A New Approach to Homeostatic Regulation: Towards a Unified View of Physiological and Ecological Concepts

Abstract

Stoichiometric homeostasis is the ability of an organism to keep its body chemical composition constant, despite varying inputs. Stoichiometric homeostasis therefore constrains the metabolic needs of consumers which in turn often feed on resources not matching these requirements. In a broader context, homeostasis also relates to the capacity of an organism to maintain other biological parameters (e.g. body temperature) at a constant level over ambient environmental variations. Unfortunately, there are discrepancies in the literature and ecological and physiological definitions of homeostasis are disparate and partly contradictory. Here, we address this matter by reviewing the existing knowledge considering two distinct groups, regulators and conformers and, based on examples of thermo- and osmoregulation, we propose a new approach to stoichiometric homeostasis, unifying ecological and physiological concepts. We suggest a simple and precise graphical way to identify regulators and conformers: for any given biological parameter (e.g. nutrient stoichiometry, temperature), a sigmoidal relation between internal and external conditions can be observed for conformers while an inverse sigmoidal response is characteristic of regulators. This new definition and method, based on well-studied physiological mechanisms, unifies ecological and physiological approaches and is a useful tool for understanding how organisms are affected by and affect their environment.

Growth rate, protein:RNA ratio and stoichiometric homeostasis of submerged macrophytes under eutrophication stress

“…Stoichiometric homeostasis is the ability of an organism to keep its body chemical composition constant, despite varying inputs (Meunier et al, 2014;Sterner and Elser, 2002). Generally, autotrophic organisms are considered to be nonhomeostatic or weakly homeostatic, whereas heterotrophs are thought to be strictly homeostatic (Persson et al, 2010;Sterner and Elser, 2002).…”

“…Plant stoichiometry varies with growth rate and the surrounding environment (Ågren and Weih, 2012). Stoichiometric homeostatic regulation reflects underlying physiological and biochemical allocations as organisms respond to their surrounding environments (Hessen et al, 2004) and thus the degree of homeostasis may be highly relevant to fitness and to a species' ecological strategy on the one hand (Frost et al, 2005) and to recycling processes of superfluous material on the other one (Meunier et al, 2014).…”

An Operational Framework for the Advancement of a Molecule-to-Biosphere Stoichiometry Theory

“…Interestingly, ontogenetic stage and body size also strongly affect autotroph stoichiometry, reflecting the fundamental links between growth rate, biomolecules and elements as described in the Growth rate hypothesis (Agren, 2008;Elser et al, 2010). Heterotrophs are classically viewed as maintaining a strict stoichiometric homeostasis, but the paradigm is slowly shifting toward the view that the degree of homeostasis differs from strict to loose in heterotrophic species (Persson et al, 2010;Meunier et al, 2014). It is still unclear what determines the degree of homeostasis (Table 1).…”

“…Potential candidates are ecological factors such as mortality rates (Wang et al, 2012) and the degree of osmotrophy (bacteria, fungi, and flagellates are often highly non-homeostatic, see Godwin and Cotner, 2015;Golz et al, 2015;Danger et al, 2016). Under natural conditions, non-homeostatic species, or conformers-to borrow the concept from physiological studies of homeostasis, are likely to show more intraspecific variability than homeostatic species, or regulators (Meunier et al, 2014). But the latter may still see substantial stoichiometric variability between different life stages or genotypes.…”

“…This vision of homeostasis in heterotrophs is increasingly challenged (Persson et al, 2010). The quest is now for a mechanistic understanding of the determinants of the degree of homeostasis in organisms (Meunier et al, 2014; Table 2). The main physiological consequence of strict homeostasis is a decrease in growth rate because one element becomes limiting (Sterner and Hessen, 1994).…”