The power and pitfalls of amino acid carbon stable isotopes for tracing the use and fate of basal resources in food webs

Natural and anthropogenic stressors are spatiotemporally complex, having indirect effects on the composition and biomass of organisms at the base of a food web, and their availability and nutritional quality. Because basal organisms synthesise the biomolecules essential for metazoan growth and survival (i.e. basal resources), understanding the connections between basal resources and consumers across diverse time scales is needed to fully comprehend their impact on food webs. Traditional approaches using bulk stable isotope ratios have provided insight into basal resource use, but lack specificity in identifying multiple basal resources and their transfer through ecosystems. The development of compound-specific stable isotope analyses now allows researchers to trace the trophic transfer of specific biomolecules. This paper provides an overview of the advances and challenges associated with tracing basal resources with carbon stable isotopes in amino acids (δ13C-AA). We develop a conceptual framework for understanding the mechanistic underpinning of δ13C-AA values. Subsequently, formal definitions of associated terminologies that have so far been lacking in the literature are proposed. We empirically highlight the diagnostic ability of the relative offsets between δ13C values of essential amino acids, termed δ13C-EAA patterns. As these offsets remain largely unaltered during trophic transfer and across varying environments, they can be used as fingerprints to trace spatiotemporal shifts in basal resource use within food webs. Given the stable preservation of amino acids in many metazoan tissues, δ13C-EAA fingerprints can provide insights into basal resource use in food webs from geological history through to the contemporary. The added value of non-essential amino acids as metabolic biomarkers are explored and demonstrated in an archaeological context. We provide thorough overviews of the analytical and statistical methodologies involved in making robust inferences in food web studies. The constraints and pitfalls of δ13C-AA data are discussed, such as issues with basal resource specificity, de novo synthesis, and problems with large compilation datasets. Taken together, δ13C-AA values provide a powerful tool for understanding the specific use of basal resources in food webs on various spatiotemporal scales, but careful consideration and characterization of basal resources is necessary to ensure accurate estimations of proportional use.