Diversity,metabolic types and δ13C carbon isotope ratios in the grass flora of Namibia in relation to growth form,precipitation and habitat conditions

The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in ¹³C values along a rainfall gradient (50–600 mm) and in different habitat conditions. However, there are significant differences in the ¹³C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest ¹³C values (–11.7 ) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower ¹³C values (–13.4 ) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (–12.5 ) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative ¹³C values than PCK species and ¹³C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, ¹³C values decreased from –11 in the inland region (600 mm precipitation) to –15 near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed.     

Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ¹³C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ¹³C between carbon pools along the plant axis and in respired CO₂. Little is known about the within-plant variation in δ¹³C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ¹³C of water soluble organic matter (δ¹³C_WSOM) of leaves, phloem and roots, as well as dark-respired δ¹³CO₂ (δ¹³C_res) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs—Halimium halimifolium and Rosmarinus officinalis—and a woody invader—Acacia longifolia). Spatial patterns in δ¹³C_WSOM along the plant vertical axis and between respired δ¹³CO₂ and its putative substrate were clearly species specific and the most δ¹³C-enriched and depleted values were found in δ¹³C of leaf dark-respired CO₂ and phloem sugars, ~?15 and ~?33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ¹³C_WSOM and δ¹³C_res varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ¹³C signatures to trace changes in photosynthetic discrimination.     

The effect of enhanced ultraviolet-B radiation on growth,photosynthesis and stable carbon isotope composition (δ13C) of two soybean cultivars (Glycine max) under field conditions

Two Chinese cultivars of Glycine max, namely Heidou and Jindou, were exposed to ambient and supplemental levels of ultraviolet-B (UV-B) radiation simulating a 24% depletion in stratospheric ozone over a 9-week growing period at an outdoor experimental site. Enhanced UV-B irradiation significantly reduced leaf, stem and root biomass, and plant height in the Heidou cultivar. These changes were associated with a diminished photosynthetic (net CO₂) rate, stomatal conductance, transpiration rate and water use efficiency, and accompanied by decreased foliar chlorophyll a and b, and total carotenoid concentrations and elevated foliar flavonoid levels. In contrast, the Jindou cultivar displayed only a significantly reduced stem mass and stomatal conductance, but no changes in pigment composition under elevated UV-B. The greater tolerance of elevated UV-B exposures by the Jindou cultivar was attributed partly to its higher foliar flavonoid content, smaller leaf size, thicker leaf cuticle and scabrous (hairy) lamina. Nevertheless both the Heidou cultivar and the less UV-B sensitive Jindou cultivar displayed an altered carbon isotope composition (δ¹³C) in their tissues following exposure to elevated UV-B. Such carbon isotope composition changes in plant tissues suggested a means of early detection of photosynthetic disruption in plants with anticipated increase in UV-B due to stratospheric ozone depletion.     

Metabolic fractionation of stable carbon isotopes: implications of different proximate compositions for studies of the aquatic food webs using δ13C data

In two laboratory experiments using tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio), we investigated the effect of lipid content in the fish carcass on the stable carbon isotope ratio (δ¹³C). In both experiments, a significant negative correlation was found between lipid content in the carcass dry matter and δ¹³C of total dry matter, but there was no influence on the δ¹³C of fat-free dry matter or lipids. As the lipid content of fish is known to vary with reproductive stage, season or nutritional state, separate analysis of fractions of the proximate composition of dry matter will lead to more reliable results than analysing the whole body. The differences in δ¹³C between diet and fish carcass (trophic shift) were different for the two species, calling for feeding trials under controlled conditions using the species and the feeds in question before applying the stable isotope tracer technique in the analysis of food webs. Received: 1 December 1997 / Accepted: 1 March 1998     

Identifying carbon sources and trophic position of coral reef fishes using diet and stable isotope (δ15N and δ13C) analyses in two contrasted bays in Moorea, French Polynesia

Stable isotope ratios (δ¹⁵N and δ¹³C) and diet of three fish species, Stegastes nigricans, Chaetodon citrinellus and Epinephelus merra, were analyzed on the fringing coral reefs of two bays that are differentially exposed to river runoff on Moorea Island, French Polynesia. S. nigricans and C. citrinellus relied mostly on turf algae and presented similar trophic levels and δ¹⁵N values, whereas E. merra fed on large invertebrates (crabs and shrimps) and had higher trophic levels and δ¹⁵N values. Discrepancies existed between stomach content and stable isotope analyses for the relative importance of food items. Bayesian mixing models indicated that sedimented organic matter was also an important additional food for S. nigricans and C. citrinellus, and fishes for E. merra. The main sources of organic matter involved in the food webs ending with these species were algal turfs and surface sediments, while water particulate organic matter was barely used. Significant spatial differences in C and N isotopic ratios for sources and fishes were found within and between bays. Lower ¹³C and higher ¹⁵N values were observed for various compartments of the studied trophic network at the end of each bay than at the entrance. Differences were observed between bays, with organic sources and consumers being, on average, slightly more ¹³C-depleted and ¹⁵N-enriched in Cook’s Bay than in Opunohu Bay, linked with a higher mean annual flow of the river at Cook’s Bay. Our results suggest that rivers bring continental material into these two bays, which is partly incorporated into the food webs of fringing coral reefs at least close to river mouths. Thus, continental inputs can influence the transfer of organic matter within coral reef food webs depending on the diet of organisms.     

Fractionation and turnover of stable carbon isotopes in animal tissues: Implications for δ13C analysis of diet

The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleoecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood processes. This study shows that tissues of the gerbil (Meriones unguienlatus) have different δ¹³C values when equilibrated on corn (C₄) or wheat (C₃) diets with constant ¹³C/¹²C contents. Lipids were depleted 3.0‰ and hair was enriched 1.0‰ relative to the C₄ diet. Tissue δ¹³C values were ranked hair>brain>muscle>liver>fat. After changing the gerbils to a wheat (C₃) diet, isotope ratios of the tissues shifted in the direction of the δ¹³C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was adequately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair. The results of this study have important implications for the use of δ¹³C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C₃ vs. C₄, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.     

What sources of organic carbon drive food webs in billabongs? A study based on stable isotope analysis

We used the stable isotopes of carbon and nitrogen to examine the food webs of three small flood-plain lakes (billabongs) in south-eastern Australia. With few exceptions, stable carbon isotope analysis could not be used to discriminate among the conspicuous potential sources of fringing, emergent or floating vegetation or benthic detritus. These primary sources showed little spatial or temporal variation in ¹³C values, with means ranging from-28.5 to-26.8 in spring and-29.1 to-25.4 in late summer. Submerged vegetation had similar ¹³C values to the above sources in spring but showed greater spatial variation and were less ¹³C-depleted, considerably so in some species, in late summer. Epiphytes and algae were ¹³C-depleted in spring compared with the other primary sources but became more ¹³C-enriched in late summer. Mean ¹³C values for primary and secondary consumers were not only far more variable (-37.4 to-22.7) but in general were more negative than the potential food sources, particularly in spring. Using the combined information from stable carbon and nitrogen isotope analysis, we could narrow down the list of potential primary sources driving food webs in these billabongs. The freshwater crayfish (Cherax) was one of the few taxa that appeared to obtain its biomass carbon from detrital material. Gastropods and leptocerid caddis larvae on emergent or submerged vegetation obtained a mixture of carbon from epiphytes and macrophytes; in both taxa, epiphytes contributed more to biomass carbon than did the macrophytes. However, other common grazers and collector/gatherers sampled from macrophytes, e.g. baetid mayflies, chironomid larvae and atyid shrimps, were often too ¹³C-depleted even to have derived their biomass carbon solely from epiphytes. Many other primary consumers, including zooplankton, and mussels (Velesunio), and most of the secondary consumers, including water mites (Hydracarina), phantom midge larvae (Chaoborus) and fish, were also ¹³C-depleted. The enormous biomass of littoral and fringing vegetation could contribute to metazoan food webs in these billabongs only if an additional highly ¹³C-depleted source was consumed simultaneously. Methane released from billabong sediments could provide such a ¹³C-depleted carbon source that is re-introduced into metazoan food webs via the consumption of methanotrophic bacteria. Alternatively, food webs in these water bodies are largely driven by an unknown and inconspicuous ¹³C-depleted primary producer, such as planktonic Chlorophyta.     

Effect of salinity and humidity on δ13C value of halophytes—Evidence for diffusional isotope fractionation determined by the ratio of intercellular/atmospheric partial pressure of CO2 under different environmental conditions

Summary Seedlings of two mangrove species, Avicennia marina and Aegiceras corniculatum, were grown in a range of salinities and humidities in controlled environment chambers, and Phaseolus vulgaris plants were grown in the glasshouse. The fractionation of carbon isotopes in the three species was correlated with the ratio of intercellular and ambient partial pressures of CO₂. The results are consistent with fractionation being due both to diffusion in air and to carboxylation in the leaf. It was concluded that the latter process discriminates against ¹³CO₂ relative to ¹²CO₂ by about 27.     

Variations in the foliar δ13C and C3/C4 species richness in the Japanese flora of Poaceae among climates and habitat types under human activity

For 383 Poaceae species harvested over the Japanese islands and stored as herbarium specimens along several decades, we determined C₃ and C₄ types of photosynthesis from leaf stable carbon isotope ratio (δ¹³C). Then, we sought the relationships between C₄ species richness and climatic factors or habitat types. Except for the two Panicum species (P. lanuginosum and P. scoparium) having the possibility of C₃–C₄ intermediate, 227 and 154 species were classified into C₃ and C₄. The C₄ species richness increased from northern to southern islands in Japan, positively correlated with mean annual air temperature. Greater C₄ species richness in the seashore habitats, and smaller C₄ species richness in the shaded, wet and highland habitats would be related to the photosynthetic responses to local environmental factors such as irradiance level and temperature regime. No difference of leaf δ-value of C₃ Poaceae was obtained between the habitats with different soil water availability, suggesting the less importance of soil water availability on leaf water-use efficiency in C₃ Poaceae species in Japan having humid climate. Additionally, possible effects of human activity around the harvested time or site on leaf δ-value were estimated, because the habitat includes the sites with high human activity. Leaf δ-value was decreased with sampling year, and it was higher in the densely inhabited district for both C₃ and C₄. They are probably due to a historical decrease in the atmospheric δ-value via increasing human activity, and high gas emission at the districts of high human density.     

Growth and migration patterns of juvenile temperate seabass Lateolabrax japonicus in the Yura River estuary,Japan—combination of stable isotope ratio and otolith microstructure analyses

It is important to know the characteristics of migration pattern and vital rates of juveniles to understand the early life history and its effect on the population dynamics of fishes. The relationship between growth and migration pattern of juvenile temperate seabass Lateolabrax japonicus in the Yura River estuary was examined by combination of stable carbon isotope ratio (δ¹³C) and otolith microstructure. Gut fullness indices were also examined to know the feeding condition of juveniles. δ¹³C values of seabass juveniles in the lower estuary and surf zone (LES) were enriched, while those in freshwater zone (FW) were depleted, consistent with δ¹³C differences in prey items. The back-calculated growth rates of juveniles in FW were significantly lower than those of juveniles that resided in the LES from 50 days to 90 days old, implying that juveniles with poor growth ascended the river while those with better growth remained in the LES. However, the growth rates of the juveniles, which resided in FW for more than 1 month, caught up with and even overtook those of juveniles in LES within 1 month after ascended the river. The higher water temperature and better feeding conditions would contribute to better growth rates of juveniles in FW than those in LES.     

Determination of saprotrophic fungi turnover in different substrates by glucosamine-specific δ13C liquid chromatography/isotope ratio mass spectrometry

A high performance anion exchange chromatography (HPAEC) isotopic ratio mass spectrometry (IRMS) method was developed for amino sugar-specific δ¹³C analysis in plant hydrolysates. The HPAEC-IRMS method provided good validation parameters and the amino sugar concentrations were similar to those obtained by reversed phase (RP) high performance liquid chromatography (HPLC) and fluorescence (Fl) detection. The limit of quantification (LOQ) was 150 μmol l^?1. This optimised HPAEC-IRMS method opens up the possibility of a glucosamine (GlcN) specific δ¹³C analysis in plant material. Thus, it was possible to determine the δ¹³C values in newly formed fungal GlcN for the first time. The formation and turnover of saprotrophic fungi was investigated by using the improved HPAEC-IRMS method for GlcN-specific δ¹³C analysis. The cultivation of saprotrophic fungi (Lentinula edodes and Pleurotus species) in beech wood mixed with maize or wheat straw showed the preferred formation of fungal biomass from maize-derived (80%) rather than from beech wood-derived C. The results indicate a faster formation of fungal biomass from maize than from wheat straw as co-substrate.     

Importance of spores and δ-endotoxin protein crystals of Bacillus thuringiensis in Galleria mellonella

Larvae of Galleria mellonella were fed on a honey-rich artificial food containing live spores or toxic crystals of Bacillus thuringiensis serotype V or various combinations of both. In this food; 1:1 combinations were 10 times more potent than live spores alone and about 10⁴ times more potent than crystals alone. Reduction in the proportion of spores, but not in that of crystals, decreased the slope of probit lines from 3.4 to 0.6. One or more factors in the spore are at least partly responsible for the potency of serotype V in G. mellonella. The results suggest than an observed gross loss of potency of this serotype in beehives is more likely to be due to death of spores than to deterioration of crystals. The reaction of G. mellonella to serotype V is nearest to that of a type 3 host species. Spores of serotype I are almost inactive in this host.     

Variation in stable isotope (δO and δC) signatures in the sagittal otolith carbonate of king threadfin, Polydactylus macrochir across northern Australia reveals multifaceted stock structure

Otoliths of king threadfin, Polydactylus macrochir were collected from 2007 to 2009 at nine locations across northern Australia representing most of their distributional range and areas where fisheries are active. Measurement of the stable isotope ratios of δ¹⁸O and δ¹³C in the sagittal otolith carbonate from assemblages of P. macrochir revealed location-specific signatures and indicated that adult fish sampled from representative sites across their range were significantly different. The significant differences in the isotopic signatures of P. macrochir demonstrated that population subdivision is evident and there is unlikely to be substantial movement of fish among these distinct adult assemblages. The stable isotopic signatures for the fish from the different locations were persistent through time, and therefore it could be concluded that they comprise separate stocks for many of the purposes of fisheries management. The spatial separation of these populations indicates a complex stock structure across northern Australia with stocks of P. macrochir associated with large coastal beaches and embayments on a fine spatial scale. These results indicate that in order to achieve optimal fisheries management, the current spatial management arrangements need to be reviewed, particularly the potential for localised depletion of stocks on small spatial scales. This study has provided further evidence that measurement of the stable isotopes ratios in teleost sagittal otolith carbonate can be a valuable tool in the delineation of fishable stocks or fishery management units of adult fish and that widely distributed fish can nonetheless show strong localised population structure.     

Effect of nutritional condition on variation in δ13C and δ15N stable isotope values in Pumpkinseed sunfish (Lepomis gibbosus) fed different diets

Stable isotope analysis is frequently used to infer resource use in natural populations of fishes. Studies have examined factors, other than diet, that influence δ¹⁵N and δ¹³C including tissue-specific rates of equilibration and starvation. Most such studies completed under laboratory conditions tightly control food quantity and its isotopic composition, but it is also necessary to evaluate the influence of these factors under more natural conditions. Using pumpkinseed sunfish (Lepomis gibbosus) we evaluated whether restricted rations below minimum daily requirements affects tissue equilibration to a change in diet by holding fish on two treatments that often reflect divergent resource use in natural populations (pelagic zooplankton or littoral macroinvertebrates). Over 42 days, δ¹⁵N values increased while δ¹³C values did not change, additionally neither were related to diet treatment. Increased δ¹⁵N values were negatively related to body condition while δ¹³C values were not, indicating that stable isotope values were more affected by decreasing body condition than by diet. Additionally, δ¹⁵N values changed more in the blood and liver tissues than in white muscle tissue, indicating that restricting food availability had greater effects on tissues with greater metabolic activity. We hypothesize that stable isotope values of consumers are subject to a tissue-specific trade-off between sensitivity to changes in resource use and resistance to the effects of low resource availability. This trade-off may require consideration in stable isotope studies of wild populations facing periodic limitations of food availability.     

Can diet composition estimates using stable isotope analysis of feathers predict growth and condition in nestling mountain bluebirds (Sialia currucoides)?

Insectivorous birds breeding in seasonal environments provision their dependent young during periods when prey diversity and abundance vary. Consequently, the composition and nutritional value of diets parents feed to their offspring may differ within and among broods, potentially affecting the condition of nestlings. In a population of mountain bluebirds (Sialia currucoides), we used two methods to estimate diet composition for individual nestlings: direct observation of provisioning using video recordings at 5 and 9 days post‐hatch, and stable isotopes of the δ¹³C and δ¹⁵N in nestling feathers and prey followed by analysis with mixing models. We determined the macronutrient content (% fat and lean mass) and estimated the metabolized energy from each type of prey. We evaluated whether different methods of estimating diet composition would produce similar results, and whether the types of prey nestlings ate at one or both ages affected their morphology, growth rates, or blood ketone concentration. We found that bluebirds fed their young 5 main types of prey: beetles, cicadas, grasshoppers, insect larvae, and spiders. Both observational and mixing model estimates of diet composition indicated that larvae are traded off with grasshoppers and that fewer larvae are provided to nestlings as the season progresses. In evaluating how diet influences individual growth and condition, estimates from direct observations had greater explanatory power than those from mixing models, indicating that diets rich in the most energy‐dense prey (greatest fat content; cicadas and larvae) were associated with larger size and higher body condition, and faster rate of mass gain and growth of tarsus. Lower value prey had more limited, specific effects on nestlings, but may still be important dietary components. While isotopic methods produced estimates of diet composition that were generally informative, when applied to explain the growth and condition of nestlings they proved less useful.     

Responses of pigmentation and δ13C in Qilian juniper to cold and drought stresses under natural conditions in the Qilian Mountains,China

The seasonal changes of pigments and stable carbon isotope composition (δ¹³C values) were investigated in needles of Qilian juniper (Sabina przewalskii Kom.) from two typical sites, one drier and the other wetter, in the Qilian Mountains, China. The anthocyanins and rhodoxanthin content from both sites were much higher in winter than in summer. Plant needles contained more carotenoids and xanthophylls in winter at the wetter site, while no seasonal difference appeared at the drier site. However, lower chlorophyll content and higher proline and δ¹³C were observed at the drier site. Average tree height was shorter at the drier site trees than at the wetter site. Our results suggested that under natural conditions, pigmentation in S. przewalskii may serve to improve the energy balance of the photosynthetic apparatus under cold and drought stresses. Proline and δ¹³C could be used as drought indicators for S. przewalskii.     

Source and age of dissolved and gaseous carbon in a peatland–riparian–stream continuum: a dual isotope (14C and δ13C) analysis

Radiocarbon isotopes are increasingly being used to investigate the age and source of carbon released from peatlands. Here we use combined ¹⁴C and δ¹³C measurements to determine the isotopic composition of soil and soil decomposition products [dissolved organic carbon (DOC), CO₂ and CH₄] in a peatland–riparian–stream transect, to establish the isotopic signature and potential connectivity between carbon pools. Sampling was conducted during two time periods in 2012 to investigate processes under different temperature, hydrological and flux conditions. Isotopic differences existed in the peatland and riparian zone soil organic matter as a result of the riparian depositional formation. The peatland had a mean radiocarbon age of 551 ± 133 years BP, with age increasing with depth, and δ¹³C values consistent with C3 plant material as the primary source. In contrast the riparian zone had a much older radiocarbon age of 1,055 ± 107 years BP and showed no age/depth relationship; δ¹³C in the riparian zone was also consistent with C3 plant material. With the exception of DOC in September, soil decomposition products were predominately >100 %modern with ¹⁴C values consistent with derivation from organic matter fixed in the previous 5 years. Emissions of CO₂ and CH₄ from the soil surface were also modern. In contrast, CO₂ and CH₄ evaded from the stream surface was older (CH₄: 310–537 years BP, CO₂: 36 years BP to modern) and contained a more complex mix of sources combining soil organic matter and geogenic carbon. The results suggest considerable vertical transport of modern carbon to depth within the soil profile. The importance of modern recently fixed carbon and the differences between riparian and stream isotopic signatures suggests that the peatland (not the riparian zone) is the most important source of carbon to stream water.     

On the relative importance of juvenile hormone and vitellogenin for oöcyte growth in the cockroach Nauphoeta cinerea

The corpora allata of castrated females of Nauphoeta grow only very slightly and do not reach a volume greater than that of the glands of normal females during gestation. These small corpora allata are, however, active and are responsible for the synthesis of vitellogenin (female specific protein) in large amounts. Besides vitellogenin the other haemolymph proteins are also synthesized and accumulated in the haemolymph in much higher concentrations than in normal females. Implanted oöcytes grow in castrated as well as in normal females at about the same rate until the tenth day of the oöcyte maturation period. Thereafter they only grow in castrated females. If castrated and normal females are decapitated, their protein content decreases. At the same time the growth stimulating capacity of their haemolymph decreases at a much faster rate. If oöcytes are implanted in castrated and decapitated females after 4 days they cannot grow any more although the vitellogenin titre of the haemolymph is still much higher than it is at any time in normal females. It can be concluded that vitellogenin alone cannot induce oöcyte growth and that juvenile hormone is necessary as well for vitellogenin synthesis as for its incorporation into the oöcytes. However, in insects rich in vitellogenin juvenile hormone leads to a more rapid oöcyte growth than in insects containing only small amounts of this protein.     

Origin of carbon in agricultural soil profiles deduced from depth gradients of C:N ratios,carbon fractions, δ13C and δ15N values

Plant and Soil - Agricultural soils in Germany store 2.54 Pg of organic carbon (C). However, information about how and when this C entered the soils is limited. This study illustrates how depth...