Stable oxygen and hydrogen isotope analyses of fossil aquatic organisms, such as the chitinous head capsules of chironomid larvae (Chironomidae: Diptera), are promising proxies for inferring paleoecological conditions. In order for analyses of stable oxygen (delta(18)O) and hydrogen isotope ratios (delta(2)H) of fossil chironomid head capsules to be used effectively in paleoecological research, it is necessary to understand the factors controlling their stable oxygen and hydrogen composition. We cultured chironomid larvae in two isotopically distinct waters under controlled, replicated laboratory conditions. Chironomid larvae were fed on identical diets, to examine the degree to which water and diet influence the delta(18)O and delta(2)H of these organisms. We used a two-end member mixing model to determine the proportional contributions of oxygen and hydrogen from water to the oxygen and hydrogen of chironomid larvae. Our experiment demonstrated that 69.0 +/- 0.4% of oxygen and 30.8 +/- 2.6% of hydrogen in chironomid larvae are derived from habitat water. Our results show that oxygen isotopes from chironomid remains can better constrain past habitat water isotopic changes compared to hydrogen, due to 69% of the chironomid oxygen being influenced by habitat water. Our data add to a small but growing suite of comparative data on the sources of oxygen and hydrogen in animal tissues, and provide the first such analyses from aquatic insects.

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Abstract. Burial of organic carbon in marine sediments has a profound influence in marine biogeochemical cycles and provides a sink for greenhouse gases such as CO 2 and CH 4 . However, tracing organic carbon from primary production sources as well as its transformations in the sediment record remains challenging. Here we examine a novel but growing tool for tracing the biosynthetic origin of amino acid carbon skeletons, based on naturally occurring stable carbon isotope patterns in individual amino acids (δ 13 C AA ). We focus on two important aspects for δ 13 C AA utility in sedimentary paleoarchives: first, the fidelity of source diagnostic of algal δ 13 C AA patterns across different oceanographic growth conditions, and second, the ability of δ 13 C AA patterns to record the degree of subsequent microbial amino acid synthesis after sedimentary burial. Using the marine diatom Thalassiosira weissflogii, we tested under controlled conditions how δ 13 C AA patterns respond to changing environmental conditions, including light, salinity, temperature, and pH. Our findings show that while differing oceanic growth conditions can change macromolecular cellular composition, δ 13 C AA isotopic patterns remain largely invariant. These results emphasize that δ 13 C AA patterns should accurately record biosynthetic sources across widely disparate oceanographic conditions. We also explored how δ 13 C AA patterns change as a function of age, total nitrogen and organic carbon content after burial, in a marine sediment core from a coastal upwelling area off Peru. Based on the four most informative amino acids for distinguishing between diatom and bacterial sources (i.e., isoleucine, lysine, leucine and tyrosine), bacterially derived amino acids ranged from 10 to 15 % in the sediment layers from the last 5000 years, and up to 35 % during the last glacial period. The greater bacterial contributions in older sediments indicate that bacterial activity and amino acid resynthesis progressed, approximately as a function of sediment age, to a substantially larger degree than suggested by changes in total organic nitrogen and carbon content. It is uncertain whether archaea may have contributed to sedimentary δ 13 C AA patterns we observe, and controlled culturing studies will be needed to investigate whether δ 13 C AA patterns can differentiate bacterial from archeal sources. Further research efforts are also needed to understand how closely δ 13 C AA patterns derived from hydrolyzable amino acids represent total sedimentary proteineincous material, and more broadly sedimentary organic nitrogen. Overall, however, both our culturing and sediment studies suggest that δ 13 C AA patterns in sediments will represent a novel proxy for understanding both primary production sources, and the direct bacterial role in the ultimate preservation of sedimentary organic matter.

The rapid expansion of the aquaculture industry with carnivorous fish such as salmon has been accompanied by an equally rapid development in alternative feed ingredients. This has outpaced the ability of prevailing authentication method to trace the diet and origins of salmon products at the retail end. To close this gap, we developed a new profiling tool based on amino acid δC fingerprints. With this tool, we discriminated with high-accuracy among wild-caught, organically, and conventionally farmed salmon groups, as well as salmon fed alternative diets such as insects and macroalgae. Substitution of fishmeal with macroalgae was detected at 5% difference level. The δC fingerprints of essential amino acids appear particularly well suited for tracing protein sources, and the non-essentials for tracing lipid origins (terrestrial vs. aquatic). In an industry constantly developing new feed proteins and functional additives, our method is a promising tool for tracing salmon and other seafood products.

Supplementation of nutrients by symbionts enables consumers to thrive on resources that might otherwise be insufficient to meet nutritional demands. Such nutritional subsidies by intracellular symbionts have been well studied; however, supplementation of de novo synthesized nutrients to hosts by extracellular gut symbionts is poorly documented, especially for generalists with relatively undifferentiated intestinal tracts. Although gut symbionts facilitate degradation of resources that would otherwise remain inaccessible to the host, such digestive actions alone cannot make up for dietary insufficiencies of macronutrients such as essential amino acids (EAA). Documenting whether gut symbionts also function as partners for symbiotic EAA supplementation is important because the question of how some detritivores are able to subsist on nutritionally insufficient diets has remained unresolved. To answer this poorly understood nutritional aspect of symbiont-host interactions, we studied the enchytraeid worm, a bulk soil feeder that thrives in Arctic peatlands. In a combined field and laboratory study, we employed stable isotope fingerprinting of amino acids to identify the biosynthetic origins of amino acids to bacteria, fungi and plants in enchytraeids. Enchytraeids collected from Arctic peatlands derived more than 80% of their EAA from bacteria. In a controlled feeding study with the enchytraeid Enchytraeus crypticus, EAA derived almost exclusively from gut bacteria when the worms fed on higher fibre diets, whereas most of the enchytraeids' EAA derived from dietary sources when fed on lower fibre diets. Our gene sequencing results of gut microbiota showed that the worms harbour several taxa in their gut lumen absent from their diets and substrates. Almost all gut taxa are candidates for EAA supplementation because almost all belong to clades capable of biosynthesizing EAA. Our study provides the first evidence of extensive symbiotic supplementation of EAA by microbial gut symbionts and demonstrates that symbiotic bacteria in the gut lumen appear to function as partners both for symbiotic EAA supplementation and for digestion of insoluble plant fibres.