Evidence for the synthesis of the multi-positional isomers of monounsaturated fatty acid in Methylococcus capsusatus by the anaerobic pathway

Abstract The biosynthesis of the positional isomers of the monounsaturated fatty acids of Methylococcus capsulatus (Bath) has been investigated by studying the incorporation of [2-¹⁴C]malonyl CoA into long-chain fatty acids in vitro. The major unsaturated products were Δ ⁹ 16:1 and Δ ¹¹ 18:1; however, Δ ⁸, Δ ¹⁰ and Δ ¹¹ 16:1, as well as, Δ ¹⁰, Δ ¹² and Δ ¹³ 18:1 were also synthesized. The exclusion of O₂ from the reaction vessel did not affect the synthesis of unsaturated fatty acids or the double bonds positions. Cerulenin inhibited the synthesis of unsaturated fatty acid more than saturated fatty acid. The use of both [1-¹⁴C] octanoate and [1-¹⁴C] decanoate as substrate resulted in the synthesis of long-chain fatty acids, however, unsaturates were only synthesized from octanoate. These results imply that the unique positional isomers of M. capsulatus are not synthesized by an aerobic mechanism.     

Release of Neurotransmitter Amino Acids from Synaptosomes: Enhancement of Calcium-Independent Efflux by Oleic and Arachidonic Acids

Abstract: The release of preloaded [¹⁴C]neuroactive amino acids (glutamic acid, proline, γ-aminobutyric acid) from rat brain synaptosomes can occur via a time-dependent, Ca²⁺ -independent process. This Ca²⁺-independent efflux is increased by compounds that activate Na⁺ channels (veratridine, scorpion venoms), by the ionophore gramicidin D, and by low concentrations of unsaturated fatty acids (oleic acid and arachidonic acid). Saturated fatty acids have no effect on the efflux process. Neither saturated nor unsaturated fatty acids have an effect on the release of [¹⁴C]leucine, an amino acid not known to possess neurotransmitter properties. The increase in the efflux of neuroactive amino acids by oleic and arachidonic acids can also be demonstrated using synaptosomal membrane vesicles. Under conditions in which unsaturated free fatty acids enhance amino acid efflux, no effect on ²²Na⁺ permeability is observed. Since Na⁺ permeability is not altered by fatty acids, the synaptosomes are not depolarized in their presence and, thus, the Na⁺ gradient can be assumed to be undisturbed. We conclude that unsaturated fatty acids represent a potentially important class of endogenous modulators of neuroactive amino acid transport in nerve endings and further postulate that their action is the result of an uncoupling of amino acid transport from the synaptosomal Na⁺ gradient.     

Temperature-induced Changes in the Synthesis of Unsaturated Fatty Acids by Acanthamoeba castellanii

ABSTRACT. Major fatty acid components of Acanthamoeba castellanii lipids extracted after growth at 30°C include myristate, palmitate, stearate and the polyunsaturates linoleate, eicosadienoate, eicosatrienoate and arachidonate, with oleate as the sole major monounsaturated fatty acid. By comparison, growth at 15°C gave increased linoleate, eicosatrienoate and arachidonate, but decreased oleate and palmitate. When the growth temperature was shifted downwards from 30°C to 15°C, increased lipid unsaturation occurred over a period of 24 h; thus decreases of oleate and eicosadienoate were accompanied by increases in linoleate, eicosatrienoate, arachidonate and eicosapentaenoate. An upwards shift from 15°C to 30°C gave negligible alterations in fatty acid composition over a similar period. At 15°C organisms rapidly use [1-¹⁴C] acetate for de novo fatty acid synthesis; stearate is converted via oleate to further desaturation and chain elongation products. Similar short term experiments at 30°C indicate only de novo synthesis and Δ9-desaturation; synthesis of polyunsaturates was a much slower process. Rapid incorporation of [1-¹⁴C] oleate at 30°C was not accompanied by metabolic conversion over two hours, whereas at 15°C n-6 desaturation to linoleate was observed. Temperature shift of organisms from 15°C to 30°C in the presence of [1-¹⁴C] acetate revealed that over half of the fatty acids in newly-synthesised lipids were saturated, but the proportions of unsaturated fatty acids increased with time until the total polyenoate components reached 17% after 22 h. A shift of temperature in the reverse direction gave a corresponding figure of 60% for polyunsaturated fatty acids. These results emphasize the importance of n-6 desaturation in the low temperature adaptation of Acanthamoeba castellanii .     

Role of Acetyl-l-Carnitine in Rat Brain Lipogenesis: Implications for Polyunsaturated Fatty Acid Biosynthesis

Abstract: This study was undertaken to explore the metabolic fate of acetyl- l -carnitine in rat brain. To measure the flux of carbon atoms into anabolic processes occurring at regional levels, we have injected [1-¹⁴C]acetyl- l -carnitine into the lateral brain ventricle of conscious rats. After injection of [1-¹⁴C]acetyl- l -carnitine, the majority of radioactivity was recovered as ¹⁴CO₂ expired (60% of that injected). The percentage of radioactivity recovered in brain was 1.95, 1.60, 1.30, and 0.93% at 1, 3, 6, and 22 h, respectively. Radioactivity distribution in various lipid components indicated that the fatty acid moiety of phospholipid contained the majority of radioactivity. The radioactive profile of these fatty acids showed that the acetyl moiety of acetyl- l -carnitine was incorporated into saturated (60%), monounsaturated (15%), and polyunsaturated (25%) fatty acids [mainly present in 20:4 (5.2%) and 22:6 (7.8%)]. Injection in the brain ventricle of radioactive glucose, the major source of acetyl-CoA in the CNS, revealed that glucose was a precursor of saturated (85%) and monounsaturated (15%) but not of polyunsaturated fatty acids. Thus, this study demonstrated distinct fates of glucose and acetyl- l -carnitine following intracerebroventricular injection. In summary, these data implicate acetyl- l -carnitine as an important member of a complex acetate trafficking system in brain lipid metabolism.     

Docosahexaenoic acid synthesis from n-3 fatty acid precursors in rat hippocampal neurons

Docosahexaenoic acid (DHA), the most abundant n-3 polyunsaturated fatty acid in the brain, has important functions in the hippocampus. To better understand essential fatty acid homeostasis in this region of the brain, we investigated the contributions of n-3 fatty acid precursors in supplying hippocampal neurons with DHA. Primary cultures of rat hippocampal neurons incorporated radiolabeled 18-, 20-, 22-, and 24-carbon n-3 fatty acid and converted some of the uptake to DHA, but the amounts produced from either [1-¹⁴C]α-linolenic or [1-¹⁴C]eicosapentaenoic acid were considerably less than the amounts incorporated when the cultures were incubated with [1-¹⁴C]22:6n-3. Most of the [1-¹⁴C]22:6n-3 uptake was incorporated into phospholipids, primarily ethanolamine phosphoglycerides. Additional studies demonstrated that the neurons converted [1-¹⁴C]linoleic acid to arachidonic acid, the main n-6 fatty acid in the brain. These findings differ from previous results indicating that cerebral and cerebellar neurons cannot convert polyunsaturated fatty acid precursors to DHA or arachidonic acid. Fatty acid compositional analysis demonstrated that the hippocampal neurons contained only 1.1–2.5 mol% DHA under the usual low-DHA culture conditions. The relatively low-DHA content suggests that some responses obtained with these cultures may not be representative of neuronal function in the brain.     

The Effect of Temperature on the Fatty Acid Composition of Tetrahymena pyriformis WH-14

SYNOPSIS. A reduction in the growth temperature of Tetrahymena pyriformis strain WH-14 from 35 C to 15 C resulted in distinct alterations in the fatty acid composition of the glycerophospholipids. The proportion of normal saturated acids declined from 26 to 19%; palmitoleic acid increased by 6%, and the composition of the polyunsaturated fatty acids increased in 18:2 Δ^6,11(n) and decreased in 18:2 Δ^9,12(n) and 18:3 Δ^6,9,12(n). The unsaturation index (the average number of double bonds/100 molecules) did not change with a shift in temperature.
Two biosynthetic pathways exist in Tetrahymena for the formation of unsaturated fatty acids. The observed changes in fatty acid composition that accompany a lowering of the environmental temperature can be accounted for by a reduction in the accumulation of products of the fatty acid pathway leading to the formation of γ-linolenic acid [16:0(n) → 18:0(n) → 18:1 Δ⁹(n) → 18:2 Δ^9,12(n) → 18:3 Δ^6,9,12(n)] and an increase in the components of the pathway leading to the formation of 18:2 Δ^6,11(n) [16:0(n) → 16:1 Δ⁹(n) → 18:1 Δ¹¹(n) → 18:2 Δ^6,11(n)]. The data suggest that the regulatory mechanism in Tetrahymena differs from that found in some bacteria where a simple substitution of unsaturated fatty acids for saturated fatty acids occurs at low culture temperatures.     

On the Role of Long-Chain Aldehydes in Mammalian Plasmalogen Biosynthesis

Abstract: [1-³H, 1-¹⁴C]Palmitaldehyde(³H:¹⁴C= 15) was injected intracerebrally to 18-day-old rats and incorporation of radioactivity into brain lipids was followed over a 24-h period. The substrate was metabolized primarily by oxidation to palmitic acid with loss of tritium and, to a lesser extent, by reduction to hexadecanol. The alkyl moieties of the ethanolamine phospholipids showed considerably lower ³H:¹⁴C ratios than the substrate, indicating a substantial participation in ether lipid synthesis by tritium-free alcohols derived from ¹⁴C-labeled fatty acids. Virtually no ³H radioactivity was found in alkenyl moieties, indicating stereospecificity of both reduction of aldehyde and dehydrogenation of alkyl to alkenyl glycerolipid. The data are consistent with the general concept that plasmalogen biosynthesis proceeds exclusively through fatty alcohols and alkyl glycerolipids and that fatty aldehydes cannot be utilized directly.     

Fatty Acids from Degenerating Myelin Lipids Are Conserved and Reutilized for Myelin Synthesis During Regeneration in Peripheral Nerve

Abstract: Following nerve crush, cholesterol from degenerating myelin is conserved and reutilized for new myelin synthesis during nerve regeneration. The possibility that other myelin lipids are salvaged and reutilized has not been investigated previously. We examined the fate of myelin phospholipids and their fatty acyl moieties following nerve crush by electron microscopic autoradiography of myelin lipids prelabeled with [³H]oleate or [2-³H]-glycerol. Both precursors were incorporated predominantly (>90%) into phospholipids; >85% of the [³H]oleate was incorporated as oleate, with the remainder in longer-chain fatty acids. Before nerve crush, both labels were restricted to myelin sheaths. Following nerve crush and subsequent regeneration, over half the label from [³H]oleate, but little from [2-³H]glycerol, remained in nerve. The oleate label was present as fatty acyl moieties in phospholipids and was localized to newly formed myelin sheaths. Among the extracellular soluble lipids within the degenerating nerve, the bulk of the labeled phospholipids floated at the same density as lipoprotein particles. These data indicate that myelin phospholipids are completely hydrolyzed during nerve degeneration, that at least half the resultant free fatty acids are salvaged and reutilized for new myelin synthesis, and that these salvaged fatty acids are transported by a lipoprotein-mediated mechanism similar to that functioning in cholesterol reutilization.     

Carbon isotope-labelling experiments indicate that ladderane lipids of anammox bacteria are synthesized by a previously undescribed, novel pathway

Ladderane lipids are unusual membrane lipids of bacteria that anaerobically oxidize ammonium to dinitrogen gas (anammox). Ladderane lipids contain linearly concatenated cyclobutane rings for which the pathway of biosynthesis is currently unknown. To investigate the possible biosynthetic routes of these lipids, 2-¹³C-labelled acetate was added to a culture of the anammox bacterium Candidatus Brocadia fulgida. Labelling patterns obtained by high-field ¹³C nuclear magnetic resonance spectroscopy of isolated lipids indicated that C . Brocadia fulgida synthesizes C_16:0 and iso C_16:0 fatty acids according to the known pathway of type II fatty acid biosynthesis. The ¹³C-labelling pattern of the C₈ alkyl chain of the C₂₀ [3] ladderane monoether also indicated the use of this route. However, carbon atoms in the cyclobutane rings and the cyclohexane ring were nonspecifically labelled and did not correspond to known patterns of fatty acid synthesis. Taken together, our results indicate that it is unlikely that ladderane lipids are formed from the cyclization of polyunsaturated fatty acids as hypothesized previously and suggest an alternative, although as yet unknown, pathway of biosynthesis.     

Accumulation of N-3 Polyunsaturated Fatty Acids by Cultured Human Y79 Retinoblastoma Cells

Abstract: The metabolism of the n-3 class of polyunsaturated fatty acids, which occur in relatively high quantities in neural tissues, was studied in human Y79 retinoblastoma cells. These cells contained low levels of n-3 polyunsaturates when grown in culture media supplemented with fetal bovine serum. The cells readily incorporated preformed docosahexaenoic acid (22:6 n-3) into phospholipids, but human skin fibroblasts did this to a similar extent. When 10 to 30 μmol/ml linolenic acid (18:3 n-3) was added, the cells also accumulated 22:6 in phospholipids. The capacity to convert appreciable amounts of 18:3 to 22:6 appears to be a unique property of the retinoblastoma cells as compared with other continuously cultured cell lines. More 18:3 than linoleic acid (18:2 n-6) was incorporated into phospholipids by the retinoblastoma cultures, and 18:3 was channeled to a larger extent into the ethanolamine glycerophospholipid fraction. These findings indicate that retinoblastoma cells handle n-3 polyunsaturated fatty acids in a manner very similar to neural tissue in vivo . Based on the results obtained with this model system, it appears that three processes may contribute to the accumulation of 22:6 in retina and neural tissue: increased ability to incorporate 18:3, the capacity to convert 18:3 to 22:6, and channeling of 18:3 and its metabolites into ethanolamine glycerophospholipids.     

Portacaval Anastomosis Results in Altered Neuron-Astrocytic Metabolic Trafficking of Amino Acids: Evidence from 13C-NMR Studies

Abstract: ¹³C-NMR spectroscopy was used to evaluate the dynamic consequences of portacaval anastomosis on neuronal and astrocytic metabolism and metabolic trafficking between neurons and astrocytes. Glutamate is predominantly labeled from [1-¹³C]glucose, whereas [2-¹³C]acetate is more efficient in labeling glutamine, in accordance with its primary metabolism in astrocytes. Alanine and succinate labeling was only observed with [1-¹³C]glucose as precursor. Brain [1-¹³C]glucose metabolism in portacaval-shunted rats was similar to that in sham-operated controls with the exception of labeled glutamine and succinate formation, which was increased in shunted rats. The ¹³C enrichment was, however, decreased owing to an increase in total glutamine and succinate. Using [2-¹³C]acetate, on the other hand, flux of astrocytic label to neurons was severely decreased because label incorporation into glutamate, aspartate, and GABA was decreased following portacaval shunting. The latter amino acids are predominantly localized in neurons. These findings demonstrate that metabolic trafficking of amino acids from astrocytes to neurons is impaired in portacaval-shunted rats.     

[2-3H]GLYCEROL AS A PRECURSOR OF PHOSPHOLIPIDS IN RAT BRAIN: EVIDENCE FOR LACK OF RECYCLING

Abstract— When [2-³H]glycerol was injected intracranially into young rats, it was presented as a pulse label, leaving the brain rapidly and giving up much of its labelled hydrogen to water. [2-³H]glycerol was efficiently incorporated into brain lipids, especially into choline and ethanolamine phospholipids. Following injection of a mixture of [³H]- and [¹⁴C]-labelled glycerol, the ratio of ³H to ¹⁴C in the phospholipids of both whole brain and the microsomal fraction decreased as a function of time after injection. This finding indicated less recycling of the tritium label. This lack of recycling was further indicated by the finding that 94 per cent of the tritium label of phosphatidyl choline was in the glycerol portion of the molecule rather than in the fatty acids. At 2 weeks following injection with [³H]glycerol, 93 per cent of the total radioactivity in brain appeared in the lipid fraction. In contrast, following injection with [¹⁴C]glycerol, only 57 per cent of the radioactivity appeared in lipid, with about 20 per cent in protein.     

Mammalian cerebral metabolism and amino acid neurotransmission during hibernation

This report demonstrates that during the torpor phase of hibernation, hamsters utilize ¹⁴C and ¹³C glucose in torpor-specific brain metabolic pathways. Microdialysis of ¹⁴C glucose into the striatum rapidly induced a steady state labeling of extracellular fluid (ECF) lactate and labeling of tissue GABA, glutamate, glutamine, and alanine in ipsilateral and contralateral striata. The same tissue metabolites were labeled in cortex, hypothalamus, and brainstem after microdialysis of ¹⁴C lactate into the lateral ventricle. Serine, aspartate, glycine, taurine, tyrosine, and methionine were not synthesized from glucose or lactate during torpor. ECF levels of amino and organic acids were low and unchanging during torpor and increased late during arousal to cenothermia. Labeled intracellular ¹⁴C GABA and glutamate were not communicated to the striatal ECF or ventricular space during torpor. ¹³C NMR demonstrated rapid formation of lactate and functional tricarboxylic acid cycles in GABAergic and glutamatergic neurons, and enrichment of glutamine and alanine after i.v. ¹³C glucose. Large changes in tissue levels of amino acids occur prior to or during entrance into torpor but not during torpor. It is proposed that cerebral intracellular dehydration, the enlargement of ECF and the biochemistries associated with brain water homeostasis may have a role in regulating hibernation.     

Links between methanotroph community composition and CH4 oxidation in a pine forest soil

The main gap in our knowledge about what determines the rate of CH₄ oxidation in forest soils is the biology of the microorganisms involved, the identity of which remains unclear. In this study, we used stable-isotope probing (SIP) following ¹³CH₄ incorporation into phospholipid fatty acids (PLFAs) and DNA/RNA, and sequencing of methane mono-oxygenase ( pmoA ) genes, to identify the influence of variation in community composition on CH₄ oxidation rates. The rates of ¹³C incorporation into PLFAs differed between horizons, with low ¹³C incorporation in the organic soil and relatively high ¹³C incorporation into the two mineral horizons. The microbial community composition of the methanotrophs incorporating the ¹³C label also differed between horizons, and statistical analyses suggested that the methanotroph community composition was a major cause of variation in CH₄ oxidation rates. Both PLFA and pmoA -based data indicated that CH₄ oxidizers in this soil belong to the uncultivated 'upland soil cluster α'. CH₄ oxidation potential exhibited the opposite pattern to ¹³C incorporation, suggesting that CH₄ oxidation potential assays may correlate poorly with in situ oxidation rates. The DNA/RNA-SIP assay was not successful, most likely due to insufficient ¹³C-incorporation into DNA/RNA. The limitations of the technique are briefly discussed.     

Changes in Membrane Fatty Acid Composition and Δ12-Desaturase Activity during Growth of Acanthamoeba castellanii in Batch Culture

ABSTRACT. Growth of Acanthamoeba castellanii in batch culture at 30° C was associated with marked changes in cellular fatty acid composition. The largest change occurred in the linoleate to oleate ratio, which was maximal in early- to mid-exponential phase cultures but decreased approximately 10-fold as cells approached stationary phase. The higher degree of lipid unsaturation in young cultures was accentuated by a greater proportion of 20-carbon polyunsaturated fatty acids than in stationary phase cultures. The unsaturation index (average number of double bonds per fatty acid) was maximal in mid-exponential phase cultures after 24 hours growth. Incorporation of [1-¹⁴C]acetate into polyunsaturated fatty acids in short-term (2 hour) experiments was high in 12 and 24 hour old cultures, where linoleate and eicosadienoate accounted for up to 26% of total labelled fatty acids. Incorporation of [1-¹⁴CJacetate into these fatty acids was negligible in stationary phase cultures. These results were correlated with changes in the specific activity of the Δ12-desaturase. Δ12-Desaturase activity was greatest in microsomal membranes isolated from early- to mid-exponential phase cells, but declined by approximately 50% as cultures progressed towards stationary phase. Membrane fractionation studies revealed that although some differences in fatty acid composition between plasma-membrane, mitochondrial (enriched), and microsomal membrane fractions were evident, the large changes in lipid unsaturation in whole cells of A. castellanii could not be accounted for by differential development of particular subcellular membranes.     

Starvation and low feeding levels result in an enrichment of 13C in lipids and 15N in protein of Nile tilapia Oreochromis niloticus L.

The influence of different feeding levels below and slightly above maintenance on whole body δ¹³C and δ¹⁵N values of Nile tilapia Oreochromis niloticus was examined. The energy budget of each fish was determined by indirect calorimetry. The δ¹³C values of the lipid-free material of Nile tilapia fed below and slightly above maintenance level did not differ between the feeding groups, but the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material showed small but significant differences. Those fish with a negative lipid retention had significantly higher δ¹³C values in the lipid fraction compared to fish that synthesized fatty acids. There was a significant negative correlation between the amount of energy metabolized by the fish and both the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material. Fasting and feeding below the maintenance level may influence the isotopic composition of animals and should therefore be considered in ecological and nutritional studies.     

COMPARTMENTATION AND LABELLING OF AMINO ACIDS IN THE DEVELOPING RAT BRAIN AFTER INJECTION OF [U-14C]RIBOSE

Abstract— [U-¹⁴C]Ribose was given by subcutaneous injection to young rats aged 2–56 days. During the first week after birth ¹⁴C in the brain was found mainly combined in glucose, fructose and sedoheptulose which contained 46–57 per cent of the ¹⁴C in the acid soluble metabolites in the rat brain. In contrast, during the critical period (10–15 days after birth) the ¹⁴C in the free sugars decreased from 24 to 3 per cent, while the ¹⁴C content of amino acids in the brain increased from 11 to 44 per cent of the total perchloric acid-soluble ¹⁴C. The increase in labelling of amino acids during the critical period was attributed to increased glycolysis and increased oxidation of pyruvate. The relative specific radioactivity of y -aminobutyrate and aspartate in the rat brain at 28 days after birth was equal to or greater than the relative specific radioactivity of glutamate. Assuming that the increase in amino acid content following the cessation of cell proliferation in the brain is located mainly in cell processes (cytoplasm of axons, dendrites, glial processes and nerve terminals), tentative values were estimated for the pool sizes of glutamate, glutamine, aspartate and y -amino butyrate.     

Regulation of n-3 and n-6 Fatty Acid Metabolism in Retinal and Cerebral Microvascular Endothelial Cells by High Glucose

Abstract: The present study was undertaken to determine whether polyunsaturated fatty acid metabolism is affected by high glucose levels in cerebral and retinal microvascular endothelial cells. The metabolism of [3-¹⁴C]22:5n-3 and [1-¹⁴C]18:2n-6 was studied in cells previously cultured for 5 days in normal (5 m M ) or high (30 m M ) glucose medium. After incubation of retinal endothelial cells with [3-¹⁴C]22:5n-3 in the high glucose condition, the formation of labeled 24:6n-3 and 22:6n-3 was increased, and that of labeled 24:5n-3 was decreased, compared with the normal glucose condition. The changes were found for fatty acids esterified in cellular lipids and those released into the medium. After incubation with [1-¹⁴C]18:2n-6, levels of all elongation/desaturation products were increased at the expense of the precursor in retinal endothelial cells cultured in high glucose medium. The changes were primarily found for esterified fatty acids, with the release of n-6 fatty acids being minor in both glucose concentrations. By contrast, high glucose levels did not affect the metabolism of [3-¹⁴C]22:5n-3 and [1-¹⁴C]18:2n-6 in cerebral endothelial cells. The changes in metabolic activity of retinal endothelial cells were not reflected in the fatty acid composition. The present data suggest that high glucose can increase the desaturation process in retinal but not cerebral endothelial cells. This may produce some lipid abnormalities in retinal microvasculature and contribute to altered vascular function observed in diabetic retinopathy.     

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.