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Compound-specific isotopic analysis of amino acids (CSIA-AA) has emerged in the last decade as a powerful approach for tracing the origins and fate of nitrogen in ecological and biogeochemical studies. This approach is based on the empirical knowledge that source AAs (i.e., phenylalanine), fractionate 15 N very little (<0.5‰) during trophic transfer, whereas trophic AAs (i.e., glutamic acid), are greatly (~6-8‰) enriched in 15 N during each trophic step. The differential fractionation of these two AA groups can provide a valuable estimate of consumer trophic position that is internally indexed to the baseline δ 15 N value of the integrated food web. In this paper, we critically review the analytical methods for determining the nitrogen isotopic composition of AAs by gas chromatography/isotope-ratio mass spectrometry. We also discuss methodological considerations for accurate trophic position assessment of organisms using CSIA-AA. We then discuss the advantages and challenges of the CSIA-AA approach by examining published studies including trophic position assessment in various ecosystems, reconstruction of ancient human diets, reconstruction of animal migration and environmental variability, and assessment of marine organic matter dynamics. It is clear that the CSIA-AA approach can provide unique insight into the sources, cycling, and trophic modification of organic nitrogen as it flows through systems. However, some uncertainty still exists in how biochemical, physiological, and ecological mechanisms affect isotopic fractionation of trophic AAs. We end this review with a call for continued exploration of the mechanisms of AA isotopic fractionation, through various studies to promote the evolution of the rapidly growing field of CSIA-AA.

Scientific drilling at a submarine mud volcano shows that geofluid migration stimulates methanogenesis in the deep biosphere.

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