbon, oxygen, and strontium isotope composition of enamel from teeth of large Miocene herbivorous mammals from Sandelzhausen (MN5, late Early/early Middle Miocene) in the North Alpine foreland basin, were analyzed to infer diet and habitat. The mean enamel δ¹³C value of −11.4 ± 1.0‰ (n = 53) for the nine taxa analyzed (including proboscideans, cervids, suids, chalicotheres, equids, rhinocerotids) indicates a pure C₃ plant diet for all mammals. ⁸⁷Sr/⁸⁶Sr ratios of ~0.710 higher than those from teeth of the western Molasse Basin (0.708–0.709) seem to indicate preferential feeding of the mammals in the northeastern Molasse Basin. The sympatric herbivores have different mean δ¹³C and δ¹⁸O values which support diet partitioning and/or use of different habitats within a C₃ plant ecosystem. Especially the three sympatric rhinoceroses Plesiaceratherium fahlbuschi, Lartetotherium sansaniense, and Prosantorhinus germanicus show clear partitioning of plants and/or habitats. The palaeomerycid Germanomeryx fahlbuschi was a canopy folivore in moderately closed environments whereas Metaschizotherium bavaricum (Chalicotheriidae) and P. germanicus (Rhinocerotidae) were browsers in more closed forest environments. The horse Anchitherium aurelianense was probably a more generalized feeder than assumed from its dental morphology. The forest hog Hyotherium soemmeringi has the highest δ¹³C and lowest δ¹⁸O value of all analyzed taxa, possibly related to a frugivorous diet. Most taxa were water-dependent browsers that record meteoric water δ¹⁸O values of about −5.6 ± 0.7‰ Vienna Standard Mean Ocean Water (VSMOW). Using a modern-day mean annual air temperature (MAT)– relation a MAT of 19.3 ± 1.5°C can be reconstructed for Sandelzhausen. A Gomphotherium subtapiroideum tusk serially sampled for δ¹⁸O values does not record a clear pattern of seasonality. Thus most taxa were C₃ browsers in a forested and humid floodplain environment in the Molasse Basin, which experienced a warm-temperate to subtropical climate and possibly low seasonality.      

Carbon isotope fractionation in plants

Plants with the C₃, C₄, and crassulacean acid metabolism (CAM) photosynthetic pathways show characteristically different discriminations against ¹³C during photosynthesis. For each photosynthetic type, no more than slight variations are observed within or among species. CAM plants show large variations in isotope fractionation with temperature, but other plants do not. Different plant organs, subcellular fractions and metabolises can show widely varying isotopic compositions. The isotopic composition of respired carbon is often different from that of plant carbon, but it is not currently possible to describe this effect in detail. The principal components which will affect the overall isotope discrimination during photosynthesis are diffusion of CO₂, interconversion of CO₂ and HCO^?₃, incorporation of CO₂ by phosphoenolpyruvate carboxylase or ribulose bisphosphate carboxylase, and respiration. Theisotope fractionations associated with these processes are summarized. Mathematical models are presented which permit prediction of the overall isotope discrimination in terms of these components. These models also permit a correlation of isotope fractionations with internal CO₂ concentrations. Analysis of existing data in terms of these models reveals that CO₂ incorporation in C₃ plants is limited principally by ribulose bisphosphate carboxylase, but CO₂ diffusion also contributes. In C₄ plants, carbon fixation is principally limited by the rate of CO₂ diffusion into the leaf. There is probably a small fractionation in C₄ plants due to ribulose bisphosphate carboxylase.     

Carbon isotope fractionation of methyl bromideduring agricultural soil fumigations

The isotopic composition ofmethyl bromide (CH₃Br) has been suggestedto be a potentially useful tracer forconstraining the global CH₃Br budget. Inorder to determine the carbon isotopiccomposition of CH₃Br emitted from the mostsignificant anthropogenic application(pre-plant fumigation) we directly measured the¹³C of CH₃Br released duringcommercial fumigation. We also measured theisotopic fractionation associated withdegradation in agricultural soil under typicalfield fumigation conditions. The isotopiccomposition of CH₃Br collected in soilseveral hours after injection of the fumigantwas –44.5 and this value increased to –20.7over the following three days. The mean kineticisotope effect (KIE) associated withdegradation of CH₃Br in agricultural soil(12) was smaller than the reported value formethylotrophic bacterial strain IMB-1, isolatedfrom previously fumigated agricultural soil,but was similar to methylotrophic bacterialstrain CC495, isolated from a pristine forestlitter zone. Using this fractionationassociated with the degradation of CH₃Brin agricultural soil and the mean¹³C of the industriallymanufactured CH₃Br (–54.4), we calculatethat the agricultural soil fumigation sourcehas a carbon isotope signature that ranges from–52.8 to –42.0. Roughly 65% ofindustrially manufactured CH₃Br is usedfor field fumigations. The remaining 35% isused for structural and post-harvestfumigations with a minor amount used duringindustrial chemical manufacturing. Assumingthat the structural and post-harvest fumigationsources of CH₃Br are emitted withoutsubstantial fractionation, we calculate thatthe ¹³C of anthropogenicallyemitted CH₃Br ranges from –53.2 to –47.5.     

Carbon isotope fractionation in species of the torrenticolous families Podostemaceae and Hydrostachyaceae

Carbon isotope ratios of herbarium material from members of the fresh-water families Podostemaceae and Hydrostachyaceae (Rosidae) were analyzed. The levels of ¹³C were highly variable (Podostemaceae −12.8‰ to −38.55‰; Hydrostachyaceae −10.78‰ to −30.42‰), across as well as within species and across a wide geographic range.

We suggest that the high variance observed is due neither to a constant attribute of the species like the photosynthetic CO₂-carboxylase (in water plants with very high discrimination of the ¹³CO₂ probably Rubisco) nor to the constant structural peculiarities of these species. Rather, it is likely due to the ‘diffusional resistance’ for the CO₂-flux from the turbulent and/or fast flowing water, causing a very variable boundary layer on the plant surface.     

Molluscs,isotopes and ecological fractionation: Examples from the upper Palaeogene of the Hampshire Basin

Carbon and oxygen isotopic analyses from upper Palaeogene molluscs collected in the Hampshire Basin (S. England) show that, in addition to long‐term trends in the data caused by climatic change, there is variation within samples collected from any one horizon. This variation is not attributable to diagenesis or other “noise”;. Linear trends in data from the meso‐ and oligohaline organisms are salinity‐dependent, as is a differentiation into clusters of the marine and brackish water animals. Within the marine organisms there is further distinction between taxa, controlled by micro‐environment. This is quite distinct from disequilibrium precipitation, as shell growth occurs in equilibrium with local isotopic ratios, though this may not necessarily be the same as contemporary mean ocean values. This “ecological fractionation”; can seriously affect the isotopic signal from a suite of fossils. Its existence should therefore be borne in mind when interpreting any biogenic isotope data from diverse taxa, localities, or micro‐habitats.     

Stable carbon isotope ratios in Asian elephant collagen: implications for dietary studies

Summary Stable carbon isotope ratios in bone collagen have been used in a variety of dietary studies in modern and fossil animals, including humans. Inherent in the stable isotope technique is the assumption that the isotopic signature is a reflection of the diet and is persistent in collagen because this is a relatively inert protein. Carbon isotope analyses of bones from a southern Indian population of Asian elephant (Elephas maximus), a long-lived mammal that alternates seasonally between a predominantly C₃ (browse) and C₄ (grass) plant diet, showed two patterns that have important implications for dietary interpretation based on isotopic studies. Relative to the quantity of the two plant types consumed on average, the δ¹³C signal in collagen indicated that more carbon was incorporated from C₃ plants, possibly due to their higher protein contribution. There was a much greater variance in δ¹³C values of collagen in sub-adult (range -10.5‰ to-22.7‰, variance=14.51) compared to adult animals (range -16.0‰ to -20.3‰, variance=1.85) pointing to high collagen turnover rates and non-persistent isotopic signatures in younger, growing animals. It thus seems important to correct for any significant relative differences in nutritive value of food types and also consider the age of an animal before drawing definite conclusions about its diet from isotope ratios.     

Silicon isotope fractionation in rice plants, an experimental study on rice growth under hydroponic conditions

Silicon (Si) isotope composition and Si distribution among different rice plant organs and different parts of rice leaf at maturity were studied, which may provide new insights into the mechanism of Si accumulation in plants and biogeochemical Si cycle. An isotope ratio mass spectrometer (IRMS) was used to examine Si isotope fractionation by rice plant grown in a hydroponic system. The observed ³⁰Si-depletion (about 0.3‰) of whole plant relative to external nutrient solutions suggested biologically mediated Si isotope fractionation occurred during uptake. However, it was not possible to judge the Si uptake mechanism with the data. For δ³⁰Si variation within plant, there was a consistent increasing trend from lower to upper tissues (stem < leaf < husk < grain; leaf sheath < leaf blade base <leaf blade middle < leaf blade top). The phenomenon, reflecting kinetic isotope effects, could be explained that isotope fractionation during Si deposition in rice plant was a Rayleigh-like behavior. The range (−2.7‰ to 2.3‰) of δ³⁰Si variation among rice plant tissues in present experiment exceeded that (−1.7‰ to 2.5‰) of phytoliths observed previously in continents, which would enhance understanding the role of phytoliths on globe Si isotope balance.     

Lipid content and carbon assimilation in Collembola: implications for the use of compound-specific carbon isotope analysis in animal dietary studies

In an effort to understand the relationships between both the lipid content and ¹³C values of Collembola and their diet, isotopically labelled (C₃ and C₄) bakers yeasts were cultured and fed to two Collembolan species, Folsomia candida and Proisotoma minuta. The fatty acid composition of Collembola generally reflected that of the diet with the addition of the polyunsaturated components 18:2(n-6), 20:4(n-6) and 20:5(n-3), which appeared to be biosynthesised by the Collembola. Whilst ergosterol was the only sterol detected in the yeast diets, only cholesterol was detected in Collembola, and although the ¹³C values of diet and consumer sterols differed by >2, the ¹³C values indicated that cholesterol was derived entirely from dietary sterol. The bulk ¹³C values of Collembola were similar to those of the diets, but fatty acid ¹³C values did not necessarily reflect those of the dietary fatty acids, indicating significant de novo biosynthesis of fatty acids within Collembola. Switching the Collembola from C₃ to C₄ yeast enabled the determination of the rates of incorporation of dietary carbon into Collembolan lipids, and showed that half-lives of the incorporation of dietary carbon varied between 1.5 and 5.8 days at 20°C. Cholesterol exhibited the slowest rate of incorporation in both species, while bulk carbon in F. candida possessed an intermediate rate. These results demonstrate that an understanding of the sources of isotopic fractionation and the role of biochemistry in regulating the ¹³C values of individual compounds is important in the application of compound-specific isotopic analysis to the study of animal trophic activities.     

Evidence for the mechanisms of zinc uptake by rice using isotope fractionation

In an earlier study, we found that rice (Oryza sativa) grown in nutrient solution well‐supplied with Zn preferentially took up light ⁶⁴Zn over ⁶⁶Zn, probably as a result of kinetic fractionation in membrane transport processes. Here, we measure isotope fractionation by rice in a submerged Zn‐deficient soil with and without Zn fertilizer. We grew the same genotype as in the nutrient solution study plus low‐Zn tolerant and intolerant lines from a recombinant inbred population. In contrast to the nutrient solution, in soil with Zn fertilizer we found little or heavy isotopic enrichment in the plants relative to plant‐available Zn in the soil, and in soil without Zn fertilizer we found consistently heavy enrichment, particularly in the low‐Zn tolerant line. These observations are only explicable by complexation of Zn by a complexing agent released from the roots and uptake of the complexed Zn by specific root transporters. We show with a mathematical model that, for realistic rates of secretion of the phytosiderophore deoxymugineic acid (DMA) by rice, and realistic parameters for the Zn‐solubilizing effect of DMA in soil, solubilization and uptake by this mechanism is necessary and sufficient to account for the measured Zn uptake and the differences between genotypes.