Production: The Formation of Organic Matter
Primary producer organisms can form organic matter by acquiring and metabolically making use of energy and carbon (plus a variety of other essential elements). In terms of energy and carbon acquisition, some definitions may be useful. Obligate autotrophs carry out photosynthesis; hence they need light as the energy source (and will not grow in the dark) and fix carbon dioxide as the source of carbon. Facultative autotrophs may carry out photosynthesis, using light and CO2, as well as use organic carbon compounds as energy and carbon sources (so may grow in the dark if organic compounds are available). Some autotrophs may carry out chemosynthesis, in which a reduced compound is the source of energy. Heterotrophs can only use organic compounds as energy and carbon sources; these are basically consumers of materials fixed by primary producers, and are treated in other chapters below.