New geochemical tools for investigating resource and energy functions at deep‐sea cold seeps using amino acid δ15N in chemosymbiotic mussels (Bathymodiolus childressi)

Abstract In order to reconstruct the ecosystem structure of chemosynthetic environments in the fossil record, geochemical proxies must be developed. Here, we present a suite of novel compound‐specific isotope parameters for tracing chemosynthetic production with a focus on understanding nitrogen dynamics in deep‐sea cold seep environments. We examined the chemosymbiotic bivalve Bathymodiolus childressi from three geographically distinct seep sites on the NE Atlantic Margin and compared isotope data to non‐chemosynthetic littoral mussels to test whether water depth, seep activity, and/or mussel bed size are linked to differences in chemosynthetic production. The bulk isotope analysis of carbon (δ 13 C) and nitrogen (δ 15 N), and δ 15 N values of individual amino acids (δ 15 N AA ) in both gill and muscle tissues, as well as δ 15 N AA‐ derived parameters including trophic level (TL), baseline δ 15 N value (δ 15 N Phe ), and a microbial resynthesis index (Σ V ), were used to investigate specific geochemical signatures of chemosynthesis. Our results show that δ 15 N AA values provide a number of new proxies for relative reliance on chemosynthesis, including TL, ∑V, and both δ 15 N values and molar percentages (Gly/Glu mol% index) of specific AA. Together, these parameters suggested that relative chemoautotrophy is linked to both degree of venting from seeps and mussel bed size. Finally, we tested a Bayesian mixing model using diagnostic AA δ 15 N values, showing that percent contribution of chemoautotrophic versus heterotrophic production to seep mussel nutrition can be directly estimated from δ 15 N AA values. Our results demonstrate that δ 15 N AA analysis can provide a new set of geochemical tools to better understand mixotrophic ecosystem function and energetics, and suggest extension to the study of ancient chemosynthetic environments in the fossil record.