The vast majority of the autotrophic production of chemical energy on Earth occurs through photosynthesis, a process that uses sunlight to provide the energy needed to take up carbon dioxide and synthesize organic compounds, principally carbohydrates.

Although its contribution to the global energy picture is smaller, chemosynthesis (also known as Chemolithotrophy), a process that uses energy from inorganic compounds to produce carbohydrates, is important to some key bacteria involved in nutrient cycling (see Concept 22.2) and in some unique ecosystems, such as hydrothermal vent communities (see the Case Study in Chapter 20). Because the energy derived from photosynthesis and chemosynthesis is stored in the carbon-carbon bonds of the organic compounds produced by these processes, ecologists often use carbon as a measure of energy.