Matt McCarthy Hilary G. Close Amanda Demopoulos Nancy G. Prouty Natasha Vokhshoori 2021 <p><span>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&nbsp;</span><i>Bathymodiolus childressi</i><span>&nbsp;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</span>¹³<span>C) and nitrogen (δ</span>¹⁵<span>N), and δ</span>¹⁵<span>N values of individual amino acids (δ</span>¹⁵<span>N</span>_AA<span>) in both gill and muscle tissues, as well as δ</span>¹⁵<span>N</span>_AA-<span>derived parameters including trophic level (TL), baseline δ</span>¹⁵<span>N value (δ</span>¹⁵<span>N</span>_Phe<span>), and a microbial resynthesis index (Σ</span><i>V</i><span>), were used to investigate specific geochemical signatures of chemosynthesis. Our results show that δ</span>¹⁵<span>N</span>_AA<span>&nbsp;values provide a number of new proxies for relative reliance on chemosynthesis, including TL, ∑V, and both δ</span>¹⁵<span>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</span>¹⁵<span>N values, showing that percent contribution of chemoautotrophic versus heterotrophic production to seep mussel nutrition can be directly estimated from δ</span>¹⁵<span>N</span>_AA<span>&nbsp;values. Our results demonstrate that δ</span>¹⁵<span>N</span>_AA<span>&nbsp;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.</span></p> application/pdf 10.1111/gbi.12458 en Wiley New geochemical tools for investigating resource and energy functions at deep-sea cold seeps using amino-acid δ15N in chemosymbiotic mussels (Bathymodiolus childressi) article