Evaluation of n-alkanes and their carbon isotope enrichments (δ13C) as diet composition markers

Pleistocene drivers of Northwest African hydroclimate and vegetation

Savanna ecosystems were the landscapes for human evolution and are vital to modern Sub-Saharan African food security, yet the fundamental drivers of climate and ecology in these ecosystems remain unclear. Here we generate plant-wax isotope and dust flux records to explore the mechanistic drivers of the Northwest African monsoon, and to assess ecosystem responses to changes in monsoon rainfall and atmospheric pCO₂. We show that monsoon rainfall is controlled by low-latitude insolation gradients and that while increases in precipitation are associated with expansion of grasslands into desert landscapes, changes in pCO₂ predominantly drive the C₃/C₄ composition of savanna ecosystems.

Carbon and hydrogen isotopes of n-alkanes in soils reconstructed after mining disturbance

Ecosystem reconstruction after mining disturbance is a challenge considering the multitude of factors that affect soil formation and revegetation. In the boreal forest of western Canada, peat material is often used as the organic amendment for land reclamation to upland forest. Carbon and water dynamics of peat-dominated ecosystems differ from natural upland forest soils. The objective of this work was to evaluate the evolution of soils reconstructed after mining disturbance using ¹³ C and ² H analyses of n-alkane tracers. Ten soils from natural ecosystems were sampled (0-10 cm) and compared with 11 soils from novel ecosystems ranging in age from 0 to 30 yr, as well as a fresh peat sample. Soils supported different vegetation, including pine (Pinus spp.), aspen (Populus spp.), and white spruce [Picea glauca (Moench) Voss]. Despite overlaps for some individual n-alkanes, we found a dominance of n-C₂₅ in reconstructed soils, also dominant in the peat material, and a dominance of n-C₂₇ in natural soils, one of the dominant n-alkanes in natural forest vegetation. In addition, there was a significant difference in odd n-alkane δ² H and δ¹³ C values between natural and reconstructed soils (p < .05). Differences in δ² H values, more negative for reconstructed soils than for natural soils, were attributed to changes in soil moisture, from wetter peat-dominated soils to drier upland forests; among forest types, δ² H values were most negative under pine vegetation. The δ¹³ C composition of odd n-alkanes, in particular n-C₂₇ , was significantly related to tree age (p < .05). Overall, both ² H and ¹³ C isotopic signatures of odd n-alkanes exhibited differences between natural and reconstructed soils. However, within the reconstructed soils, neither isotopic signature showed a clear evolution with age since reclamation.

Stable isotope-labelled feed nutrients to assess nutrient-specific feed passage kinetics in ruminants

Knowledge of digesta passage kinetics in ruminants is essential to predict nutrient supply to the animal in relation to optimal animal performance, environmental pollution and animal health. Fractional passage rates (FPR) of feed are widely used in modern feed evaluation systems and mechanistic rumen models, but data on nutrient-specific FPR are scarce. Such models generally rely on conventional external marker techniques, which do not always describe digesta passage kinetics in a satisfactory manner. Here the use of stable isotope-labelled dietary nutrients as a promising novel tool to assess nutrient-specific passage kinetics is discussed. Some major limitations of this technique include a potential marker migration, a poor isotope distribution in the labelled feed and a differential disappearance rate of isotopes upon microbial fermentation in non-steady state conditions. Such limitations can often be circumvented by using intrinsically stable isotope-labelled plant material. Data are limited but indicate that external particulate markers overestimate rumen FPR of plant fibre compared with the internal stable isotope markers. Stable isotopes undergo the same digestive mechanism as the labelled feed components and are thus of particular interest to specifically measure passage kinetics of digestible dietary nutrients.

Stable isotope labeled n-alkanes to assess digesta passage kinetics through the digestive tract of ruminants

We describe the use of carbon stable isotope (¹³C) labeled n-alkanes as a potential internal tracer to assess passage kinetics of ingested nutrients in ruminants. Plant cuticular n-alkanes originating from intrinsically ¹³C labeled ryegrass plants were pulse dosed intraruminally in four rumen-cannulated lactating dairy cows receiving four contrasting ryegrass silage treatments that differed in nitrogen fertilization level (45 or 90 kg nitrogen ha⁻¹) and maturity (early or late). Passage kinetics through the gastrointestinal tract were derived from the δ¹³C (i.e. the ratio ¹³C:¹²C) in apparently undigested fecal material. Isotopic enrichment was observed in a wide range of long-chain n-alkanes (C₂₇–C₃₆) and passage kinetics were determined for the most abundant C₂₉, C₃₁ and C₃₃n-alkanes, for which a sufficiently high response signal was detected by combustion isotope ratio mass spectrometry. Basal diet treatment and carbon chain length of n-alkanes did not affect fractional passage rates from the rumen (K₁) among individual n-alkanes (3.71–3.95%/h). Peak concentration time and transit time showed a quantitatively small, significant (p≤0.002) increase with carbon chain length. K₁ estimates were comparable to those of the ¹³C labeled digestible dry matter fraction (3.38%/h; r = 0.61 to 0.71; p≤0.012). A literature review has shown that n-alkanes are not fermented by microorganisms in the rumen and affirms no preferential depletion of ¹³C versus ¹²C. Our results suggest that ¹³C labeled n-alkanes can be used as nutrient passage tracers and support the reliability of the δ¹³C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.

Evaluation of a tandem gas chromatography/time-of-flight mass spectrometry metabolomics platform as a single method to investigate the effect of starvation on whole-animal metabolism in rainbow trout (Oncorhynchus mykiss)

This study was conducted to evaluate the use of a two-dimensional gas chromatography/time-of-flight mass spectrometry (GC×GC/TOF-MS) metabolomic platform to comprehensively analyze the effect of starvation on whole-animal metabolism in rainbow trout (Oncorhynchus mykiss). Trout were either fed a commercial diet at 2% body mass twice daily or starved for 4 weeks. Metabolomic analysis was conducted on serum, liver and muscle tissue from each fish. Database searching and statistical analysis revealed that concentrations of more than 50 positively identified molecules changed significantly (P<0.05) as a result of starvation. Our results indicate that starving rainbow trout for 4 weeks promotes increased utilization of select tissue fatty acids in liver and muscle. However, starvation did not significantly affect protein catabolism in peripheral tissues, as indicated by reductions in the level of serum amino acids in starved fish. In contrast, starvation appears to promote protein catabolism in liver as the level of methionine, proline and lysine metabolite 2-piperidine carboxylic acid increased significantly. Also, starvation resulted in significant changes in the level of numerous xenobiotics that could indicate the origin of particular feed ingredients and selective retention of these molecules in tissues. We suggest that metabolomic analysis using GC×GC/TOF-MS is an effective tool in studying whole-animal metabolism and the fate of important xenobiotic compounds in rainbow trout as numerous polar and non-polar metabolites were rapidly and accurately profiled using a single method.