Eitaro Wada

Stepwise enrichment of 15N along food chains: Further evidence and the relation between δ15N and animal age

The isotopic composition of nitrogen was measured in marine and fresh-water animals from the East China Sea, The Bering Sea, Lake Ashinoko and Usujiri intertidal zone. Primary producers, showed average δ15Nversus atmospheric nitrogen of +5.0%. (+3.4 to +7.5) in the Bering Sea and Lake Ashinoko, and +6.8%. (+6.0 to +7.6) in Usujiri intertidal zone. Blue green algae from the East China Sea show an average −0.55%. (−0.8 to +1.2). All consumers, Zooplankton, fish and bird exhibited Stepwise enrichment of 15N with increasing trophic level. The 15N enrichment at a single feeding process ranged from +1.3 to +5.3 averaging +3.4 ± 1.1%.. This isotopic fractionation seems to be independent of habitat. n nThe effect of age in animals was obtained by analyzing two marine mussels. The soft tissue nitrogen showed +2.0%. enrichment relative to that of primary producers, and the magnitude was almost constant with shell ages ranging from 0 to 8 years. n nA similar 15N enrichment occurs in all Molluscs, Crustaceans, Insecta, Amphibia, Fish, Ave and Mammal species regardless of the difference in the form of excreted nitrogen and in laboratory cultured fish, brine shrimp and mice (+2.9 to +4.9%.). The excreted ammonia from guppy was sufficiently light to balance the concentration of 15N to animal body.

15N and 13C abundances in the Antartic Ocean with emphasis on the biogeochemical structure of the food web

Abstract Distributions of δ15N and δ13C for biogenic substances in the Antarctic Ocean were investigated to construct a biogeochemical framework for assessing the Antarctic ecosystem. Phytoplankton exhibited particularly low δ15N (0.5%) and 13C (-26.9%) values in pelagic plankton samples. High nitrate concentrations, and high PCO2 in the surface waters on the southern side of the polar front and the resulting slow growth rate of phytoplankton under low light intensity are suggested as possible factors in causing the low isotopic compositions. Mean fractionation factors of 1.029 and 1.006 were estimated for photosynthetic carbon fixation and for the assimilation of inorganic nitrogeneous compounds (ammonium plus nitrate) during algal growth, respectively. Enrichment of 15N with increasing trophic level was confirmed for Antarctic ecosystems: δ15Nanimal% = 3.3 (trophic level −1) + δ5Nalgae, whereas 13C content did not increase in the same manner. Differences in lipid content among animals may be the main factor in causing this δ13C anomaly. 15N and 13C abundance of sedimentary organic nitrogen differed from phytoplankton and settling particles. An exact mechanism for explaining the high δ15N (around 5%) is not known. The very high δ13C value of −20.5% at Sta. 3B may originate in ice algae that had grown under CO2-limited conditions. Particles collected by sediment traps gave characteristically low δ15N values (−3.0 to 0.9%), strongly suggesting a phytoplankton origin. The δ15N and δ13C values of settling material showed similar vertical profiles with depth which might arise from temporal variation of algal growth.

Nitrogen isotope ratios of red tide organisms in the East China Sea: A characterization of biological nitrogen fixation

Nitrogen isotope fractionation in the process of N2 fixation was examined in detail by culturing the blue green alga Anabaena cylindrica. The δ15N value of the alga was almost constant (− 0.6 ± 0.9‰ on average) irrespective of growth conditions, similar to other N2 fixing microorganisms previously reported. Nitrogen isotope ratios of marine plankton from the East China Sea have been studied to evaluate the contribution of N2 fixation to the red tide organisms, predominated by the N2 fixing blue-green alga Trichodesmium spp. The δ15N of Trichodesmium (− 2.1 to + 1.0‰) showed the same range of that estimated if the biological atmospheric N2 fixation was the main nitrogen source of the red tide organisms. Mixed plankton collected by towing a phytoplankton net (96 μm) gave from 2 to 4‰ higher δ15N than that of sorted phytoplankton. Zooplankton and fish collected from the same red tide area showed systematic enrichment of 15N along the food chain, and exhibited relatively lower δ15N than previous results for zooplankton from other regions, reflecting biological N2 fixation. The δ15N of mixed plankton in the surface seawater from 32°N to 20°N latitude along 125°E longitude decreased from 6 to 1‰. A symmetrical relation was found between the horizontal distribution of δ15N of plankton, the concentration of Trichodesmium and the trend of salinity.

The use of stable isotopes for food web analysis

General aspects in isotope biogeochemistry was summarized with emphasis on delta 15N and delta 13C contents in plants and animals in natural ecosystems. In the estuary, the variation of isotope ratios were principally governed by the mixing of land-derived organic matter, marine phytoplankton, and seagrasses. A clear cut linear relationship between animal delta 15N and its trophic level was obtained in the Antarctic food chain system. Several current efforts to use the stable isotopes for food web analysis were demonstrated for some terrestrial and marine systems as well as human food web.

Carbon isotopic composition of bacterial methane in a soil incubation experiment: Contributions of acetate and CO2H2

Anaerobic incubations of paddy soil collected from Konosu, Japan, were carried out for 10 weeks to clarify the general principles that govern the variation of carbon isotopic composition of bacterial methane from freshwater areas. n nThe concentrations and isotopic compositions of CH4, CO2, and acetate produced in the incubated system were measured. The δ13C value of biogenic CH4 was highly variable ranging from −60 to −33%., corresponding to changes in its formation pathways: acetate fermentation and CO2H2reduction. n nAcetate accumulated only during the first week. After it became depleted, acetate production completely limited CH4 production. The δ13C value of the methyl carbon of acetate, which was accumulated with addition of inhibitor for methanogenesis, ranged from −43 to −30%.. The δ13C value of methane from acetate was estimated to be −43 to −30%., after acetate depletion. n nThe δ13C value of CH4 from CO2H2 reduction was estimated to be −77 to −60%.. An enrichment in 13C of CO2 localized around the site of methanogenesis was suggested. CO2 produced in situ during methanogenesis (e.g., CO2 from carboxyl group of acetate) was possibly utilized as well. n nUsing the above δ13C values of two endmembers of CH4, the change of acetate contribution for CH4 production was calculated: less than 12% until week 1, 65 to 100% at weeks 1 to 3, 16 to 28% at weeks 3 to 5, and 35 to 40% after week 5. Variability of CH4δ13C resulted from the difference in contribution of each biological process. Also, the δ13C value of CH4 was a useful indicator for assessing the contribution of each process to methane production in sulfate depleted freshwater areas.

Biogeochemical studies on the transport of organic matter along the Otsuchi River watershed, Japan

The distributions and stable isotope ratios of biogenic nitrogen and carbon were investigated in detail along a small watershed in order to establish a biogeochemical framework for assessing the fate of organic matter. Forest ecosystems supply soluble and particulate materials to river systems which are depleted in 15N and 13C. The number of suspended particles and the concentrations of δ15N and δ13C in the river sediments increased along the watershed, indicating a change from river to marine ecosystems. Dramatic variation of δ15N and δ13C were observed in the intertidal sediments, where the progress of denitrification, discharge of domestic sewage, and the accumulation and the decomposition of macroalgae and seagrasses took place. n nThe contribution of land-derived organic matter to estuarine sediments has been estimated from δ13C and from δ15N data. The contribution the landderived organo-silty-clay mineral was 70–100% in the inner bay sediments and 34–42% at the open bay. Possible factors that influence the variation of stable isotope ratios along the watershed are discussed. The relationship between the sizes of particles and isotope ratios clearly demonstrated that organo-silty-clay minerals with diameter smaller than 64 μm were the major source of land-derived refractory organics.

High nitrogen isotope ratio for soils of seabird rookeries

Soils from rookeries of penguin, of gull, and of albatross were examined for their nitrogen isotope ratio. The ratio was far higher than any so far reported for soils. Furthermore, there was an apparent dependence of the ratio on the latitude of rookery locations. The high ratio seemed to have had resulted from a relatively high ratio for incoming nitrogen to the rookeries, and from the large fractionation factor during the ammonia volatilization from the rookeries. The isotope ratio for ammonium nitrogen of the penguin rookery soils averaged 45 per mil, while that of the gull rookery soils gave the mean of 27 per mil during the breeding season of the birds. Soils of gull rookeries and of albatross rookery gave, on the average, a similar ratio of 17 per mil for Kjeldahl nitrogen, though its content of soils of gull rookeries was nearly two orders of magnitude higher than that of albatross rookery. Soils from penguin rookery showed the ratio of 32 per mil for Kjeldahl nitrogen, the enrichment of15N being two and a half times as large as that for soils of other rookeries.

Use of15N/14N rations to evaluate the food source of the mysid,Neomysis intermedia Czerniawsky, in a eutrophic lake in Japan

The stable isotope ratios of nitrogen were measured in the mysid,Neomysis intermedia, together with various biogenic materials in a eutrophic lake, Lake Kasumigaura, in Japan throughout a year of 1984/85. The mysid, particulate organic matter (POM, mostly phytoplankton), and zooplankton showed a clear seasonal change in δ15N with high values in spring and fall, but the surface bottom mud did not. A year to year variation as well as seasonal change in δ15N was found in the mysid. The annual averages of δ15N of each material collected in 1984/85 are as follows: surface bottom mud, 6.3‰ (range: 5.7–6.9‰); POM, 7.9‰ (5.8–11.8‰); large sized mysid, 11.6‰ (7.7–14.3‰); zooplankton, 12.5‰ (10.0–16.4‰); prawn, 13.2‰ (9.9–15.4‰); goby, 15.1‰ (13.8–16.7‰). The degree of15N enrichment by the mysid was determined as 3.2‰ by the laboratory rearing experiments. The apparent parallel relationship between the POM and the mysid in the temporal patterns of δ15N with about 3‰ difference suggests the POM (mostly phytoplankton) as a possible food source ofN. intermedia in this lake through the year.

Effect of high atmospheric CO2 concentration on δ13 of algae

Precambrian reduced carbon is more depleted in13C than what would be expected from the carbon isotopic composition of modern marine algae and algal mats. Since the photosynthetic carbon fixation by algae is the most likely source of the reduced carbon, the depletion has been considered an anomaly.We examined factors that might have contributed to the carbon isotope fractionation from inorganic sources through algae to organic matter in a sedimentary rock, and related laboratory obtainable data to those from Precambrian rocks. Laboratory culture experiments were then performed with nine strains of algae at various concentrations of carbon dioxide, and the result was interpreted according to the relationship.It indicated that the depletion could be understood in terms of a combined effect of fractionation factors, most depletion occurring at the fractionation during the photosynthetic carbon fixation. It also suggested that all but one algal strain incorporated bicarbonate as the source of carbon for its growth. The exception was a thermophilic, acidophilic alga, which must have used carbon dioxide as the carbon source.The present study suggests that Precambrian atmosphere was enriched in carbon dioxide roughly two orders of magnitude more than its present atmospheric level.

Carbon isotope composition of CH4 from rice paddies in Japan

Carbon isotope ratios (δ13C) for bubble CH4 in a submerged paddy soil were studied in Yokohama, Japan, throughout a growing period, and its variation was found. Bubble CH4 collected from other 33 paddy fields in Japan was also measured for its δ13C and the results agreed with Yokohama. Furthermore, the variation occurred irrespective of the amount and the type of supplied organic substances to the fields (whole rice straw, rice stubble, or compost). The δ13C value (average value of -55.9 ± 4.24‰) from these paddy fields was higher than those of the CH4 emitted from African and North American paddies. The higher value was little affected by their difference in the supplied organic substances. CH4 oxidation likely occurs for bubble CH4 in the shallow paddy fields. A rough estimate of the total CH4 production, using isotope mass balance, showed that 17 to 22% of organic carbon supplied to Japanese paddies transforms to CH4.