Search

We determined hydrogen, carbon and nitrogen isotopic compositions of chlorophylls a and b isolated from leaves of five C3 higher plant species (Benthamidia japonica, Prunus japonica, Acer carpinifolium, Acer argutum and Querus mongloica), and hydrogen and carbon isotopic compositions of phytol and chlorophyllides in the chlorophylls to understand isotopic fractionations associated with chlorophyll biosynthesis in these species. Chlorophylls are depleted in D relative to ambient water by approximately 189 per thousand and enriched in (13)C relative to bulk tissue by approximately 1.6 per thousand. These data can be explained by the contribution of isotopic fractionations during phytol and chlorophyllide biosyntheses. Phytol is more depleted in both D (by approximately 308 per thousand) and (13)C (by approximately 4.3 per thousand), while chlorophyllides are less depleted in D (by approximately 44 per thousand) and enriched in (13)C (by approximately 4.8 per thousand). Such inhomogeneous distribution of isotopes in chlorophylls suggests that (1) the phytol in chlorophylls reflects strong D- and (13)C-depletions due to the isotopic fractionations during the methylerythritol phosphate pathway followed by hydrogenation, and (2) the chlorophyllides reflect D- and (13)C-enrichments in tricarboxylic acid cycle. On the other hand, chlorophylls are slightly ( approximately 1.2 per thousand) depleted in (15)N relative to the bulk tissue, indicating that net isotopic fractionation of nitrogen during chlorophyll biosynthesis is small compared with those of hydrogen and carbon.

Compound-specific hydrogen isotopic compositions (deltaD) of saturated, monounsaturated and polyunsaturated fatty acids have been determined for natural marine macroalgae including two brown algae (Heterokontophyta) and two red algae (Rhodophyta). deltaD values of individual fatty acids from four macroalgae exhibit a wide variation ranging from -189% to +48%. Generally, stearic (18:0), arachidic (20:0) and behenic acids (22:0) are much more enriched in D by up to approximately 180% relative to myristic (14:0), palmitic (16:0), octatetraenoic [18:4(n-3)] and eicosapentaenoic acids [20:5(n-3)]. Other fatty acids such as oleic [18:1(n-9)], lenoleic [18:2(n-6)] and linolenic acids [18:3(n - 3)] fall isotopically between these fatty acids. This wide deltaD variation of fatty acids is probably explained by the hydrogen isotopic fractionation during desaturation being much larger than that during elongation in the network of polyunsaturated fatty acid biosynthesis. A large hydrogen isotopic fractionation during desaturation may cause D-enrichment in the remaining hydrogen of the residual fatty acids, which could be controlled by the relative flux into their desaturates.

Stable hydrogen and carbon isotopic compositions of individual n-alkanes were determined for various terrestrial plants (33 samples including 27 species) and aquatic plants (six species) in natural environments from Japan and Thailand. In C3 plants, n-alkanes extracted from angiosperms have a deltaD value of -152+/-26 per thousand (relative to Standard Mean Ocean Water [SMOW]) and delta13C value of -36.1+/-2.7 per thousand (relative to Peedde Belemnite [PDB]), and those from gymnosperms have a deltaD value of -149+/-16 per thousand and delta13C value of -31.6+/-1.7 per thousand. Angiosperms have n-alkanes depleted in 13C relative to gymnosperms. n-Alkanes from C4 plants have a deltaD value of -171+/-12 per thousand and delta13C value of -20.5+/-2.1 per thousand, being a little depleted in D and much enriched in 13C compared to C3 plants. n-Alkanes of CAM plants are a little depleted in D and vary widely in delta13C relative to those of C3 and C4 plants. In aquatic plants, n-alkanes from freshwater plants have a deltaD value of -187+/-16 per thousand and delta13C value of -25.3+/-1.9 per thousand, and those from seaweeds have a deltaD value of -155+/-34 per thousand and delta13C value of -22.8+/-1.0 per thousand. All n-alkanes from various plant classes are more depleted in D and 13C relative to environmental water and bulk tissue, respectively. In addition, the hydrogen and carbon isotopic fractionations during n-alkane synthesis are distinctive for these various plant classes. While C3 plants have smaller isotopic fractionations in both D and 13C, seaweed has larger isotopic fractionations.

A suite of isotopic methods were combined to provide a comprehensive investigation of organic matter transfer and consumer-resource links in a large lake food web. We applied compound specific isotope analysis (CSIA) of nitrogen within amino acids of organisms, a relatively new method and one not yet widely applied to large lake systems, to determine the trophic positions of several dominant species. Comparison of this CSIA trophic designation to those calculated by traditional bulk stable carbon and nitrogen isotope ratios, as well as gut content analyses, revealed this CSIA method to be the most representative of known trophic links in Lake Superior. Limnocalanus macrurus, an omnivorous copepod, was found to occupy a trophic position higher than would have been predicted based on gut analyses, and one full trophic position above primary consumer Daphnia. Radiocarbon analysis of bulk zooplankton and fish tissue was employed as a modern carbon tracer to elucidate the relative ages of carbon incorporated into, and propagated through, the food web. Comparison of animal tissue, non-living carbon pools (DOC, DIC), and POC radiocarbon content indicate the food web is supported predominately by newly synthesized autochthonous material. The uniquely depleted radiocarbon signature of the benthic amphipod Diporeia, attributed to consumption of aged sedimentary or allochthonous material, was not reflected in its known predators, indicating little overall incorporation of that aged carbon source into the pelagic food web. This multi-method isotopic investigation extends our understanding of food web structure and organic matter flow in this large lake. [ABSTRACT FROM AUTHOR]