Nutrient dynamics

Energy enters the biosphere principally as sunlight and is transformed into energy-rich organic substrates by photosynthesis and chemosynthesis. This organic matter, consumed either living or dead, fuels the metabolism of heterotrophs and in the process is returned to its inorganic state as respired carbon dioxide (CO2). Various chemical compounds of life must be acquired by plants in order for synthesis of new organic matter to take place, and by animals and microbes to sustain their growth and metabolism. Although energy exhibits a one-way flow from synthesis to dissipation via ecosystem metabolism, the chemical constituents of living organisms are continuously reutilized as they cycle between the biota and the environment. These chemical constituents are referred to as nutrients because they are necessary to sustain life, and the supply of nutrients often determines the biological productivity of an ecosystem.