| Summary: | Analytical methodologies have been developed for the compound-specific isotope analysis of amino acids (CSIA-AA). Amino acids are the molecules of choice as they are the building blocks of proteins and ultimately, the building blocks of life. The isotopic signature can also be manipulated to generate patterns, calculate trophic levels and provide other tools to further understanding of C and N origins and flows from their source and through the food web. Although many recent studies have been developing, using and expanding the AA method, there are still a lot of possible applications of the method to be explored. In the present study, the AA method was tested in multiple challenging scenarios with samples of different matrices, different sizes and from different ecosystems. At the edge between marine-terrestrial environments, the AA method was applied to unravel the trophic relationship that links a scarab larva and mites. Suspected to be parasitic, the interpretation of C and N isotopic signature in AAs (δ13CAA and δ15NAA respectively) and associated patterns revealed that the scarab larva - mite trophic relationship was instead commensalistic. Similar δ13CAA indicated the affiliation to the same C3-plant based food web, the trophic level results clearly showed that the mite was two trophic levels higher than the larva, hence proving its none-parasitic behaviour; and finally, a shift in δ15NPhe baseline redirected the mite source of C and N to the larva’s faeces. The AA method was then applied on suspended particulate organic matter (POM) from oligotrophic areas in the North and South Atlantic oceans, in order to identify the origins of C and N fuelling the base of the marine food web. δ13CAA patterns revealed the domination of autotrophic-sourced material in the North and South Atlantic ocean. Furthermore, while δ15NAA patterns indicated that the primary production in shallow waters of the studied area was dominated by prokaryotes in the North Atlantic Ocean, the total fractionation coefficient (εtot) pointed out the eukaryotic and prokaryotic origins of N across the 40 oS transect in the South Atlantic Ocean. This information was backed up by the depth variations observed in δ15NPhe values. Complementarily, the heterotrophic recycling parameter (ΣV) indicated that the main mechanism driving 15N enrichment of suspended POM in both oceans appeared to be the isotope fractionation associated with heterotrophic degradation; especially at depth below 100 m. Heterotrophic bacteria re-synthesis was more active in the South Atlantic Ocean. Furthermore, AAs role in oligotrophic ocean were investigated using an analytical methodology developed specially to measure dissolved free amino acids (DFAA) concentrations in seawater. The results of all research confirmed in polyvalent use of the AA method and encourage future research to use it as a reliable tool for understanding the N-cycle in oligotrophic open oceans.
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