Introduction
In addition to energy, all organisms require specific chemical elements to function and grow. Organisms get these elements by absorbing them from the environment or by consuming other organisms, living or dead.
Iron, for example, is needed by all organisms for several important metabolic functions, but how those organisms get their iron and where it comes from vary substantially. Phytoplankton in the Atlantic Ocean may take up iron that came from dust that blew in from the Sahara Desert. Lions in African savannas get their iron from the prey they kill and consume. Aphids get their iron in the sap they suck from a plant, whereas the plant takes up water containing dissolved iron from the soil. The ultimate source of all of this iron, however, is solid minerals in Earth's crust, which are subjected to chemical transformations as they move through the different physical and biological components of ecosystems.The study of the physical, chemical, and biological factors that influence the movements and transformations of elements is known as biogeochemistry. An understanding of biogeochemistry is important for determining the availability of nutrients, which are defined as the chemical elements an organism requires for its metabolism and growth. Nutrients must be present in certain chemical forms to be available for uptake by organisms. The rate at which physical and chemical transformations occur determines the supply of nutrients, which in turn influences organismal function and population growth. Biogeochemistry also encompasses the study of non-nutrient elements that can serve as tracers in ecosystems and of pollutant compounds, such as persistent organic pollutants and heavy metals, that cause environmental damage. Biogeochemistry is a discipline that integrates contributions from soil science, hydrology, and atmospheric science as well as ecology.
In this chapter, we will consider the biological, chemical, and physical factors that control the supply and availability of nutrients in ecosystems. We will emphasize nutrient requirements and acquisition by autotrophs because they in turn are the principal source of nutrients for heterotrophs. We will describe what nutrients are most important, their sources, and how they enter ecosystems, and review some of the important chemical and biological transformations that constitute the cycling of nutrients in ecosystems. In Concept 25.1, we will consider the global-scale cycling of some of these elements.
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