Stable isotope characteristics across narrow savanna/woodland ecotones in Wolfe Creek Meteorite Crater,Western Australia

The stable isotopic composition (δ¹³C) of sediments from lakes are frequently analyzed to reconstruct the proportion of the regional vegetation that used either the C₃ or C₄ photosynthetic pathways, often without conducting a detailed survey of the current local vegetation. We performed a study on the modern vegetation composition within the Wolfe Creek Meteorite Crater to complement our future paleoecological investigation of the crater. A bull’s-eye pattern exists where C₄ grasses dominate an outer ring and salt tolerant species, including shrubs, herbs, chenopods, and halophytic algae, dominate the inner pan of the crater. The ecotone between the inner and outer zones is narrow and occupied by tall (>7 m) Acacia ampliceps, with some C₄ grasses in the understory. Along with the highest water table and most saline soils the center of the crater has C₃ plants present with the highest δ¹³C and δ¹⁵N values. The range of δ¹³C and δ¹⁵N values from the analysis of surface soil organic matter (OM) was much smaller compared with the range of values from plant materials implying that either: (1) the current plant OM has not yet been integrated into the soils, or (2) processes within the soil have acted to homogenize isotopic variability within the crater. The application of a two end member mixing model to calculate %C₄ and %C₃ biomass from the δ¹³C of surface soil OM was complicated by: (1) the crater containing both a dry habitat with C₄ grasses and a central pan with C₄ halophytic plants and, (2) the large variation in the δ¹³C of the plants and soil OM.     

Functional groups based on leaf physiology: are they spatially and temporally robust?

The functional grouping concept, which suggests that complexity in ecosystem function can be simplified by grouping species with similar responses, was tested in the Florida scrub habitat. Functional groups were identified based on how species regulate exchange of carbon and water with the atmosphere as indicated by both instantaneous gas exchange measurements and integrated measures of function (%N, δ¹³C, δ¹⁵N, C:N ratio) in fire-maintained Florida scrub, which was considered the natural state for scrub habitat. Using cluster analysis, five distinct physiologically based functional groups were identified in the fire-maintained scrub and were determined to be distinct clusters and not just arbitrary divisions in a continuous distribution by the non-parametric multivariate analysis of similarities (ANOSIM; R=0.649, P=0.005). These functional groups were tested for robustness spatially, temporally, and with management regime using ANOSIM. The physiological functional groups remained distinct clusters in this broader array of sites (R=0.794, P=0.001) and were not altered by plot differences, primarily, water table depth (R=−0.115, P=0.893) or by the three different management regimes: prescribed burn, mechanically treated and burned, and fire-suppressed (R=0.018, P=0.349). The physiological groupings also remained robust between the two climatically different years, with 1999 being a much wetter year than 2000 (R=−0.027, P=0.725). Easy-to-measure morphological characteristics, if they indicate the same functional groups, would be more practical for scaling and modeling ecosystem processes than detailed gas exchange measurements; therefore, we tested a variety of morphological characteristics as functional indicators. A combination of non-parametric multivariate techniques were used to compare the ability of life form, leaf thickness (LT), and specific leaf area (SLA) classifications to identify the physiologically based functional groups. Life form classifications (ANOSIM; R=0.629, P=0.001) were able to depict the physiological groupings more adequately than either SLA (ANOSIM; R=0.426, P=0.001) or LT (ANOSIM; R=0.344, P=0.001). The ability of life forms to depict the physiological groupings was improved by separating the parasitic Ximenia americana from the shrub category (ANOSIM; R=0.794, P=0.001). Therefore, a life form classification including parasites was determined to be a good indicator of the physiological processes of scrub species and would be a useful method of grouping species for scaling physiological processes to the ecosystem level.     

Association between tree-ring and needle δ<Superscript>13</Superscript>C and leaf gas exchange in <Emphasis Type="Italic">Pinus halepensis</Emphasis> under semi-arid conditions

Associations between δ¹³C values and leaf gas exchanges and tree-ring or needle growth, used in ecophysiological compositions, can be complex depending on the relative timing of CO₂ uptake and subsequent redistribution and allocation of carbon to needle and stem components. For palaeoenvironmental and dendroecological studies it is often interpreted in terms of a simple model of δ¹³C fractionation in C₃ plants. However, in spite of potential complicating factors, few studies have actually examined these relationships in mature trees over inter- and intra-annual time-scales. Here, we present results from a 4 years study that investigated the links between variations in leaf gas-exchange properties, growth, and dated δ¹³C values along the needles and across tree rings of Aleppo pine trees growing in a semi-arid region under natural conditions or with supplemental summer irrigation. Sub-sections of tissue across annual rings and along needles, for which time of formation was resolved from growth rate analyses, showed rapid growth and δ¹³C responses to changing environmental conditions. Seasonal cycles of growth and δ¹³C (up to ~4‰) significantly correlated (P<0.01) with photosynthetically active radiation, vapour pressure deficit, air temperature, and soil water content. The irrigation significantly increased leaf net assimilation, stomatal conductance and needle and tree-ring growth rate, and markedly decreased needle and tree-ring δ¹³C values and its sensitivity to environmental parameters. The δ¹³C estimates derived from gas-exchange parameters, and weighted by assimilation, compared closely with seasonal and inter-annual δ¹³C values of needle- and tree-ring tissue. Higher stomatal conductances of the irrigated trees (0.22 vs. 0.08 mol m⁻² s⁻¹ on average) corresponded with ~2.0‰ lower average δ¹³C values, both measured and derived. Derived and measured δ¹³C values also indicated that needle growth, which occurs throughout the stressful summer was supported by carbon from concurrent, low rate assimilation. For Aleppo pine under semi-arid and irrigated conditions, the δ¹³C of tree-ring and needle material proved, in general, to be a reasonable indicator of integrated leaf gas-exchange properties.     

Metabolic flux analysis in complex isotopolog space. Recycling of glucose in tobacco plants

Tobacco plants grown in vitro were supplied with a mixture of [U-13C6]glucose and unlabelled sucrose via the root system. After 20 days, leaves were harvested and extracted with water. Glucose was isolated from the extract and was analysed by 13C NMR spectroscopy. All 13C signals appeared as complex multiplets due to 13C-13C coupling. The abundance of 21 isotopologous glucose species was determined from the 13C NMR signal integrals by numerical deconvolution using a genetic algorithm. The relative fractions of specific isotopologs in the overall excess of 13C-labelled specimens establish flux contributions via glycolysis/glucogenesis, pentose phosphate pathway, citric acid cycle and Calvin cycle including 13CO2 refixation. The fluxes were modelled and reconstructed in silico by a novel rule-based approach yielding the contributions of circular pathways and the degree of multiple cycling events. The data indicate that the vast majority of the proffered [U-13C6]glucose molecules had been modified by catabolism and subsequent glucogenesis from catabolic fragments, predominantly via passage through the citric acid cycle and the pentose phosphate pathway.     

Dissimilatory Fe(III) reduction by an electrochemically active lactic acid bacterium phylogenetically related to Enterococcus gallinarum isolated from submerged soil

AIMS: The isolation and identification of a glucose-oxidizing Fe(III)-reducing bacteria (FRB) with electrochemical activity from an anoxic environment, and characterization of the role of Fe(III) in its metabolism. METHODS AND RESULTS: A Gram-positive (Firmicutes), nonmotile, coccoid and facultative anaerobic FRB was isolated based on its ability to reduce Fe(III). Using the Vitek Gram-positive identification card kit and 16S rRNA gene sequence analysis, the isolate was identified as Enterococcus gallinarum, designated strain MG25. On glucose this isolate produced lactate plus small amounts of acetate, formate and CO2 and its growth rates were similar in the presence and absence of Fe(O)OH. These results suggest that MG25 can couple glucose oxidation to Fe(III) reduction, but without conservation of energy to support growth. Cyclic voltammetry showed that strain MG25 was electrochemically active. CONCLUSIONS: An electrochemically active and FRB, E. gallinarum MG25, was isolated from submerged soil. Fe(III) is used in the bacterial metabolism as an electron sink. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report concerning the electrochemical activity of glucose-oxidizing FRB, E. gallinarum. This organism and others like it could be used as new biocatalysts to improve the performance of a mediator-less microbial fuel cell.     

Plant responses to elevated CO<Subscript>2</Subscript> concentration at different scales: leaf,whole plant,canopy, and population

Elevated CO₂ enhances photosynthesis and growth of plants, but the enhancement is strongly influenced by the availability of nitrogen. In this article, we summarise our studies on plant responses to elevated CO₂. The photosynthetic capacity of leaves depends not only on leaf nitrogen content but also on nitrogen partitioning within a leaf. In Polygonum cuspidatum, nitrogen partitioning among the photosynthetic components was not influenced by elevated CO₂ but changed between seasons. Since the alteration in nitrogen partitioning resulted in different CO₂-dependence of photosynthetic rates, enhancement of photosynthesis by elevated CO₂ was greater in autumn than in summer. Leaf mass per unit area (LMA) increases in plants grown at elevated CO₂. This increase was considered to have resulted from the accumulation of carbohydrates not used for plant growth. With a sensitive analysis of a growth model, however, we suggested that the increase in LMA is advantageous for growth at elevated CO₂ by compensating for the reduction in leaf nitrogen concentration per unit mass. Enhancement of reproductive yield by elevated CO₂ is often smaller than that expected from vegetative growth. In Xanthium canadense, elevated CO₂ did not increase seed production, though the vegetative growth increased by 53%. As nitrogen concentration of seeds remained constant at different CO₂ levels, we suggest that the availability of nitrogen limited seed production at elevated CO₂ levels. We found that leaf area development of plant canopy was strongly constrained by the availability of nitrogen rather than by CO₂. In a rice field cultivated at free-air CO₂ enrichment, the leaf area index (LAI) increased with an increase in nitrogen availability but did not change with CO₂ elevation. We determined optimal LAI to maximise canopy photosynthesis and demonstrated that enhancement of canopy photosynthesis by elevated CO₂ was larger at high than at low nitrogen availability. We also studied competitive asymmetry among individuals in an even-aged, monospecific stand at elevated CO₂. Light acquisition (acquired light per unit aboveground mass) and utilisation (photosynthesis per unit acquired light) were calculated for each individual in the stand. Elevated CO₂ enhanced photosynthesis and growth of tall dominants, which reduced the light availability for shorter subordinates and consequently increased size inequality in the stand.     

Carbon cycling and budget in a forested basin of southwestern Hokkaido,northern Japan

Quantification of annual carbon sequestration is very important in order to assess the function of forest ecosystems in combatting global climate change and the ecosystem responses to those changes. Annual cycling and budget of carbon in a forested basin was investigated to quantify the carbon sequestration of a cool-temperate deciduous forest ecosystem in the Horonai stream basin, Tomakomai Experimental Forest, northern Japan. Net ecosystem exchange, soil respiration, biomass increment, litterfall, soil-solution chemistry, and stream export were observed in the basin from 1999–2001 as a part of IGBP-TEMA project. We found that 258 g C m^–2 year^–1 was sequestered annually as net ecosystem exchange (NEE) in the forested basin. Discharge of carbon to the stream was 4 g C m^–2 year^–1 (about 2% of NEE) and consisted mainly of dissolved inorganic carbon (DIC). About 43% of net ecosystem productivity (NEP) was retained in the vegetation, while about 57% of NEP was sequestered in soil, suggesting that the movement of sequestered carbon from aboveground to belowground vegetation was an important process for net carbon accumulation in soil. The derived organic carbon from aboveground vegetation that moved to the soil mainly accumulated in the solid phase of the soil, with the result that the export of dissolved organic carbon to the stream was smaller than that of dissolved inorganic carbon. Our results indicated that the aboveground and belowground interaction of carbon fluxes was an important process for determining the rate and retention time of the carbon sequestration in a cool-temperate deciduous forest ecosystem in the southwestern part of Hokkaido, northern Japan.     

Carbon isotope composition of canopy leaves in a tropical forest in Panama throughout a seasonal cycle

The seasonal variation in ¹³C values was measured in leaves from 17 upper canopy, five mid- canopy and in four gap tree species, as well as in five epiphyte and five vine species, in a seasonally dry lowland tropical forest at Parque Natural Metropolitano near Panama City, Republic of Panama. No seasonal variation was detected in the ¹³C values of mature exposed leaves from either the upper or mid- canopy. However, canopy position did influence the ¹³C value. The mean isotopic composition of leaves from the mid- canopy was more negative than that of the upper canopy throughout the year. The ¹³C value was also influenced by leaf development, with juvenile leaves on average 1.5 less negative than mature leaves. The five epiphyte species exhibited ¹³C values that were typical of crassulacean acid metabolism (CAM). Codonanthe uleana, with isotopic values of –19.9 to –22.1, is only the second species in the Gesneriaceae reported to express CAM, whereas values between –14.6 and –22.0 indicate that Peperomia macrostachya can exhibit different degrees of CAM. The isotopic composition of exposed mature leaves from the vines showed little interspecific variation and was similar to the upper-canopy leaves of the trees.     

Mutational study of the bacterial hemoglobin distal heme pocket

Ligand binding experiments on three mutants in the distal heme pocket of Vitreoscilla hemoglobin (GlnE7His, ProE8Ala, and GlnE7His,ProE8Ala) were used to probe the role of GlnE7 and ProE8 in the pocket's unusual structure. The oxygen dissociation constants for the wild type, E8Ala mutant, and E7His mutant proteins were 4.5, 4.7, and 1.7microM, respectively; the K(d) for the double mutant was not determinable by our technique. Visible-Soret spectra of the carbonyl and cyanyl forms and FT-IR of the carbonyl form of the E8 mutant were similar to those of the wild type; the opposite was true for the GlnE7His and GlnE7His,ProE8Ala mutants, which also differed from wild type in the visible-Soret spectra of their oxidized forms. Models of the effects of the mutations on distal pocket structure were consistent with the experimental findings, particularly the larger effects of the GlnE7His change.