The effect of drought on C and N stable isotopes in different fractions of leaves, stems and roots of sensitive and tolerant beech ecotypes

Beech seedlings from 11 German climatic provenances were exposed to a realistically timed drought treatment in a greenhouse experiment. The stable isotope composition of carbon (C) and nitrogen (N) was analysed in pooled bulk material of roots, stems and leaves, as well as in the aqueous extracts and starch fractions. The delta 13C values increased in bulk samples (BS) of roots, stems and leaves by drought, although no leaf growth occurred during the experimental period. A clear drought effect on delta 13C in aqueous extracts was detected in leaves. In aqueous extracts of stems and roots as well as in starch fractions of all organs, abundance of delta 13C also tended to be increased by drought, but this effect was not statistically significant. For both delta 13C and delta 15N, enrichment was observed from the site of uptake/ source to the site of use/sink. A gradient for delta 13C in all fractions from leaves (-29.49, -28.89 and -27.85 per thousand) to stems (-28.81, -27.48 and -26.98 per thousand) and to roots (-27.60, -26.37 and -26.48 per thousand) was detected in BS, aqueous extracts and starch, respectively. An opposite gradient for delta 15N was found in BS: 1.59 per thousand, 1.84 per thousand and 3.05 per thousand in roots, stems and leaves, respectively. delta 15N was neither affected by drought in the BS nor in aqueous extracts, but an effect of provenance was observed. Particularly in roots and stems, drought-sensitive provenances showed the strongest shifts in delta 13C induced by drought and the lowest delta 15N values. In the present experiment, delta 13C values were more affected by the environmental factor drought, while delta 15N values were more affected by the genetic factor provenance.     

13C gas chromatography-combustion isotope ratio mass spectrometry analysis of N-pivaloyl amino acid esters of tissue and plasma samples

We present the analysis of the stable carbon isotope compositions of 14 individual N-pivaloyl-isopropyl (NPP) amino acid esters by gas chromatography-combustion isotope ratio mass spectrometry (GC-C-IRMS). The mean reproducibility of derivatization procedure and GC-C-IRMS analysis was 0.45 per thousand (range, 0.12-0.68), whereas the mean analytical error was 0.26 per thousand delta(13)C (range, 0.13-0.42). Furthermore, the delta(13)C values of N-pivaloyl-isopropyl and N-acetyl-n-propyl (NAP) amino acid esters were compared. Due to a reproducible isotopic fractionation introduced by the derivatization process an empirical correction factor for each individual amino acid was derived separately for both derivatives (NPP, -1.13 to -2.52 (lysine, +2.09) per thousand delta(13)C; NAP, -2.36 to -3.97 (lysine, +1.91) per thousand delta(13)C), and the original delta(13)C value of the underivatized amino acid was calculated. Further, we performed an animal study where rats (n = 5) ingested a mixed meal containing uniformly (13)C-labeled casein (indispensable amino acids 1.3 to 1.7 at.%). One hour after the meal delta(13)C values of protein-bound amino acids from small intestinal mucosa and liver and of free amino acids from mucosa and plasma were determined. Significant (13)C enrichments of indispensable amino acids of the free pools of mucosa and plasma (range, 0.0518 to 0.1700 at.% excess) and in mucosa and liver proteins (range, 0.0021 and 0.0161 at.% excess) were observed. The feasibility of various derivatives for the measurement of carbon isotopic composition is discussed.     

高山林线急尖长苞冷杉不同器官的稳定碳同位素组成分布特征

通过分析青藏高原东南部色季拉山林线物种急尖长苞冷杉不同年龄叶片、嫩枝、枝条、树干及根系的稳定碳同位素比值（6BC）及其空间分布特征,研究了植物光合作用后稳定碳同位素分馏及其影响因素．结果表明：植物不同器官δ^13C值差异显著（P〈0．001）,为树干（-24．19‰）〉枝条（-24．56‰）〉根部（-25．05‰）〉嫩枝（-25．12‰）〉叶片（-27．25‰）,说明从光合作用器官到非光合作用器官有明显的碳同位素分馏,且非光合作用器官之间也存在差异．随着急尖长苞冷杉叶片或嫩枝年龄的增加,叶片δ^13C值降低,而嫩枝δ^13C值升高（P〈0．01）．冠层上部叶片δ^13C值明显高于冠层下部（P〈0．01）,嫩枝δ^13C值则无显著性差异（P〉0．05）．远离树干2．5m的枝条δ^13C值有明显的高度变化（P〈0．01）,而离树干较近（1．5或0．5m）的枝条及树干在不同高度之间无差异（P〉0．05）．在同一冠层高度,随着与树干距离加大,枝条δ^13C值降低,且在中部和下部枝条尤为明显．说明林线地区冷杉光合作用后存在明显的碳同位素分馏;特定冠层高度树干与枝条生长所需的碳并不是全部来源于同高度的叶片光合作用合成的碳．     

Effects of sample preparation on stable isotope ratios of carbon and nitrogen in marine invertebrates: implications for food web studies using stable isotopes

Trophic ecology has benefitted from the use of stable isotopes for the last three decades. However, during the last 10 years, there has been a growing awareness of the isotopic biases associated with some pre-analytical procedures that can seriously hamper the interpretation of food webs. We have assessed the extent of such biases by: (1) reviewing the literature on the topic, and (2) compiling C and N isotopic values of marine invertebrates reported in the literature with the associated sample preparation protocols. The factors considered were: acid-washing, distilled water rinsing (DWR), sample type (whole individuals or pieces of soft tissues), lipid content, and gut contents. Two-level ANOVA revealed overall large and highly significant effects of acidification for both delta(13)C values (up to 0.9 per thousand decrease) and delta(15) N values (up to 2.1 per thousand decrease in whole individual samples, and up to 1.1 per thousand increase in tissue samples). DWR showed a weak overall effect with delta(13)C increments of 0.6 per thousand (for the entire data set) or decrements of 0.7 per thousand in delta(15) N values (for tissue samples). Gut contents showed no overall significant effect, whereas lipid extraction resulted in the greatest biases in both isotopic signatures (delta(13)C, up to -2.0 per thousand in whole individuals; delta(15)N, up to +4.3 per thousand in tissue samples). The study analyzed separately the effects of the various factors in different taxonomic groups and revealed a very high diversity in the extent and direction of the effects. Maxillopoda, Gastropoda, and Polychaeta were the classes that showed the largest isotopic shifts associated with sample preparation. Guidelines for the standardization of sample preparation protocols for isotopic analysis are proposed both for large and small marine invertebrates. Broadly, these guidelines recommend: (1) avoiding both acid washing and DWR, and (2) performing lipid extraction and gut evacuation in most cases.     

Isotopic discrimination between food and blood and feathers of captive penguins: implications for dietary studies in the wild

Using measurements of naturally occurring stable isotopes to reconstruct diets or source of feeding requires quantifying isotopic discrimination factors or the relationships between isotope ratios in food and in consumer tissues. Diet-tissue discrimination factors of carbon ((13)C/(12)C, or delta (13)C) and nitrogen ((15)N/(14)N, or delta (15)N) isotopes in whole blood and feathers, representing noninvasive sampling techniques, were examined using three species of captive penguins (king Aptenodytes patagonicus, gentoo Pygoscelis papua, and rockhopper Eudyptes chrysocome penguins) fed known diets. King and rockhopper penguins raised on a constant diet of herring and capelin, respectively, had tissues enriched in (15)N compared to fish, with discrimination factors being higher in feathers than in blood. These data, together with previous works, allowed us to calculate average discrimination factors for (15)N between whole lipid-free prey and blood and feathers of piscivorous birds; they amount to +2.7 per thousand and +4.2 per thousand, respectively. Both fish species were segregated by their delta (13)C and delta (15)N values, and importantly, lipid-free fish muscle tissue was consistently depleted in (13)C and enriched in (15)N compared to whole lipid-free fish. This finding has important implications because previous studies usually base dietary reconstructions on muscle of prey rather than on whole prey items consumed by the predator. We tested the effect of these differences using mass balance calculations to the quantification of food sources of gentoo penguins that had a mixed diet. Modeling indicated correct estimates when using the isotopic signature of whole fish (muscle) and the discrimination factors between whole fish (muscle) and penguin blood. Conversely, the use of isotopic signatures of muscle together with discrimination factors between whole fish and blood (or the reverse) leads to spurious estimates in food proportions. Consequently, great care must be taken in the choice of isotopic discrimination factors to apply to wild species for which no controlled experiments on captive individuals have been done. Finally, our results also indicate that there is no need to remove lipids before isotopic analysis of avian blood.     

Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios

Carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) stable isotope ratios were longitudinally measured in fingernail and hair samples from mother-infant pairs where infants were exclusively breastfed (n = 5), breast- and formula-fed (n = 2), or exclusively formula-fed (n = 1) from birth. All exclusively breastfed infants had a dual enrichment in carbon ( approximately 1 per thousand) and nitrogen ( approximately 2-3 per thousand) when compared to maternal values. In contrast, breast- and formula-fed subjects had reduced enrichments compared to exclusively breastfed subjects, and the exclusively formula-fed infant showed no increase in delta(13)C or delta(15)N values. This finding of a carbon trophic level effect in breastfeeding infants suggests that (13)C-enrichments of approximately 1 per thousand in archaeological populations are not necessarily the result of the consumption of C(4)-based weaning foods such as maize or millet. During the weaning process, the delta(13)C results for breastfed infants declined to maternal levels more rapidly than the delta(15)N results. This suggests that delta(13)C values have the potential to track the introduction of solid foods into the diet, whereas delta(15)N values monitor the length of time of breast milk consumption. These findings can be used to refine the isotopic analysis of breastfeeding and weaning patterns in past and modern populations.     

Biosynthetic controls on the 13C contents of organic components in the photoautotrophic bacterium Chloroflexus aurantiacus

To assess the effects related to known and proposed biosynthetic pathways on the (13)C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The bulk cell material was 13 per thousand depleted in (13)C relative to the dissolved inorganic carbon. Evidently, inorganic carbon fixation by the main carboxylating enzymes used by C. aurantiacus, which are assumed to use bicarbonate rather than CO(2), results in a relatively small carbon isotopic fractionation compared with CO(2) fixation by the Calvin cycle. Even carbon numbered fatty acids, odd carbon numbered fatty acids, and isoprenoid lipids were 14, 15, and 17-18 per thousand depleted in (13)C relative to the carbon source, respectively. Based on the (13)C contents of alkyl and isoprenoid lipids, a 40 per thousand difference in (13)C content between the carboxyl and methyl carbon from acetyl-coenzyme A has been calculated. Both sugars and polyhydroxyalkanoic acid were enriched in (13)C relative to the alkyl and isoprenoid lipids. To the best of our knowledge this is the first report in which the stable carbon isotopic composition of a large range of biosynthetic products in a photoautotrophic organism has been investigated and interpreted based on previously proposed inorganic carbon fixation and biosynthetic pathways. Our results indicate that compound-specific stable carbon isotope analysis may provide a rapid screening tool for carbon fixation pathways.     

Biosynthetic controls on the 13C contents of organic components in the photoautotrophic bacterium Chloroflexus aurantiacus.

To assess the effects related to known and proposed biosynthetic pathways on the (13)C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The bulk cell material was 13 per thousand depleted in (13)C relative to the dissolved inorganic carbon. Evidently, inorganic carbon fixation by the main carboxylating enzymes used by C. aurantiacus, which are assumed to use bicarbonate rather than CO(2), results in a relatively small carbon isotopic fractionation compared with CO(2) fixation by the Calvin cycle. Even carbon numbered fatty acids, odd carbon numbered fatty acids, and isoprenoid lipids were 14, 15, and 17-18 per thousand depleted in (13)C relative to the carbon source, respectively. Based on the (13)C contents of alkyl and isoprenoid lipids, a 40 per thousand difference in (13)C content between the carboxyl and methyl carbon from acetyl-coenzyme A has been calculated. Both sugars and polyhydroxyalkanoic acid were enriched in (13)C relative to the alkyl and isoprenoid lipids. To the best of our knowledge this is the first report in which the stable carbon isotopic composition of a large range of biosynthetic products in a photoautotrophic organism has been investigated and interpreted based on previously proposed inorganic carbon fixation and biosynthetic pathways. Our results indicate that compound-specific stable carbon isotope analysis may provide a rapid screening tool for carbon fixation pathways.     

Interspecific variation in bulk tissue, fatty acid and monosaccharide delta(13)C values of leaves from a mesotrophic grassland plant community

The leaves of 37 grass, herb, shrub and tree species were collected from a mesotrophic grassland to assess natural variability in bulk, fatty acid and monosaccharide delta(13)C values of leaves from one plant community. The leaf tissue mean bulk delta(13)C value was -29.3 per thousand. No significant differences between tissue bulk delta(13)C values with life form were determined (P=0.40). On average, C(16:0), C(18:2) and C(18:3) constituted 89% of leaf tissue total fatty acids, whose delta(13)C values were depleted compared to whole leaf tissues. A general interspecific (between different species) trend for fatty acids delta(13)C values was observed, i.e. delta(13)C(16:0)delta(13)C(xylose)>delta(13)C(glucose)>delta(13)C(galactose), was consistently observed. Therefore, we have shown (i) diversity in compound-specific delta(13)C values contributing to leaf bulk delta(13)C values; (ii) interspecific variability between bulk and compound-specific delta(13)C values of leaves of individual grassland species, and (iii) trends between individual fatty acid and monosaccharide delta(13)C values common to leaves of all species within one plant community.     

In vitro study of lipid biosynthesis in an anaerobically methane-oxidizing microbial mat

The anaerobic oxidation of methane (AOM) is a key process in the global methane cycle, and the majority of methane formed in marine sediments is oxidized in this way. Here we present results of an in vitro 13CH4 labeling study (delta13CH4, approximately 5,400 per thousand) in which microorganisms that perform AOM in a microbial mat from the Black Sea were used. During 316 days of incubation, the 13C uptake into the mat biomass increased steadily, and there were remarkable differences for individual bacterial and archaeal lipid compounds. The greatest shifts were observed for bacterial fatty acids (e.g., hexadec-11-enoic acid [16:1Delta11]; difference between the delta13C at the start and the end of the experiment [Deltadelta13C(start-end)], approximately 160 per thousand). In contrast, bacterial glycerol diethers exhibited only slight changes in delta13C (Deltadelta13C(start-end), approximately 10 per thousand). Differences were also found for individual archaeal lipids. Relatively high uptake of methane-derived carbon was observed for archaeol (Deltadelta13C(start-end), approximately 25 per thousand), a monounsaturated archaeol, and biphytanes, whereas for sn-2-hydroxyarchaeol there was considerably less change in the delta13C (Deltadelta13C(start-end), approximately 2 per thousand). Moreover, an increase in the uptake of 13C for compounds with a higher number of double bonds within a suite of polyunsaturated 2,6,10,15,19-pentamethyleicosenes indicated that in methanotrophic archaea there is a biosynthetic pathway similar to that proposed for methanogenic archaea. The presence of group-specific biomarkers (for ANME-1 and ANME-2 associations) and the observation that there were differences in 13C uptake into specific lipid compounds confirmed that multiple phylogenetically distinct microorganisms participate to various extents in biomass formation linked to AOM. However, the greater 13C uptake into the lipids of the sulfate-reducing bacteria (SRB) than into the lipids of archaea supports the hypothesis that there is autotrophic growth of SRB on small methane-derived carbon compounds supplied by the methane oxidizers.     

Annual rainfall does not directly determine the carbon isotope ratio of leaves of Eucalyptus species

Leaf carbon isotope discrimination (delta13C) was widely considered to directly reflect the rainfall environment in which the leaf developed, but recent observations have queried this. The relationship between delta13C and rainfall was explored in Eucalyptus species growing along a rainfall gradient in Australia. The leaves of 43 species of Eucalyptus and the closely related Corymbia species produced in 2003 were sampled in September 2004 at 50 sites and grouped into 15 locations along a rainfall gradient in southwest Western Australia. At 24 sites, the same species and same trees were sampled as in a study in September 2003 when leaves produced in 2002 were sampled. The rainfall in 2004 was on average 190 mm (range 135-270 mm) higher at all locations than in 2003. In the leaves sampled in 2004, the mean carbon isotope discrimination (delta13C) across the 15 locations decreased 2.9 per thousand per 1000 mm of rainfall, the specific leaf area (SLA) increased by 2.9 m2 kg(-1) per 1000 mm of rainfall and the nitrogen (N) content decreased by 1.56 g m(-2) per 1000 mm of rainfall. In contrast, a comparison between the leaves produced in the drier 2002 year compared with the wetter 2003 year showed that there was a strong correlation (r2= 0.85) between the SLA values between years and a trend for higher values with increasing SLA, but the values of delta(13)C were on average only 0.38 per thousand lower (more negative) at all locations in the wetter year, equivalent to a decrease of 2.0 per thousand per 1000 mm of rainfall. The results suggest that while there may be constitutive differences in leaf morphology, SLA and N content per unit area, increasing rainfall or cloudiness associated with higher rainfall increases SLA and decreases N content per unit area. We conclude that rainfall does not directly influence delta13C, but induces leaf morphological and physiological changes that affect the resultant delta13C.     

Isotopic evidence for breastfeeding and possible adult dietary differences from Late/Sub-Roman Britain

Historical documents indicate that breastfeeding and weaning practices have fluctuated in England through history. In order to obtain evidence for general breastfeeding patterns in Late/Sub-Roman Britain, stable carbon and nitrogen isotope values were measured in juvenile and adult skeletons (n = 87) from the cemetery of Queenford Farm, Dorchester-on-Thames, Oxfordshire. As the site contained few individuals between 0-1.5 years of age, it was not possible to determine the initial timing for the introduction of weaning foods. Between ages 2-4 years, the mean +/- SD delta(13)C results (-20.2 +/- 0.3 per thousand) are significantly more negative (t = -4.03, P < 0.001) compared to adult females (-19.7 +/- 0.3 per thousand). This is interpreted as evidence of a different diet being fed to children during weaning. After age 2, the delta(15)N values gradually decline, indicating complete cessation of breastfeeding by 3-4 years. Among adults, stature (males = 1.68 +/- 0.06 m; females = 1.58 +/- 0.07 m) and sexual dimorphism (106) were low, suggesting that the population was possibly under environmental stress. The delta(13)C results for adults are similar, but females show a small but statistically significantly (t = -2.86, P < 0.01) lower mean delta(15)N value (9.9 +/- 0.9 per thousand) compared to males (10.6 +/- 0.5 per thousand). These lower female delta(15)N values possibly reflect the different physiology of the sexes (pregnancy and/or lactation) or the reduced consumption of animal/fish protein by women, and this may have been influenced by individual preference, family needs, or societal values of the era.     

Carbon pools and isotopic trends in a hypersaline cyanobacterial mat

The fine-scale depth distribution of major carbon pools and their stable carbon isotopic signatures (delta(13)C) were determined in a cyanobacterial mat (Salin-de-Giraud, Camargue, France) to study early diagenetic alterations and the carbon preservation potential in hypersaline mat ecosystems. Particular emphasis was placed on the geochemical role of extracellular polymeric substances (EPS). Total carbon (C(tot)), organic carbon (C(org)), total nitrogen (N(tot)), total hydrolysable amino acids (THAA), carbohydrates, cyanobacteria-derived hydrocarbons (8-methylhexadecane, n-heptadec-5-ene, n-heptadecane) and EPS showed highest concentrations in the top millimetre of the mat and decreased with depth. The hydrocarbons attributed to cyanobacteria showed the strongest decrease in concentration with depth. This correlated well with the depth profiles of oxygenic photosynthesis and oxygen, which were detected in the top 0.6 and 1.05 mm, respectively, at a high down-welling irradiance (1441 micromol photons m(-2) s(-1)). At depths beneath the surface layer, the C(org) was composed mainly of amino acids and carbohydrates. A resistance towards microbial degradation could have resulted from interactions with diverse functional groups present in biopolymers (EPS) and with minerals deposited in the mat. A (13)C enrichment with depth for the total carbon pool (C(tot)) was observed, with delta(13)C values ranging from -16.3 per thousand at the surface to -11.3 per thousand at 9-10 mm depth. Total lipids depicted a delta(13)C value of -17.2 per thousand in the top millimetre and then became depleted in (13)C with depth (-21.7 to -23.3 per thousand). The delta(13)C value of EPS varied only slightly with depth (-16.1 to -17.3 per thousand) and closely followed the delta(13)C value of C(org) at depths beneath 4 mm. The EPS represents an organic carbon pool of preservation potential during early stages of diagenesis in recent cyanobacterial mats as a result of a variety of possible interactions. Their analyses might improve our understanding of fossilized microbial remains from mat ecosystems.     

Using stable isotope analysis to obtain dietary profiles from old hair: a case study from Plains Indians

Stable isotope composition of human tissue reflects that of foods consumed, and can provide information about diet independent of artifactual remains. Here we refine and test this method by analyzing nitrogen (delta(15)N) and carbon (delta(13)C) isotope ratios in historic North American Plains Indians hair. Gas-source isotope-ratio mass spectrometry provides high-precision data for both delta(15)N and delta(13)C (+/-0.2 per thousand, 1 sigma) in single hair strands as short as 2 cm (100-150 mug). Because hair contains more carbon than nitrogen, if only delta(13)C data are needed, shorter strands (<1 cm) can be analyzed. This reduction in sample size opens new opportunities for analysis of small hair fragments found in archaeological excavations, as well as for analysis of seasonal variations in long hair strands. We find distinct isotope profiles (delta(15)N vs. delta(13)C) for two cultural groups, the Lower Brule reservation Sioux of 1892 and the reservation Blackfoot of 1892 and 1935. The resultant dietary profiles indicate a higher consumption of meat by the Blackfoot and a higher consumption of maize (or of animals that had fed on maize or other C(4) plants) by the Lower Brule. The two groups of Blackfoot yield similar isotopic profiles despite the passage of four decades, suggesting a strong role for cultural preference even as food sources change. Such stable isotope profiles can be used to link samples from the same cultural tradition based on their similar diets.     

Stable isotope composition of organic compounds transported in the phloem of European beech--evaluation of different methods of phloem sap collection and assessment of gradients in carbon isotope composition during leaf-to-stem transport

The analysis of stable isotope composition (delta13C, delta15N, delta18O) of phloem-transported organic matter is a useful tool for assessing short-term carbon and water balance of trees. A major constraint of the general application of this method to trees at natural field sites is that the collection of phloem sap with the "phloem bleeding" technique is restricted to particular species and plant parts. To overcome this restriction, we compared the contents (amino compounds and sugars) and isotope signatures (delta13C, delta15N, delta18O) of phloem sap directly obtained from incisions in the bark (bleeding technique) with phloem exudates where bark pieces were incubated in aqueous solutions (phloem exudation technique with and without chelating agents [EDTA, polyphosphate] in the initial sampling solution, which prevent blocking of sieve tubes). A comparable spectrum of amino compounds and sugars was detected using the different techniques. O, C, or N compounds in the initial sampling solution originating from the chelating agents always decreased precision of determination of the respective isotopic signatures, as indicated by higher standard deviation, and/or led to a significant difference of mean delta as compared to the phloem bleeding technique. Hence, depending on the element from which the ratio of heavy to light isotope is determined, compounds lacking C, N, and/or O should be used as chelating agents in the exudation solution. In applying the different techniques, delta13C of organic compounds transported in the phloem of the twig (exudation technique with polyphosphate as chelating agent) were compared with those in the phloem of the main stem (phloem bleeding technique) in order to assess possible differences in carbon isotope composition of phloem carbohydrates along the tree axis. In July, organic compounds in the stem phloem were significantly enriched in 13C by > 1.3 per thousand as compared to the twig phloem, whereas this effect was not observed in September. Correlation analysis between delta13C and stomatal conductance (Gs) revealed the gradient from the twigs to the stem observed in July may be attributed to temporal differences rather than to spatial differences in carbon isotope composition of sugars. As various authors have produced conflicting results regarding the enrichment/depletion of 13C in organic compounds in the leaf-to-stem transition, the different techniques presented in this paper can be used to provide further insight into fractionation processes associated with transport of C compounds from leaves to branches and down the main stem.     

Correlations between the 13C Content of Primary and Secondary Plant Products in Different Cell Compartments and That in Decomposing Basidiomycetes

Relative carbon isotope ratio ([delta]13C values) of primary and secondary products from different compartments of annual plants, pine needles, wood, and decomposing Basidiomycetes have been determined. An enrichment in 13C was found for storage tissues of annual plants, because of the high level of the primary storage products sucrose and starch; however, the enrichment was even greater in leaf starch. All of these compounds had the same relative 13C enrichment in positions 3 and 4 of glucose. Secondary products in conifer needles (lignin, lipids) were depleted in 13C by 1 to 2 [per mille (thousand) sign] relative to carbohydrates from the same origin. Air pollution caused a small decrease in [delta]13C values; however, the relative content of plant products, especially of the soluble polar compounds, was also affected. Decomposing fungi showed a global accumulation of 13C by 4[per mille (thousand) sign] relative to their substrates in wood. Their chitin was enriched by 2[per mille (thousand) sign] relative to the cellulose of the wood. Hence, Basidiomycetes preferentially metabolize "light" molecules, whereas "heavy" molecules are preferentially polymerized. Our results are discussed on the basis of a kinetic isotope effect on the fructose-1,6-bisphosphate aldolase reaction and of metabolic branching on the level of the triose phosphates with varying substrate fluxes.     

Trophic diversity in the evolution and community assembly of loricariid catfishes

ABSTRACT: BACKGROUND: The Neotropical catfish family Loricariidae contains over 830 species that display extraordinary variation in jaw morphologies but nonetheless reveal little interspecific variation from a generalized diet of detritus and algae. To investigate this paradox, we collected delta13C and delta15N stable isotope signatures from 649 specimens representing 32 loricariid genera (82 species) from 19 local assemblages distributed across South America. We calculated vectors representing the distance and direction of each specimen relative to the delta15N/delta13C centroid for its local assemblage, and then examined the evolutionary diversification of loricariids across assemblage isotope niche space by regressing the mean vector for each genus in each assemblage onto a phylogeny reconstructed from osteological characters. RESULTS: Loricariids displayed a total range of delta15N assemblage centroid deviation spanning 4.9per thousand, which is within the tissue-diet discrimination range known for Loricariidae, indicating that they feed at a similar trophic level and that delta15N largely reflects differences in their dietary protein content. Total range of delta13C deviation spanned 7.4per thousand, which is less than the minimum range reported for neotropical river fish communities, suggesting that loricariids selectively assimilate a restricted subset of the full basal resource spectrum available to fishes. Phylogenetic regression of assemblage centroid-standardized vectors for delta15N and delta13C revealed that loricariid genera with allopatric distributions in disjunct river basins partition basal resources in an evolutionarily conserved manner concordant with patterns of jaw morphological specialization and with evolutionary diversification via ecological radiation. CONCLUSIONS: Trophic partitioning along elemental/nutritional gradients may provide an important mechanism of dietary segregation and evolutionary diversification among loricariids and perhaps other taxonomic groups of apparently generalist detritivores and herbivores. Evolutionary patterns among the Loricariidae show a high degree of trophic niche conservatism, indicating that evolutionary lineage affiliation can be a strong predictor of how basal consumers segregate trophic niche space.     

Nutrient routing in omnivorous animals tracked by stable carbon isotopes in tissue and exhaled breath

Omnivorous animals feed on several food items that often differ in macronutrient and isotopic composition. Macronutrients can be used for either metabolism or body tissue synthesis and, therefore, stable C isotope ratios of exhaled breath (delta(13)C(breath)) and tissue may differ. To study nutrient routing in omnivorous animals, we measured delta(13)C(breath) in 20-g Carollia perspicillata that either ate an isotopically homogeneous carbohydrate diet or an isotopically heterogeneous protein-carbohydrate mixture. The delta(13)C(breath) converged to the delta(13)C of the ingested carbohydrates irrespective of whether proteins had been added or not. On average, delta(13)C(breath) was depleted in (13)C by only ca. -2 per thousand in relation to the delta(13)C of the dietary carbohydrates and was enriched by +8.2 per thousand in relation to the dietary proteins, suggesting that C. perspicillata may have routed most ingested proteins to body synthesis and not to metabolism. We next compared the delta(13)C(breath) with that of wing tissue (delta(13)C(tissue)) in 12 free-ranging, mostly omnivorous phyllostomid bat species. We predicted that species with a more insect biased diet--as indicated by the N isotope ratio in wing membrane tissue (delta(15)N(tissue))--should have higher delta(13)C(tissue) than delta(13)C(breath) values, since we expected body tissue to stem mostly from insect proteins and exhaled CO(2) to stem from the combustion of fruit carbohydrates. Accordingly, delta(13)C(tissue) and delta(13)C(breath) should be more similar in species that feed predominantly on plant products. The species-specific differences between delta(13)C(tissue) and delta(13)C(breath) increased with increasing delta(15)N(tissue), i.e. species with a plant-dominated diet had similar delta(13)C(tissue) and delta(13)C(breath) values, whereas species feeding at a higher trophic level had higher delta(13)C(tissue) than delta(13)C(breath) values. Our study shows that delta(13)C(breath) reflect the isotope ratio of ingested carbohydrates, whereas delta(13)C of body tissue reflect the isotope ratio of ingested proteins, namely insects, supporting the idea of isotopic routing in omnivorous animals.     

Stable carbon isotope fractionations of the hyperthermophilic crenarchaeon Metallosphaera sedula

The stable carbon isotopic compositions of the inorganic carbon source, bulk cell material, and isoprenoid lipids of the hyperthermophilic crenarchaeon Metallosphaera sedula, which uses a 3-hydroxypropionate-like pathway for autotrophic carbon fixation, have been measured. Bulk cell material was approximately 3 per thousand enriched in 13C relative to the dissolved inorganic carbon, and 2 per thousand depleted in 13C relative to isoprenoid membrane lipids. The isotope data suggested that M. sedula uses mainly bicarbonate rather than CO(2) as inorganic carbon source, which is in accordance with a 3-hydroxypropionate-like carbon fixation pathway. To the best of our knowledge this is the first report of 13C fractionation effects of such a hyperthermophilic crenarchaeon.     

Characterization of symbiont populations in life-history stages of mussels from chemosynthetic environments

The densities of chemoautotrophic and methanotrophic symbiont morphotypes were determined in life- history stages (post-larvae, juveniles, adults) of two species of mussels (Bathymodiolus azoricus and B. heckerae) from deep-sea chemosynthetic environments (the Lucky Strike hydrothermal vent and the Blake Ridge cold seep) in the Atlantic Ocean. Both symbiont morphotypes were observed in all specimens and in the same relative proportions, regardless of life-history stage. The relative abundance of symbiont morphotypes, determined by transmission electron microscopy, was different in the two species: chemoautotrophs were dominant (13:1-18:1) in B. azoricus from the vent site; methanotrophs were dominant (2:1-3:1) in B. heckerae from the seep site. The ratio of CH4:H2S is proposed as a determinant of the relative abundance of symbiont types: where CH4:H2S is less than 1, as at the Lucky Strike site, chemoautotrophic symbionts dominate; where CH4:H2S is greater than 2, as at the seep site, methanotrophs dominate. Organic carbon and nitrogen isotopic compositions of B. azoricus (delta 13C = -30 per thousand; delta 15N = -9 per thousand) and B. heckerae (delta 13C = -56 per thousand; delta 15N = -2 per thousand) varied little among life-history stages and provided no record of a larval diet of photosynthetically derived organic material in the post-larval and juvenile stages.