Hilary Stuart-Williams

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.

Comprehensive inter-laboratory calibration of reference materials for delta O-18 versus VSMOW using various on-line high-temperature conversion techniques

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high-temperature conversion techniques (HTC)a in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125 per thousand, an artificially enriched reference water (delta(18)O of +78.91 per thousand) and two barium sulfates (one depleted and one enriched in (18)O) were prepared and calibrated relative to VSMOW2b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded: Reference material delta(18)O and estimated combined uncertainty IAEA-602 benzoic acid+71.28 +/- 0.36 per thousand USGS 35 sodium nitrate+56.81 +/- 0.31 per thousand IAEA-NO-3 potassium nitrate+25.32 +/- 0.29 per thousand IAEA-601 benzoic acid+23.14 +/- 0.19 per thousand IAEA-SO-5 barium sulfate+12.13 +/- 0.33 per thousand NBS 127 barium sulfate+8.59 +/- 0.26 per thousand VSMOW2 water 0 per thousand IAEA-600 caffeine-3.48 +/- 0.53 per thousand IAEA-SO-6 barium sulfate-11.35 +/- 0.31 per thousand USGS 34 potassium nitrate-27.78 +/- 0.37 per thousand SLAP water-55.5 per thousand The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for delta(18)O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios (delta(18)O) of nitrates, sulfates, or organic material should explicitly state in their reports the delta(18)O values of two or more internationally distributed nitrates (USGS 34, IAEA-NO-3, and USGS 35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the delta(18)O values at a later time should it become necessary.The high-temperature reduction technique for analyzing delta(18)O and delta(2)H is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

Measurement and Interpretation of the Oxygen Isotope Composition of Carbon Dioxide Respired by Leaves in the Dark

We measured the oxygen isotope composition (δ18O) of CO2 respired by Ricinus communis leaves in the dark. Experiments were conducted at low CO2 partial pressure and at normal atmospheric CO2 partial pressure. Across both experiments, the δ18O of dark-respired CO2 (δR) ranged from 44‰ to 324‰ (Vienna Standard Mean Ocean Water scale). This seemingly implausible range of values reflects the large flux of CO2 that diffuses into leaves, equilibrates with leaf water via the catalytic activity of carbonic anhydrase, then diffuses out of the leaf, leaving the net CO2 efflux rate unaltered. The impact of this process on δR is modulated by the δ18O difference between CO2 inside the leaf and in the air, and by variation in the CO2 partial pressure inside the leaf relative to that in the air. We developed theoretical equations to calculate δ18O of CO2 in leaf chloroplasts (δc), the assumed location of carbonic anhydrase activity, during dark respiration. Their application led to sensible estimates of δc, suggesting that the theory adequately accounted for the labeling of CO2 by leaf water in excess of that expected from the net CO2 efflux. The δc values were strongly correlated with δ18O of water at the evaporative sites within leaves. We estimated that approximately 80% of CO2 in chloroplasts had completely exchanged oxygen atoms with chloroplast water during dark respiration, whereas approximately 100% had exchanged during photosynthesis. Incorporation of the δ18O of leaf dark respiration into ecosystem and global scale models of C18OO dynamics could affect model outputs and their interpretation.

Stable isotopes in leaf water of terrestrial plants

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ(18) O and δ(2) H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.

Biosynthetic origin of the saw-toothed profile in δ13C and δ2Η of n-alkanes and systematic isotopic differences between n-, iso- and anteiso-alkanes in leaf waxes of land plants

The n-fatty acids containing an even number of carbons (ECN-n-FAs) in higher plants are biosynthesised by repetitive addition of a two carbon unit from malonyl-ACP. The n-alkanes containing an odd number of carbon atoms (OCN-n-alkanes) are generally formed by the decarboxylation of ECN-n-FAs, but it is unknown how the less abundant even-carbon-numbered alkanes (ECN-n-alkanes) are biosynthesised in higher plants. There is a distinctive compositional pattern of incorporation of stable carbon ((13)C) and hydrogen ((2)H) isotopes in co-existing ECN- and OCN-n-alkanes in leaves of higher plants, such that the OCN n-alkanes are relatively enriched in (13)C but relatively depleted in (2)H against the ECN-n-alkanes. This is consistent with the OCN-n-fatty acids having a propionate precursor which is derived from reduction of pyruvate. A tentative pathway is presented with propionate produced by enzymatic reduction of pyruvate which is then thio-esterified with CoSH (coenzyme A thiol) in the chloroplast to form the terminal precursor molecule propionyl-CoA. This is then repetitively extended/elongated with the 2-carbon unit from malonyl-ACP to form the long chain OCN-n-fatty acids. The anteiso- and iso-alkanes in Nicotiana tabacum leaf waxes have previously been found to be systematically enriched in (13)C compared with the n-alkanes by Grice et al. (2008). This is consistent with the isotopic composition of their putative respective precursors (pyruvate as precursor for n-alkanes, valine for iso-alkanes and isoleucine for anteiso-alkanes). The current study complements that of Grice et al. (2008) and looks at the distribution of hydrogen isotopes. The n-alkanes were found to be more enriched in deuterium ((2)H) than the iso-alkanes which in turn were more enriched than the anteiso-alkanes. We propose therefore that the depletion of (2)H in the iso-alkanes, relative to the n-alkanes is the consequence of accepting highly (2)H-depleted hydrogen atoms from NADPH during their biosynthesis. The anteiso-alkanes are further depleted again because there are three NADPH-derived hydrogen atoms in their precursor isoleucine, as compared with only one NADPH-derived hydrogen in valine, the precursor of the iso-alkanes.

Biosynthetic and environmental effects on the stable carbon isotopic compositions of anteiso- (3-methyl) and iso- (2-methyl) alkanes in tobacco leaves

Nicotiana tabacum is the only plant known to synthesise large quantities of anteiso- (3-methyl) alkanes and iso- (2-methyl) alkanes. We investigated the carbon isotope ratios of individual long-chain n-alkanes, anteiso- and iso-alkanes (in the C(29)-C(33) carbon number range) extracted from tobacco grown in chambers under controlled conditions to confirm the pathway used by the tobacco plant to synthesise these particular lipids and to examine whether environmental data are recorded in these compounds. Tobacco was grown under differing temperatures, water availabilities and light intensities in order to control its stable carbon isotope ratios and evaluate isotopic fractionations associated with the synthesis of these particular lipids. The anteiso-alkanes were found to have a predominant even-carbon number distribution (maximising at C(32)), whereas the iso-alkanes exhibit an odd-carbon number distribution (maximising at C(31)). Iso-alkanes were relatively more abundant than the anteiso-alkanes and only two anteiso-alkanes (C(30) and C(32)) were observed. The anteiso-alkanes and iso-alkanes were found to be enriched in (13)C by 2.8-4.3 per thousand and 0-1.8 per thousand compared to the n-alkanes, respectively, consistent with different biosynthetic precursors. The assumed precursor for the odd-carbon-numbered iso-alkanes is iso-butyryl-CoA (a C(4) unit derived from valine) followed by subsequent elongation of C(2) units and then decarboxylation. The assumed precursor for even-carbon-numbered anteiso-alkanes is alpha-methylbutyryl-CoA (a C(5) unit derived from isoleucine) and subsequent elongation by C(2) units followed by decarboxylation. The ratio of carbon atoms derived from alpha-methylbutyryl-CoA and subsequent C(2) units (from malonyl-CoA) is 1:5 for the biosynthesis of a C(30)anteiso-alkane. The ratio of carbon atoms derived from iso-butyryl-CoA and subsequent C(2) units (from malonyl-CoA) is 4:25 for the synthesis of a C(29)iso-alkane. An order of (13)C depletion n-alkanes>iso-alkanes>anteiso-alkanes is evident from compound specific isotope data. This trend can probably be attributed to the ratio of the two different sources of carbon atoms in the final wax components. Higher water availability generally results in more depleted stable carbon isotope ratios due to maximised discrimination during carboxylation, associated with less diffusional limitation. This was confirmed in the present study by compound specific isotope analyses of iso-alkanes, anteiso-alkanes and n-alkane lipids extracted from the tobacco leaves. Likewise, light intensity has been shown to influence plant bulk delta(13)C in previous studies. The carbon isotope ratios of n-alkanes in tobacco grown under low-light conditions were about 2 per thousand more depleted in (13)C than those of lipids extracted from tobacco grown under elevated light conditions. A similar order of difference is observed for the iso-alkanes and anteiso-alkanes (1.8 per thousand and 1.9 per thousand, respectively). A negligible depletion in carbon isotope ratios was observed for the iso-alkanes and anteiso-alkanes extracted from tobacco grown under elevated temperatures. These results are consistent with the work of Farquhar [Farquhar, G.D., 1980. Carbon isotope discrimination by plants: effects of carbon dioxide concentration and temperature via the ratio of intercellular and atmospheric CO(2) concentrations. In: Pearman, G.I. (Ed.), Carbon Dioxide and Climate: Australian Research. Springer, Berlin, pp. 105-110] where temperature appears to have only a minor effect on plant bulk delta(13)C.

Environmental Effects on Oxygen Isotope Enrichment of Leaf Water in Cotton Leaves

The oxygen isotope enrichment of bulk leaf water (Δb) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δb increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (ea/ei) and to stomatal conductance (gs). The oxygen isotope enrichment of lamina water relative to source water \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(({\bar{{\Delta}}}_{1}),\) \end{document} which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δb (and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}),\) \end{document} as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δb and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} with an L of 7.9 mm, much less than the total radial effective length Lr of 43 mm. The fitted values of L for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\Delta}}}_{1}\) \end{document} of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.

Determinants of maximum tree height in Eucalyptus species along a rainfall gradient in Victoria, Australia

We present a conceptual model linking dry-mass allocational allometry, hydraulic limitation, and vertical stratification of environmental conditions to patterns in vertical tree growth and tree height. Maximum tree height should increase with relative moisture supply and both should drive variation in apparent stomatal limitation. Carbon isotope discrimination (Δ) should not vary with maximum tree height across a moisture gradient when only hydraulic limitation or allocational allometry limit height, but increase with moisture when both hydraulic limitation and allocational allometry limit maximum tree height. We quantified tree height and Δ along a gradient in annual precipitation from 300 to 1600 mm from mallee to temperate rain forest in southeastern Australia; Eucalyptus on this gradient span almost the entire range of tree heights found in angiosperms worldwide. Maximum tree height showed a strong, nearly proportional relationship to the ratio of precipitation to pan evaporation. Δ increased with ln ...

Allocate carbon for a reason: Priorities are reflected in the 13C/12C ratios of plant lipids synthesized via three independent biosynthetic pathways

It has long been theorized that carbon allocation, in addition to the carbon source and to kinetic isotopic effects associated with a particular lipid biosynthetic pathway, plays an important role in shaping the carbon isotopic composition ((13)C/(12)C) of lipids (Park and Epstein, 1961). If the latter two factors are properly constrained, valuable information about carbon allocation during lipid biosynthesis can be obtained from carbon isotope measurements. Published work of Chikaraishi et al. (2004) showed that leaf lipids isotopic shifts from bulk leaf tissue Δδ(13)C(bk-lp) (defined as δ(13)C(bulkleaftissue)-δ(13)C(lipid)) are pathway dependent: the acetogenic (ACT) pathway synthesizing fatty lipids has the largest isotopic shift, the mevalonic acid (MVA) pathway synthesizing sterols the lowest and the phytol synthesizing 1-deoxy-D-xylulose 5-phosphate (DXP) pathway gives intermediate values. The differences in Δδ(13)C(bk-lp) between C3 and C4 plants Δδ(13)C(bk-lp,C4-C3) are also pathway-dependent: Δδ(13)C(ACT)(bk-lp,C4-C3) > Δδ(13)C(DXP(bk-lp,C4-C3) > Δδ(13)C(MVA)(bk-lp,C4-C3). These pathway-dependent differences have been interpreted as resulting from kinetic isotopic effect differences of key but unspecified biochemical reactions involved in lipids biosynthesis between C3 and C4 plants. After quantitatively considering isotopic shifts caused by (dark) respiration, export-of-carbon (to sink tissues) and photorespiration, we propose that the pathway-specific differences Δδ(13)C(bk-lp,C4-C3) can be successfully explained by C4-C3 carbon allocation (flux) differences with greatest flux into the ACT pathway and lowest into the MVA pathways (when flux is higher, isotopic shift relative to source is smaller). Highest carbon allocation to the ACT pathway appears to be tied to the most stringent role of water-loss-minimization by leaf waxes (composed mainly of fatty lipids) while the lowest carbon allocation to the MVA pathway can be largely explained by the fact that sterols act as regulatory hormones and membrane fluidity modulators in rather low concentrations.

An innovative molybdenum column liner for oxygen and hydrogen stable isotope analysis by pyrolysis.

The most widely used method for pyrolysing samples for hydrogen or oxygen isotopic analysis involves heating them to greater than 1300 degrees C in a helium stream passed through a glassy carbon tube in an alumina casing. There are a number of difficulties with this. Glassy carbon tubes are expensive and interaction between the carbon tube and the outer casing produces unwanted carbon monoxide by reduction of the alumina at high temperatures. The latter effect is overwhelming if temperatures of 1400 degrees C or greater are used for pyrolysis. We experimented with lining alumina casings with pure molybdenum sheet. It is relatively cheap, conforms well to the interior of the reactor tube (to avoid carrier and sample bypassing of the carbon pack), resists high temperatures and neither oxidises excessively nor absorbs the gases. The main disadvantages are that silver sample cups must be used and that the molybdenum degrades over time by formation of the carbide. We can maintain sharp peaks, high precision and good accuracy over more than 700 solid samples for both hydrogen and oxygen. The reactors last longer for water injections. The molybdenum in the columns does not contribute greatly to memory effects. The precision of analysis is dependent on other factors as well as the pyrolysis column, but for oxygen we typically achieve approximately <0.2 per thousand (sucrose), <0.25 per thousand (water) and <0.25 per thousand (leaf), sometimes using only a linear correction of drift, after dividing the run into 1 to 3 segments.