Functional importance of the family 1 glucosyltransferase UGT72B1 in the metabolism of xenobiotics in Arabidopsis thaliana

The Arabidopsis type 1 UDP-glucose-dependent glucosyltransferase UGT72B1 is highly active in conjugating the persistent pollutants 3,4-dichloroaniline (DCA) and 2,4,5-trichlorophenol (TCP). To determine its importance in detoxifying xenobiotics in planta, mutant plants where the respective gene has been disrupted by T-DNA insertion have been characterized. Extracts from the knockout ugt72B1 plants showed radically reduced conjugating activity towards DCA and TCP and the absence of immunodetectable UGT72B1 protein. In contrast, activities towards phenolic natural products were unaffected. When aseptic root cultures were fed [14C]-DCA, compared with wild types, the ugt72B1 plants showed a reduced rate of uptake of the xenobiotic and very little metabolism to soluble DCA-glucose or associated polar conjugates. Instead, the knockouts accumulated non-extractable radioactive residues, most probably associated with lignification. When the feeding studies were carried out with [14C]-TCP, rates and routes of metabolism were identical in the wild type and knockouts, with TCP-glucoside a major product in both cases. Similar differential effects on the metabolism of DCA and TCP were obtained in whole plant studies with wild type and ugt72B1 mutants, demonstrating that while UGT72B1 had a central role in metabolizing chloroanilines in Arabidopsis, additional UGTs could compensate for the conjugation of TCP in the knockout. TCP was equally toxic to wild type and ugt72B1 plants, while surprisingly, the knockouts were less sensitive to DCA. From this it was concluded that the glucosylation of DCA may not be as effective in xenobiotic detoxification as bound-residue formation.     

Effect of UV-B radiation on the shoot dry matter production and stable carbon isotope composition of two Arabidopsis thaliana genotypes

An application of stable carbon isotope analysis to the mechanistic interpretation of ultraviolet-B (UV-B) effects on growth inhibition is described that is particularly useful for small plants such as Arabidopsis thaliana that are not well suited for gas exchange studies. Many investigators use tissue δ¹³C, relative abundance of ¹³C and ¹²C, as a proxy for water use efficiency and as an indicator of environmental effects on stomatal behaviour and on photosynthesis during growth. Discrimination against ¹³C is enhanced by both high stomatal conductance and damage to photosynthetic machinery. Because the thinning of the stratospheric ozone layer is permitting more UV-B to enter the biosphere, the mechanisms of action of UV-B radiation on plants are of particular current interest. Arabidopsis thaliana wild-type Landsberg erecta (L er ) and the UV-B-sensitive mutant fah I , deficient in UV-absorbing sinapate esters, were grown in a controlled environment and exposed to UV-B^BE doses of 0 or 6–7 kJ m⁻² day⁻¹. UV-B exposure decreased dry matter production and δ¹³C in both genotypes, but growth inhibition was generally greater in fah I than in L er . The fah I mutant also had less leaf greenness than L er . Changes in leaf tissue δ¹³C were detected before growth inhibition and were evident in treatments of both genotypes that did not cause marked growth effects. This suggests that the effects of UV-B contributing to increased carbon isotope discrimination in L er may have been primarily associated with high stomatal conductance, and in fah I with both high stomatal conductance and damage to photosynthetic machinery.     

Distribution and metabolism of root-applied cytokinins in Pisum sativum

When N ⁶ [8–¹⁴C] furfuryladenine was applied to the intact root system of Pisum sativum L. cv. Meteor seedlings it was almost completely metabolised to other compounds within 24 h. Of the total activity recovered from the plants 94.5% was retained in the root system itself. ¹⁴C was recovered in a number of ethanol-soluble compounds and in ribonucleic acid, deoxyribonucleic acid and protein fractions of roots, stems, leaves and axillary buds. In rapidly growing axillary buds released from apical dominance by removal of the shoot apex the combined nucleic acid fractions accounted for 63.3% of the total ¹⁴C recovered from these organs. Xylem exudate collected from decapitated plants 0 to 12 h after supplying N ⁵[8–¹⁴C]furfuryladenine to the roots consistently contained a single major ¹⁴C-labelled compound which, in three different solvent systems, had the same Rf values as a major endogenous cytokinin isolated from the xylem of unlabelled plants. The content of N ⁶ [8–¹⁴C] furfuryladenine itself in the xylem exudate was always low and in some experiments it could not be detected.
It is suggested that part of the label from N ⁶ [8- ¹⁴CJfurfuryladenine taken up by the intact root system may have become incorporated in an endogenous cylokinin before export to the shoot.     

Incorporation of label from root-applied N6[8-14C]furfiuryIadenine into the guanine nucleotide fraction of pea bud ribonucleic acid

The pattern of incorporation of label into the nucleotides of axillary bud ribonucleic acid was investigated in Pisum sativum L. cv. Meteor following the application of N ⁶[8-^I4C]furfuryladenine or of [8-¹⁴C]adenine to the root system of decapitated plants and to cultured excised buds. When N ⁶[8-¹⁴C]furifaryladenine was applied to the root system label was confined to the guanine nucleotide moiety of the axillary bud ribonucleic acid; label from [8-¹⁴C]adenine was incorporated preferentially into adenine nucleotide in the molar ratio adenine nucleotide/guanine nucleotide = 3.23. When isolated buds were incubated in media containing [8-¹⁴C]adenine or N ⁶[8-¹⁴C]furfuryladenine, label was incorporated into both purine moieties of the ribonucleic acid. However, the relative incorporation into the guanine nucleotide fraction was considerably greater for N ⁶[8-^I4C]furfuryladenine (adenine nucleotide/guanine nucleotide = 2.23) than for [8-¹⁴C]adenine (ratio = 4.67).
It was concluded that the pattern of metabolism of adenine to guanine and its incorporation into the guanine nucleotide moiety of pea axillary bud ribonucleic acid, is influenced by the presence of a substitution in the N ⁶ position of the adenine base.     

Effects of water stress and nocturnal temperature on carbon allocation in the perennial grass, Leymus chinensis

Water deficit and high temperature often occur simultaneously, but their effects on plants are usually investigated separately. The aim of this study was to test how interactions between water stress and nocturnal warming affect carbon allocation in the perennial grass, Leymus chinensis . Plant biomass, dry mass allocation, ¹⁴C partitioning and carbon isotope composition (δ¹³C) were measured. Severe and extreme water stress during nocturnal warming decreased the allocation of dry mass and ¹⁴C partitioning below ground to the roots, but moderate water stress significantly increased the below-ground allocation of dry mass and ¹⁴C, especially at the lower night temperature. The δ¹³C values were more positive at day/night temperatures of 30/20°C than at 30/25°C, and greater in the roots than in the leaves. By plotting the δ¹³C values of the leaves against the δ¹³C values of the roots, the slopes of regressions were steeper at low than at high night temperature, also indicating that nocturnal warming reduces carbon allocation below ground to the roots. The results suggest that nocturnal warming may weaken acclimation during water stress in this species by regulating carbon allocation between source and sink organs.     

Metabolism of Cysteine in Astroglial Cells: Synthesis of Hypotaurine and Taurine

Abstract: The synthesis of hypotaurine and taurine was investigated in astroglia-rich primary cultures obtained from brains of neonatal Wistar rats using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Cell extracts of astroglial cultures analyzed by ¹H NMR spectroscopy show prominent signals of hypotaurine. To identify cysteine as precursor for hypotaurine and taurine synthesis in astroglial cells, primary cultures were incubated with [3-¹³C]cysteine for 24 or 72 h. Cell extracts and incubation media were then analyzed with ¹³C NMR spectroscopy. Labeled hypotaurine, taurine, glutathione, and lactate were identified in the cell extracts. Within 72 h, 35.0% of the total intracellular hypotaurine and 22.5% of taurine were newly synthesized from [3-¹³C]cysteine. The presence of [1-¹³C]hypotaurine and [1-¹³C]taurine in the incubation medium proves the release of those products of cysteine metabolism into the medium. Minor amounts of the [3-¹³C]cysteine were used for the synthesis of glutathione in astroglial cells or metabolized to [3-¹³C]lactate, which was found in cell extracts and media. These results indicate that the formation of hypotaurine and taurine is a major pathway of cysteine metabolism in astroglial cells.     

The effects of reed canary grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) on wetland habitat and arthropod community composition in an urban freshwater wetland

Reed canary grass (Phalaris arundinacea L.) is an aggressive invader that dominates wetlands throughout the US. We examined the effects of reed canary grass on wetland habitat, both vegetation canopy architecture and soil environment, and its impacts the arthropod community in an urban wetland in Portland, OR, USA. Reed canary grass dominance resulted in reduced vegetation canopy complexity through reductions in native vegetation diversity and canopy height. In addition, reed canary grass dominance significantly changed the wetland soil environment, decreasing soil organic content and increasing soil moisture. The arthropod community responded to these habitat changes, being distinct between plots dominated by reed canary grass and those dominated by native vegetation. In addition, diversity measures were significantly lower in plots dominated by reed canary grass. Variables describing both vegetation canopy complexity and soil environment were more important predictors than relative abundance of reed canary grass in multiple regression models developed for dominant arthropod taxa and community metrics. Our results suggest that the mechanism by which reed canary grass affects the wetland arthropod community is primarily indirect, through habitat changes, rather than by directly altering its food source.     

Altered resource allocation during seed development in Arabidopsis caused by the abi3 mutation

The regulation of whole-plant resource allocation during seed development in Arabidopsis thaliana was investigated by examining growth rate and partitioning of ¹⁴CO₂ in wild-type plants and those carrying the abi3 mutation. Plants carrying the abi3 mutation partitioned more resources into seed development than the wild type. The extra resources were available as a result of delayed senescence of the cauline leaves in the mutant. After supply of ¹⁴CO₂ at later stages of reproductive development differences in patterns of ¹⁴C distribution between mutant and wild type were consistent with long-term changes in growth and allocation. The role of long-distance signals in the regulation of seed yield in Arabidopsis is discussed.     

How to account for the lipid effect on carbon stable-isotope ratio (δ13C): sample treatment effects and model bias

This study investigated the impact of lipid extraction, CaCO₃ removal and of both treatments combined on fish tissue δ¹³C, δ¹⁵N and C:N ratio. Furthermore, the suitability of empirical δ¹³C lipid normalization and correction models was examined. δ¹⁵N was affected by lipid extraction (increase of up to 1·65‰) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in δ¹⁵N represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for δ¹⁵N analysis. C:N and δ¹³C were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (ΔC:N and Δδ¹³C) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue δ¹³C, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and δ¹³C, as well as of the underlying biochemical processes.     

Translocation of photoassimilate from leaves of two popyploid genotypes of tall fescue differing in photosynthetic rates

The photosynthetic rate of a decaploid genotype (1-16-2) of tall fescue ( Festuca arundinacea Schreb.) is about twice that of a common hexaploid genotype (V6-802) (Plant Physiol. 72: 16–21, 1983). Translocation of photosynthate out of the leaves is a possible means of regulating carbon assimilation. To evaluate this possibility, we have examined a) translocation velocity, b) time course of translocation from leaves, c) photoassimilate partitioning pattern into whole plants in pulse and chase experiments, and d) interveinal distances between two ploidy genotypes. Most of the ¹⁴C accumulated in sucrose, and the labelled carbon moved down the leaf blades at similar velocities (6 to 10 cm h⁻¹) in both genotypes. Recent ¹⁴C assimilate was rapidly translocated from the fed area of the leaf blade. For example, the decaploid and the common hexaploid had translocated 40 and 26% of the ¹⁴C, respectively, at 6 h, and 79 and 49% of the ¹⁴C, respectively, at 24 h. Partitioning of ¹⁴C among plant organs was considerably different between the genotypes after a 24 h chase. For example, out of the total ¹⁴C recovered from the whole plant, the decaploid had retained 40% in the labelled leaf with 10, 33 and 29% in other leaves, stem bases and roots, respectively; whereas the hexaploid had retained 91% in the labelled leaf with 4, 3 and 2% in other leaves, stem bases and roots, respectively. However, the higher rate of translocation was correlated with greater interveinal distances in the decaploid genotype. These results suggested that the higher translocation percentage in the decaploid than the hexaploid genotype was due to greater sink activity.     

Assimilate partitioning in Pachyrhizus erosus tubers under short days

The distribution of ¹⁴C products in yam bean ( Pachyrhizus erosus ) plants was measured as a function of time after a ¹⁴CO₂ pulse-chase experiment under short-day conditions. The ¹⁴C declined by more than 70% in leaves during the first two hours, indicating that they actively exported carbon. Tubers were strong storage sinks for carbon and accumulated more than 80% of the total incorporated ¹⁴C after a 72-h chase. The data show that, although sucrose represented about 21% of the tuber non-structural carbohydate, i.e. 15% of the tuber dry weight, ¹⁴C did not accumulate in sucrose but in glucose, fructose and starch. The data indicate the existence of a separate sucrose pool which is affected only very slowly by recent assimilates. As a result, recent photosynthates, temporarily stored, may contribute to the reservoir of carbon available for nitrogen fixation. The data also suggest the existence of two distinct pools of amino acids in the tuber, one utilized mostly for protein synthesis and the other probably stored in the vacuole.     

Chlorophyll-protein levels and degree of thylakoid stacking in radish chloroplasts from high-light, low-light and bentazon-treated plants

Lemna gibba plants were incubated aseptically on medium containing labelled 10^-7 M indole-3-acetic acid (IAA-1-¹⁴C). Most of the radioactivity disappeared from the culture medium during a 24 h light period. A high percentage of the loss was due to photolysis and only a low percentage of the radioactivity was recovered in the plants. Uptake of ¹⁴C by the plants was strongly stimulated by light. The radioactivity taken up by the plants was the sum of photosynthetically taken up ¹⁴CO₂ and ¹⁴C taken up in IAA. Analyses with the indolo-α-pyrone fluorescence method revealed that the free IAA content was almost the same in plants grown in control and in IAA media for 5 h, whereas the amount of IAA which could be liberated by alkaline hydrolysis was doubled by the presence of IAA in the medium.     

Characterization of l-Arginine and Aminoguanidine Uptake into Isolated Rat Choroid Plexus: Differences in Uptake Mechanisms and Inhibition by Nitric Oxide Synthase Inhibitors

Abstract: Recent reports suggest that nitric oxide (NO) may contribute to several neurodegenerative diseases, e.g., focal cerebral ischemia, N -methyl- d -aspartate-mediated neurotoxicity, and experimental autoimmune encephalomyelitis. Accordingly, an understanding of the CNS transport processes of NO synthase (NOS) inhibitors has important therapeutic implications. The objective of the present study was to characterize the in vitro transport processes governing the uptake of l -[¹⁴C]arginine and the NOS inhibitor [¹⁴C]aminoguanidine in rat choroid plexus tissue. Consistent with previous reports, the uptake of l -[¹⁴C]arginine was mediated by both saturable and nonsaturable processes and was inhibited by the NOS inhibitors N ^G-methyl- l -arginine, N ^G-amino- l -arginine, and N ⁵-imidoethyl- l -ornithine. l -[¹⁴C]Arginine uptake was not inhibited by aminoguanidine or N ^G-nitro- l -arginine. Because aminoguanidine is an organic cation that bears some structural similarity to l -arginine, aminoguanidine might be transported by either an organic cation transporter or by the basic amino acid transporter governing arginine uptake. However, there was no evidence of a saturable uptake process for [¹⁴C]aminoguanidine in isolated rat choroid plexus, in contrast to that observed for l -[¹⁴C]arginine.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Allocation of carbon to growth, storage and respiration in the vegetative barley plant

Abstract. It is proposed that the growing plant can be divided into three compartments with reference to carbon: soluble, storage and structural. Experiments carried out at 10, 15, 20 and 30°C in the light followed changes in size of these compartments in barley plants 10–24 days old. The redistribution of ^I4C photo-assimilated by 10 day old plants was monitored simultaneously. The soluble and storage compartments are a higher percentage of plant weight at lower temperatures, and are turned over rapidly at all temperatures; they form the source of respired ¹⁴C. About 30% of the ¹⁴C fixed enters structural material; in the first 24 h after labelling, for each unit of ¹⁴C entering the structural compartment, between 0–9 (at 15°C) and 3.2 (at 30°C) units of ¹⁴C are lost by respiration. At 15°C in the dark, respiratory loss of ¹⁴C is initially from soluble and storage compartments; thereafter respiration of ^I4C occurs at the expense of structural material.     

Reed Canary Grass (Phalaris arundinacea) as a Biological Model in the Study of Plant Invasions

Invasive species pose a serious threat to native plant communities and are an important contributor to loss of biodiversity. In the case of Phalaris arundinacea, L. (Poaceae), reed canary grass, a cool-season, long-lived perennial plant native to Eurasia and North America, nonnative agronomically important genotypes were introduced to North America for numerous uses such as forage and soil stabilization. Following repeated introductions, reed canary grass became an aggressive invader that takes over natural wet prairies, stream-banks and wetlands. Reed canary grass can outcompete native plant species, resulting in monospecific stands with concomitant loss of plant and insect diversity and ultimately to alteration in ecosystem function. Abiotic factors such as disturbance, changes in hydrological regime, and particularly nutrient runoff to wetlands can enhance reed canary grass establishment and vegetative spread. In addition, the species' capacity for early season growth, rapid vegetative spread, high stem elongation potential, wide physiological tolerance, and high architectural plasticity make the species highly aggressive under a wide range of ecological conditions. The change in life-history and environmental conditions responsible for the enhanced aggressiveness observed between native and invasive genotypes are not yet understood. Hence, reed canary grass provides an ideal study system to test a number of ecological and genetic hypotheses to explain why some plant species become extremely aggressive when transported into a new geographical area. To date, genetic studies have found that invasive populations have high genetic diversity and that genotypes differ in their phenotypic plasticity and response to ecological conditions, which may contribute to their invasion success. Finally comparative studies currently underway on European native and American invasive genotypes of reed canary grass should shed light on the mechanisms responsible for reed canary grass's aggressiveness and should provide an experimental protocol to test ecological and genetic hypotheses about what makes a species invasive.     

Meeting US biofuel goals with less land: the potential of Miscanthus

Biofuels from crops are emerging as a Jekyll & Hyde – promoted by some as a means to offset fossil fuel emissions, denigrated by others as lacking sustainability and taking land from food crops. It is frequently asserted that plants convert only 0.1% of solar energy into biomass, therefore requiring unacceptable amounts of land for production of fuel feedstocks. The C₄ perennial grass Miscanthus × giganteus has proved a promising biomass crop in Europe, while switchgrass ( Panicum virgatum ) has been tested at several locations in N. America. Here, replicated side-by-side trials of these two crops were established for the first time along a latitudinal gradient in Illinois. Over 3 years of trials, Miscanthus × giganteus achieved average annual conversion efficiencies into harvestable biomass of 1.0% (30 t ha⁻¹) and a maximum of 2.0% (61 t ha⁻¹), with minimal agricultural inputs. The regionally adapted switchgrass variety Cave-in-Rock achieved somewhat lower yields, averaging 10 t ha⁻¹. Given that there has been little attempt to improve the agronomy and genetics of these grasses compared with the major grain crops, these efficiencies are the minimum of what may be achieved. At this 1.0% efficiency, 12 million hectares, or 9.3% of current US cropland, would be sufficient to provide 133 × 10⁹ L of ethanol, enough to offset one-fifth of the current US gasoline use. In contrast, maize grain from the same area of land would only provide 49 × 10⁹ L, while requiring much higher nitrogen and fossil energy inputs in its cultivation.     

Transfer of nitrogen from a tropical legume tree to an associated fodder grass via root exudation and common mycelial networks

Symbiotic dinitrogen fixation by legume trees represents a substantial N input in agroforestry systems, which may benefit the associated crops. Applying ¹⁵N labelling, we studied N transfer via common mycelial networks (CMN) and root exudation from the legume tree Gliricidia sepium to the associated fodder grass Dichantium aristatum . The plants were grown in greenhouse in shared pots in full interaction (treatment FI) or with their root systems separated with a fine mesh that allowed N transfer via CMN only (treatment MY). Tree root exudation was measured separately with hydroponics. Nitrogen transfer estimates were based on the isotopic signature of N ( δ ¹⁵N) transferred from the donor. We obtained a range for estimates by calculating transfer with δ ¹⁵N of tree roots and exudates. Nitrogen transfer was 3.7–14.0 and 0.7–2.5% of grass total N in treatments FI and MY, respectively. Root δ ¹⁵N gave the lower and exudate δ ¹⁵N the higher estimates. Transfer in FI probably occurred mainly via root exudation. Transfer in MY correlated negatively with grass root N concentration, implying that it was driven by source-sink relationships between the plants. The range of transfer estimates, depending on source δ ¹⁵N applied, indicates the need of understanding the transfer mechanisms as a basis for reliable estimates.     

The effect of water status and season on the incorporation of 14CO2 and [14C]-acetate into resin and rubber fractions in guayule

The effects of drought stress and season on both allocation of photosynthates to stems and leaves and potential for stem rubber synthesis were studied in guayule ( Parthenium argentatum Gray USDA line 11604). Two-year-old plants grown under field conditions in the Negev desert of Israel were subjected to different irrigation regimes, and water status was assessed by measuring the relative water content (RWC). Undetached plant tips were exposed to a 1 h pulse of ¹⁴CO₂, followed by a 24 h chase. ¹⁴C fixed and translocated to different plants parts and notably ¹⁴C incorporation into rubber and resin fractions was determined. The potential of detached branch slices to incorporate [¹⁴C]-acetate into rubber was also studied. A higher percentage of fixed ¹⁴C was translocated from shoot tips in winter (28–30%) than in summer (15–18%). The percentage of [¹⁴C]-acctate incorporated into the rubber fraction by stem slices was maximal in winter (20%) and minimal in summer (3–5%) in both cases in the absence of drought stress. In summer the translocation of photosynthates into stems was inversely related to plant RWC, dropping from 18% three days after irrigation to 3% 14 days later, and the potential of stems to synthesise rubber was high under drought conditions and low in well irrigated plants.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.