Introduction

Physiological processes are responsible for the multitude of functions that are essential for living organisms. These processes include the acquisition, incorporation, turnover, and release of energy and nutrients (Figure 1), and they ultimately affect the elemental composition (stoichiometry) of organisms. Whereas some of the processes are regulated so as to maintain a homeostatic stoichiometry (e.g., selective nutrient uptake or excretion, and synthesis of basic macromolecular structures), other processes will invariably result in changes in stoichiometry (e.g., excess nutrient uptake and storage). Rates of the physiological processes are affected by environmental factors (both abiotic and biotic) and are of great importance for the ecological role and fitness of organisms. In addition to the impact of these physiological processes on organismal stoichiometry, they are also subject to evolution by natural selection and ultimately affect the turnover of nutrients in entire ecosystems. Many physiological processes are ubiquitous in organisms because they share basic metabolic pathways and cell structures. Each of these metabolic pathways and cell structures involves and/or produces a specific setup of macromolecules or other compounds, and because these different types of compounds are characterized by specific elemental stoi-chiometries, the allocation to different compounds will affect organismal stoichiometry. Thus, there are inevitably strong links between ecological performance, physiological function, allocation patterns, and elemental stoichiometry of organisms.

In this article, basic principles regarding regulation of the elemental stoichiometry of organisms and their molecular constituents (both organic and inorganic) are presented in

Autotrophs

CO2 PO4-

Ca2+

Acquisition

Uptake of inorganic nutrients Gas diffusion

Light energy

Light energy

Ca2+

Incorporation and turnover Release

Gene expression Ion release

Photosynthesis Exudation of organic matter

Biomolecule synthesis Gas diffusion

Storage

Respiration

Heterotrophs

Acquisition

Food selectivity Ingestion of particles Digestion Assimilation

Gas diffusion and transport

Ca2+

PO4-

Incorporation and turnover

Gene expression Biomolecule synthesis Storage Respiration

Release

Excretion of ions and organic matter Egestion of organic matter Gas diffusion and transport

Figure 1 Conceptual outline of the acquisition, incorporation, turnover, and release of nutrients by autotrophs (plants) and heterotrophs (metazoan animals). The physiological processes of the organisms involve key metabolic pathways and molecular structures that are ubiquitous (e.g., synthesis of nucleic acids and proteins, and respiration), restricted to autotrophs (e.g., uptake of inorganic nutrients, and photosynthesis), or restricted to heterotrophs (e.g., feeding on particles). The incorporation and turnover comprise a biosynthetic machinery required for anabolic and catabolic processes, as well as organic and inorganic compounds that are synthesized and/or transformed. The physiological processes are similar in that they are performed in response to abiotic and biotic environmental stressors, and they affect the ecological performance of the organism, as well as their elemental stoichiometry. Modified from Frost PC, Evans-White MA, Finkel ZV, Jensen TC, and Matzek V (2005) Are you what you eat? Physiological constraints on organismal stoichiometry in an elementally imbalanced world. Oikos 109: 18-28.

the first section. Then the acquisition, incorporation, turnover, and release of elements are discussed with special emphasis on physiological processes that link molecular processes and structures with elemental stoichiometry and the maintenance of homeostasis.

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