The impact of metabolism on stable isotope dynamics: a theoretical framework

Abstract

Stable isotope analysis is a powerful tool used for reconstructing individual life histories, identifying food-web structures and tracking flow of elemental matter through ecosystems. The mechanisms determining isotopic incorporation rates and discrimination factors are, however, poorly understood which hinders a reliable interpretation of field data when no experimental data are available. Here, we extend dynamic energy budget (DEB) theory with a limited set of new assumptions and rules in order to study the impact of metabolism on stable isotope dynamics in a mechanistic way. We calculate fluxes of stable isotopes within an organism by following fluxes of molecules involved in a limited number of macrochemical reactions: assimilation, growth but also structure turnover that is here explicitly treated. Two mechanisms are involved in the discrimination of isotopes: (i) selection of molecules occurs at the partitioning of assimilation, growth and turnover into anabolic and catabolic sub-fluxes and (ii) reshuffling of atoms occurs during transformations. Such a framework allows for isotopic routing which is known as a key, but poorly studied, mechanism. As DEB theory specifies the impact of environmental conditions and individual state on molecule fluxes, we discuss how scenario analysis within this framework could help reveal common mechanisms across taxa.

Food habits of the farmer damselfishStegastes nigricansinferred by stomach content, stable isotope, and fatty acid composition analyses

“…This high trophic-level fractionation value seems to be explained by differences in the diet quality (in C:N terms) and metabolism between herbivores and carnivores (Mill et al 2007;Pecquerie et al 2010). In fact, Δ 15 N value of animals increases following the increase of C:N value of their diet (Adams and Sterner 2000).…”

Dynamic energy budget theory restores coherence in biology

“…The complementarity between the two approaches is important in bringing new insights to unsolved problems that link the sub-individual and the individual levels such as the mechanisms underlying gene expression or the mechanisms underlying ageing (van Leeuwen et al 2010). Pecquerie et al (2010) develop DEB theory further to provide a framework for stable isotope dynamics. The fundamental processes of the standard DEB model-assimilation, dissipation and growth-are further detailed into their anabolic and catabolic transformations to account for the mass balance of stable isotopes.…”

“…The links that DEB theory establishes between the sub-and supra-individual levels together with the high amount of throughput data becoming available at the sub-individual level have allowed the use, test and development of DEB models at this organization level (van Leeuwen et al 2010;Pecquerie et al 2010;Vinga et al 2010). Vinga et al (2010) use DEB theory for a top-down approach to understand the dynamic behaviour of metabolites.…”

“…The mass, energy and entropy description of all fluxes, provided by DEB theory, are most useful to study the internal concentration of specific compounds such as isotopes (Pecquerie et al 2010), reactive oxygen species (van Leeuwen et al 2010) and toxins (Jager & Klok 2010;Ducrot et al 2010) that affect the performance of organisms at the individual level. The mode of action of a compound is, in the context of DEB theory, defined by the parameters that are affected.…”

“…More specifically they discuss the relationship between chlorophyll, biomass and the production of exopolimeric substances; these are key features in the interpretation of remote sensing data. Pecquerie et al (2010) evaluate how variations in isotope concentrations in the environment work out for the organism. They do not make the common assumption that isotope dynamics is at equilibrium, and include the full metabolism in their analysis.…”