Internode length in Zea mays L.

[¹³C, ³H]Gibberellin A₂₀ (GA₂₀) has been fed to seedlings of normal (tall) and dwarf-5 and dwarf-1 mutants of maize (Zea mays L.). The metabolites from these feeds were identified by combined gas chromatography-mass spectrometry. [¹³C, ³H]Gibberellin A₂₀ was metabolized to [¹³C, ³H]GA₂₉-catabolite and [¹³C, ³H]GA₁ by the normal, and to [¹³C, ³H]GA₂₉ and [¹³C, ³H]GA₁ by the dwarf-5 mutant. In the dwarf-1 mutant, [¹³C, ³H]GA₂₀ was metabolized to [¹³C, ³H]GA₂₉ and [¹³C, ³H]GA₂₉-catabolite; no evidence was found for the metabolism of [¹³C, ³H]GA₂₀ to [¹³C, ³H]GA₁. [¹³C, ³H]Gibberellin A₈ was not found in any of the feeds. In all feeds no dilution of ¹³C in recovered [¹³C, ³H]GA₂₀ was observed. Also in the dwarf-5 mutant, the [¹³C]label in the metabolites was apparently undiluted by endogenous [¹³C]GAs. However, dilution of the [¹³C]label in metabolites from [¹³C, ³H]GA₂₀ was observed in normal and dwarf-1 seedlings. The results from the feeding studies provide evidence that the dwarf-1 mutation of maize blocks the conversion of GA₂₀ to GA₁.Abbreviations GA_n gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - RP reverse phase     

Synthesis of a 13C-Labeled Tracer for Stream DOC: Labeling Tulip Poplar Carbon with 13CO2

Ecosystem tracer-level additions would benefit from a stable isotope-labeled source of complex organic molecules. We tested a method to label tree C with ¹³C and create a stable isotope tracer for stream dissolved organic carbon (DOC) using tulip poplar (Liriodendron tulipifera L.) seedlings. In 2000, seedlings were grown with 0.82 moles of ¹³CO₂ to assess the distribution and level of ¹³C enrichment in the tree tissues. In 2001, seedlings were grown with 25 times more ¹³CO₂ to generate tissues with a ¹³C signal strong enough for a ¹³C-DOC stream tracer addition. ¹³C enrichment in the trees varied in each year and by tissue age and type. Tissues formed during labeling (new) were more enriched in ¹³C than tissues established prior to the ¹³CO₂ injection (old). Stems were most enriched in ¹³C in both new and old tissues. A higher percentage of ¹³CO₂ was incorporated into seedlings in 2000 (59% ±1) than 2001 (43% ±0). Percent ¹³C incorporation among tree tissue types paralleled biomass distributions. Although tree C and ¹³C were equally soluble in both years, a greater percentage of tree C went into solution in 2001 (30%) than 2000 (20%). The water-soluble tree C accounted for approximately 12% of the injected ¹³CO₂ and had both humic and polysaccharide components. Results from a whole-stream ¹³C-DOC tracer addition demonstrated that tree C could be sufficiently labeled with ¹³CO₂ to create a stream DOC isotope tracer with some polymeric constituents.     

Utilization of 13C-13C Coupling in Structural and Biosynthetic Studies Double Labeling Method

A new method which utilizes ¹³C-¹³C coupling for structural and biosynthetic studies on acetate-derived metabolites is described. The ¹³C-NMR spectra of dihydrolatumcidins separately labeled with ¹³CH₃¹³C0₂Na and with a 1: 1 mixture of ¹³CH₃CO₂-Na and CH313C02-Na gave enough information to establish its structure.     

Effects of D-glucose upon D-fuctose metabolism in rat hepatocytes: A 13C NMR study

Isolated hepatocytes from fed rats were exposed for 120 min to D-glucose (10 mM) and either D-[1-¹³C]fructose, D-[2-¹³C]fructose or D-[6-¹³C]fructose (also 10 mM) in the presence of D₂O. The identification and quantification of ¹³C-enriched D-fructose and its metabolites (D-glucose, L-lactate, L-alanine) in the incubation medium and the measurement of their deuterated isotopomers indicated, by comparison with a prior study conducted in the absence of exogenous D-glucose, that the major effects of the aldohexose were to increase the recovery of ¹³C-enriched D-fructose, decrease the production of ¹³C-enriched D-glucose, restrict the deuteration of the ¹³C-enriched isotopomers of D-glucose to those generated by cells exposed to D-[2-¹³C]fructose, and to accentuate the lesser deuteration of the C₂ (as compared to C₅) of ¹³C-enriched D-glucose derived from D-[2-¹³C]fructose. The ratio between C2-deuterated and C₂-hydrogenated L-lactate, as well as the relative amounts of the CH₃-, CH₂D-, CHD₂ and CD₃- isotopomers of ¹³C-enriched L-lactate were not significantly different, however, in the absence or presence of exogenous D-glucose. These findings indicate that exogenous D-glucose suppressed the deuteration of the C₁ of D-[1-¹³C]glucose generated by hepatocytes exposed to D-[1-¹³C]fructose or D-[6-¹³C]fructose, as otherwise attributable, in part at least, to gluconeogenesis from fructose-derived [3-¹³C]pyruvate, and apparently favoured the phosphorylation of D-fructose by hexokinase isoenzymes, probably through stimulation of D-fructose phosphorylation by glucokinase.     

The biosynthesis of the antioxidant caffeic acid derivative subulatin by the liverwort Jungermannia subulata cultured in vitro

The in vitro cultured liverwort Jungermannia subulata produces the unique molecule subulatin. In this study, we examined the incorporation of [1-¹³C] and [1,2-¹³C₂] glucose, [2-¹³C] arabinose, [2-¹³C] caffeic acid, and [1-¹³C] phenylalanine into subulatin. The trilobatinoic acid C unit of subulatin incorporated ¹³C atoms from [1-¹³C] and [1,2-¹³C₂] glucose and from [2-¹³C] arabinose but not from any other of the other precursors. Based on these results and labeling patterns, the trilobatinoic acid C unit of subulatin appears to be biosynthesized from arabinose-5-phosphate and phosphoenolpyruvate.     

The intramolecular ¹³C-distribution in ethanol reveals the influence of the CO₂ -fixation pathway and environmental conditions on the site-specific ¹³C variation in glucose

Efforts to understand the cause of ¹²C versus ¹³C isotope fractionation in plants during photosynthesis and post‐photosynthetic metabolism are frustrated by the lack of data on the intramolecular ¹³C‐distribution in metabolites and its variation with environmental conditions. We have exploited isotopic carbon‐13 nuclear magnetic resonance (¹³C NMR) spectrometry to measure the positional isotope composition (δ¹³C_i, ‰) in ethanol samples from different origins: European wines, liquors and sugars from C₃, C₄ and crassulacean acid metabolism (CAM) plants. In C₃‐ethanol samples, the methylene group was always ¹³C‐enriched (～2‰) relative to the methyl group. In wines, this pattern was correlated with both air temperature and δ¹⁸O of wine water, indicating that water vapour deficit may be a critical defining factor. Furthermore, in C₄‐ethanol, the reverse relationship was observed (methylene‐C relatively ¹³C‐depleted), supporting the concept that photorespiration is the key metabolic process leading to the ¹³C distribution in C₃‐ethanol. By contrast, in CAM‐ethanol, the isotopic pattern was similar to but stronger than C₃‐ethanol, with a relative ¹³C‐enrichment in the methylene‐C of up to 13‰. Plausible causes of this ¹³C‐pattern are briefly discussed. As the intramolecular δ¹³C_i‐values in ethanol reflect that in source glucose, our data point out the crucial impact on the ratio of metabolic pathways sustaining glucose synthesis.     

Inorganic carbon supply to algae submerged in acid wetland pools: Analysis using the natural abundance of carbon isotopes

Summary

The inorganic C supply to macroalgae in two acid pools in a highland wetheath was analysed using ¹³C/¹²C natural abundance measurements. The inorganic C in the pools (pH 3.9 – 4.5) is all present as CO₂, and is more than three times the air-equilibrium concentration. The ¹³C/¹²C value of the pool CO₂ was predicted from the ¹³C/¹²C of rainwater CO₂ and that of CO ₂derived from terrestrial plant (peat) respiration in a quantity adequate to account for the over- saturation of CO₂. This ¹³C/¹²C value, at least in one pool, is lower than the measured value; this could relate to preferential removal of ¹²CO₂ in submerged photosynthesis and methanogenesis in producing the measured CO₂ concentration and ¹³C/¹²C in the pool. The algae Batrachospermum keratophytum and Mougeotia capucina appear to be dependent on CO₂ diffusion followed by C₃ biochemistry; the algal ¹³C/¹²C ratio relative to pool CO₂ ¹³C/ ¹²C predicts a fractional limitation of photosynthesis by CO₂ diffusion of 0.85 – 0.96. This is much higher than the limitation of photosynthesis by CO₂ diffusing in algae in lotic environments with similar CO₂ oversaturation values, presumably due to the thicker diffusion boundary layers in the lentic pool environment.     

Controlling for anthropogenically induced atmospheric variation in stable carbon isotope studies

Increased use of stable isotope analysis to examine food-web dynamics, migration, transfer of nutrients, and behavior will likely result in expansion of stable isotope studies investigating human-induced global changes. Recent elevation of atmospheric CO₂ concentration, related primarily to fossil fuel combustion, has reduced atmospheric CO₂ δ¹³C (¹³C/¹²C), and this change in isotopic baseline has, in turn, reduced plant and animal tissue δ¹³C of terrestrial and aquatic organisms. Such depletion in CO₂ δ¹³C and its effects on tissue δ¹³C may introduce bias into δ¹³C investigations, and if this variation is not controlled, may confound interpretation of results obtained from tissue samples collected over a temporal span. To control for this source of variation, we used a high-precision record of atmospheric CO₂ δ¹³C from ice cores and direct atmospheric measurements to model modern change in CO₂ δ¹³C. From this model, we estimated a correction factor that controls for atmospheric change; this correction reduces bias associated with changes in atmospheric isotopic baseline and facilitates comparison of tissue δ¹³C collected over multiple years. To exemplify the importance of accounting for atmospheric CO₂ δ¹³C depletion, we applied the correction to a dataset of collagen δ¹³C obtained from mountain lion (Puma concolor) bone samples collected in California between 1893 and 1995. Before correction, in three of four ecoregions collagen δ¹³C decreased significantly concurrent with depletion of atmospheric CO₂ δ¹³C (n ≥ 32, P ≤ 0.01). Application of the correction to collagen δ¹³C data removed trends from regions demonstrating significant declines, and measurement error associated with the correction did not add substantial variation to adjusted estimates. Controlling for long-term atmospheric variation and correcting tissue samples for changes in isotopic baseline facilitate analysis of samples that span a large temporal range.     

An NMR study of the loss of carbon-20 in the biosynthesis of gibberellin A3 by Gibberella fujikuroi

Resuspension cultures of Gibberella fujikuroi, strain GF-1a, were shown to metabolise potassium [3′-¹³C] mevalonate to ¹³C-enriched C₁₉-gibberellins, plus ¹³CO₂ (derived from the loss of carbon-20). The formation of [¹³C]-gibberellins could be observed in vivo using ¹³C NMR; however that of ¹³CO₂ could not. In contrast, removal of the mycelium and concentration of the filtrate at pH 12 enabled the ¹³CO₂ produced to be observed using ¹³C NMR. During incubations of H¹⁴CO₂Na with this fungus, complete conversion to other radioactive products was observed, and the significance of these results in the light of previous work is discussed.     

Variation in bulk‐leaf 13C discrimination,leaf traits and water‐use efficiency–trait relationships along a continental‐scale climate gradient in Australia

Large spatial and temporal gradients in rainfall and temperature occur across Australia. This heterogeneity drives ecological differentiation in vegetation structure and ecophysiology. We examined multiple leaf‐scale traits, including foliar ¹³C isotope discrimination (Δ¹³C), rates of photosynthesis and foliar N concentration and their relationships with multiple climate variables. Fifty‐five species across 27 families were examined across eight sites spanning contrasting biomes. Key questions addressed include: (i) Does Δ¹³C and intrinsic water‐use efficiency (WUE_i) vary with climate at a continental scale? (ii) What are the seasonal and spatial patterns in Δ¹³C/WUE_i across biomes and species? (iii) To what extent does Δ¹³C reflect variation in leaf structural, functional and nutrient traits across climate gradients? and (iv) Does the relative importance of assimilation and stomatal conductance in driving variation in Δ¹³C differ across seasons? We found that MAP, temperature seasonality, isothermality and annual temperature range exerted independent effects on foliar Δ¹³C/WUE_i. Temperature‐related variables exerted larger effects than rainfall‐related variables. The relative importance of photosynthesis and stomatal conductance (g_s) in determining Δ¹³C differed across seasons: Δ¹³C was more strongly regulated by g_s during the dry‐season and by photosynthetic capacity during the wet‐season. Δ¹³C was most strongly correlated, inversely, with leaf mass area ratio among all leaf attributes considered. Leaf N_mass was significantly and positively correlated with MAP during dry‐ and wet‐seasons and with moisture index (MI) during the wet‐season but was not correlated with Δ¹³C. Leaf P_mass showed significant positive relationship with MAP and Δ¹³C only during the dry‐season. For all leaf nutrient‐related traits, the relationships obtained for Δ¹³C with MAP or MI indicated that Δ¹³C at the species level reliably reflects the water status at the site level. Temperature and water availability, not foliar nutrient content, are the principal factors influencing Δ¹³C across Australia.     

Carbon-13 NMR studies of C2H5N13C binding to various hemoproteins

The addition of an excess of C₂H₅N¹³C to myoglobin and human adult and fetal hemoglobins, gives three characteristic NMR spectra with new ¹³C resonances respectively at δ = ?10,56 ppm, δ = ?7,03 and ?7,95 ppm and δ = ?6,28 and ?7,95 ppm (CH₃CO₂Na as external standard). These signals correspond to the C₂H₅N¹³C bound to the Fe(II) of the different heme units, according to CO exchange experiments. Characteristic resonances can be assigned to C₂H₅N¹³C bound to α, β and γ subunits. C₂H₅N¹³C appears as a more sensitive probe than ¹³CO for hemoprotein NMR studies.     

Phytogenic biosynthesis and emission of methyl acetate

Acetylation of plant metabolites fundamentally changes their volatility, solubility and activity as semiochemicals. Here we present a new technique termed dynamic ¹³C‐pulse chasing to track the fate of C_1–3 carbon atoms of pyruvate into the biosynthesis and emission of methyl acetate (MA) and CO₂. ¹³C‐labelling of MA and CO₂ branch emissions respond within minutes to changes in ¹³C‐positionally labelled pyruvate solutions fed through the transpiration stream. Strong ¹³C‐labelling of MA emissions occurred only under pyruvate‐2‐¹³C and pyruvate‐2,3‐¹³C feeding, but not pyruvate‐1‐¹³C feeding. In contrast, strong ¹³CO₂ emissions were only observed under pyruvate‐1‐¹³C feeding. These results demonstrate that MA (and other volatile and non‐volatile metabolites) derive from the C_2,3 atoms of pyruvate while the C₁ atom undergoes decarboxylation. The latter is a non‐mitochondrial source of CO₂ in the light generally not considered in studies of CO₂ sources and sinks. Within a tropical rainforest mesocosm, we also observed atmospheric concentrations of MA up to 0.6 ppbv that tracked light and temperature conditions. Moreover, signals partially attributed to MA were observed in ambient air within and above a tropical rainforest in the Amazon. Our study highlights the potential importance of acetyl coenzyme A (CoA) biosynthesis as a source of acetate esters and CO₂ to the atmosphere.     

Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the ¹³C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if ¹³C discrimination during respiration could contribute to this pattern, we compared δ¹³C signatures of respired CO₂ from sieved mineral soil, litter layer and litterfall with measurements of δ¹³C and δ¹⁵N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in δ¹³C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As ¹³C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ¹³C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ¹³C of atmospheric CO₂ since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO₂ explains only a portion of the additional 1.0‰ increase in δ¹³C below a depth of 20 cm. The δ¹³C of the respired CO₂ was similar to that of the organic matter in the upper soil layers but became increasingly ¹³C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this ¹³C enrichment of the CO₂ as well as the residual increase in δ¹³C of the organic matter below a soil depth of 20 cm results from the increased contribution of ¹³C-enriched microbially derived C with depth. Our results suggest that ¹³C discrimination during microbial respiration does not contribute to the ¹³C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ¹³C of respired CO₂ are used to separate different components of soil respiration or ecosystem respiration.     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Internode length in Pisum

The influence of the Na and Le genes in peas on gibberellin (GA) levels and metabolism were examined by gas chromatographic-mass spectrometric analysis of extracts from a range of stem-length genotypes fed with [¹³C, ³H]GA₂₀. The substrate was metabolised to [¹³C, ³H]GA₁, [¹³C, ³H]GA₈ and [¹³C, ³H]GA₂₉ in the immature, expanding apical tissue of all genotypes carrying Le. In contrast, [¹³C, ³H]GA₂₉ and, in one line, [¹³C, ³H]GA₂₉-catabolite, were the only products detected in plants homozygous for the le gene. These results confirm that the Le gene in peas controls the 3-hydroxylation of GA₂₀ to GA₁. Qualitatively the same results were obtained irrespective of the genotype at the Na locus. In all Na lines the [¹³C, ³H]GA₂₀ metabolites were considerably diluted by endogenous [¹²C]GAs, implying that the metabolism of [¹³C, ³H]GA₂₀ mirrored that of endogenous [¹²C]GA₂₀. In contrast, the [¹³C, ³H]GA₂₀ metabolites in na lines showed no dilution with [¹²C]GAs, confirming that the na mutation prevents the production of C₁₉-GAs. Estimates of the levels of endogenous GAs in the apical tissues of Na lines, made from the ¹²C:¹³C isotope ratios and the radioactivity recovered in respective metabolites, varied between 7 and 40 ng of each GA per plant in the tissue expanded during the 5 d between treatment with [¹³C, ³H]GA₂₀ and extraction. No [¹²C]GA₁ and only traces of [¹²C]GA₈ (in one line) were detected in the two Na le lines examined. These results are discussed in relation to recent observations on dwarfism in rice and maize.Abbreviations GA_n gibberellin A_n - GC-MS gas chromatography-mass spectrometry - HPLC high-pressure liquid chromatography     

Gibberellin A(3) Is Biosynthesized from Gibberellin A(20) via Gibberellin A(5) in Shoots of Zea mays L

[17-¹³C,³H]-Labeled gibberellin A₂₀ (GA₂₀), GA₅, and GA₁ were fed to homozygous normal (+/+), heterozygous dominant dwarf (D8/+), and homozygous dominant dwarf (D8/D8) seedlings of Zea mays L. (maize). ¹³C-Labeled GA₂₉, GA₈, GA₅, GA₁, and 3-epi-GA₁, as well as unmetabolized [¹³C]GA₂₀, were identified by gas chromatography-selected ion monitoring (GC-SIM) from feeds of [17-¹³C, ³H]GA₂₀ to all three genotypes. ¹³C-Labeled GA₈ and 3-epi-G₁, as well as unmetabolized [¹³C]GA₁, were identified by GC-SIM from feeds of [17-¹³C, ³H]GA₁ to all three genotypes. From feeds of [17-¹³C, ³H]GA₅, ¹³C-labeled GA₃ and the GA₃-isolactone, as well as unmetabolized [¹³C]GA₅, were identified by GC-SIM from +/+ and D8/D8, and by full scan GC-MS from D8/+. No evidence was found for the metabolism of [17-¹³C, ³H]GA₅ to [¹³C]GA₁, either by full scan GC-mass spectrometry or by GC-SIM. The results demonstrate the presence in maize seedlings of three separate branches from GA₂₀, as follows: (a) GA₂₀ → GA₁ → GA₈; (b) GA₂₀ → GA₅ → GA₃; and (c) GA₂₀ → GA₂₉. The in vivo biogenesis of GA₃ from GA₅, as well as the origin of GA₅ from GA₂₀, are conclusively established for the first time in a higher plant (maize shoots).     

Influence of Ozone on the Stable Carbon Isotope Composition, deltaC, of Leaves and Grain of Spring Wheat (Triticum aestivum L.)

The relative composition of stable carbon isotopes, δ¹³C, was determined in flag leaves and grain of spring wheat (Triticum aestivum L. cv Albis) grown in open-top field fumigation chambers and exposed to different O₃ levels during the growing season. The aim of the study was to establish exposure-response relationships for the radiation-weighted seasonal mean O₃ concentration and δ¹³C (relative deviation of the ¹³C/¹²C ratio) values of the two plant parts. Samples were collected at harvest in 1986, 1987, and 1988. With increasing O₃ concentration, δ¹³C values increased (became less negative) proportionally. Year to year δ¹³C differences at equivalent O₃ concentrations were small. The shift in δ¹³C caused by O₃ was more pronounced in grain than in leaves. According to models of ¹³C discrimination in C₃ plants, these results indicate increasing limitation of photosynthesis by CO₂ diffusion relative to limitation by carboxylation with increasing O₃ exposure. This conclusion is not in agreement with results from gas exchange analysis. Water use efficiency in green flag leaves tended to decrease with increasing O₃, indicating a dominating effect of O₃ on CO₂ carboxylation.     

Interpretation of soil δ13C as an indicator of vegetation change in African savannas

Question: The relationship between carbon‐13 in soil organic matter and C₃ and C₄ plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ¹³C data from soils cannot be used to infer past C₃ and C₄ plant abundance; only the proportion of carbon derived from C₃ and C₄ plants can be estimated. In this paper, we compare δ¹³C of surface soil samples with vegetation data, in order to establish whether the ratio of C₃:C₄ plants (rather than the proportion of carbon from C₃ and C₄ plants) can be inferred from soil δ¹³C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ¹³C of organic matter in surface soil samples and derive regression equations relating the δ¹³C of soil organic matter to C₃:C₄ plant abundance. We use these equations to interpret δ¹³C data from soil profiles in terms of changes in inferred C₃:C₄ plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ¹³C of organic matter in surface soil samples and the natural logarithm of the ratio of C₃:C₄ plants in the 100m² surrounding the soil sample. Conclusions: We suggest that interpretation of δ¹³C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ¹³C of organic matter in surface soil samples. Our results suggest that past C₃ plant abundance might be under‐estimated if a mass balance approach is used.     

Malate Metabolism in the Dark After CO(2) Fixation in the Crassulacean Plant Kalanchoë tubiflora

The metabolism of [¹³C]malate was studied in the Crassulacean plant Kalanchoë tubiflora following exposure to ¹³CO₂ for 2 hour intervals during a 16 hour dark cycle. Nuclear magnetic resonance spectroscopy of [¹³C]malate extracted from labeled tissue revealed that the transient flux of malate to the mitochondria, estimated by the randomization of [4-¹³C]malate to [1- ¹³C]malate by fumarase, varied substantially during the dark period. At both 15 and 25°C, the extent of malate label randomization in the mitochondria was greatest during the early and late parts of the dark period and was least during the middle of the night, when the rate of ¹³CO₂ uptake was highest. Randomization of labeled malate continued for many hours after malate synthesis had initially occurred. Internally respired ¹²CO₂ also served as a source of carbon for malate formation. At 15°C, 15% of the total malate was formed from respired ¹²CO₂, while at 25°C, 49% of the accumulated malate was derived from respired ¹²CO₂. Some of the malate synthesized from external ¹³CO₂ was also respired during the night. The proportion of the total [¹³C]malate respired during the dark period was similar at 15 and 25°C, and respiration of newly formed [¹³C]malate increased as the night period progressed. These data are discussed with regard to the relative fluxes of malate to the mitochondria and the vacuole during dark CO₂ fixation.     

Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

Microbial mats that inhabit gypsum deposits in ponds at Guerrero Negro, Baja California Sur, Mexico, developed distinct pigmented horizons that provided an opportunity to examine the fixation and flow of carbon through a trophic structure and, in conjunction with previous phylogenetic analyses, to assess the diagenetic fates of molecular δ¹³C biosignatures. The δ¹³C values of individual biomarker lipids, total carbon, and total organic carbon (TOC) were determined for each of the following horizons: tan‐orange (TO) at the surface, green (G), purple (P), and olive‐black (OB) at the bottom. δ¹³C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ¹³C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO₂ fixation. δ¹³C_TOC of the underlying G was 3‰ greater than that of TO. The most δ¹³C‐depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C₁₇ n‐alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C₂₀ isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ?[δ¹³C_TOC ? δ¹³C_∑IPFA] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that ¹³C‐enriched residual organic matter accumulated at depth. The permanently anoxic P horizon was dominated by anoxygenic phototrophs and sulfate‐reducing bacteria. P hosted an active sulfur‐dependent microbial community. IPFA and bishomohopanol were ¹³C‐depleted relative to upper crust by 7 and 4‰, respectively, and C₂₀ isoprenoids were somewhat ¹³C‐enriched. Synthesis of alkanes in P was evidenced only by ¹³C‐depleted n‐octadecane and 8‐methylhexadecane. In OB, the marked increase of total inorganic carbon δ¹³C (δ¹³C_TIC) of >6‰ perhaps indicated terminal mineralization. This δ¹³C_TIC increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of ¹³C‐depleted methane from the mat.