Nutrition refers to how an organism obtains two resources for synthesizing organic compounds: energy and a source of carbon. Species that use light energy are termed phototrophs. Chemotrophs obtain their energy from chemicals taken from the environment. IF an organism needs only the inorganic compound CO2 as a carbon source, it is called an autotrophy. Heterotrophs require atleast one organic nutrient - glucose, for instance - as a source of carbon for making other organic compounds. We can combine the phototroph-versus-chemotroph (energy source) and autotrophy-versus-heterotroph (carbon source) criteria to classify prokaryotes based on four major modes of nutrition:

1. Photoautotrophs are photosynthetic organisms that harness light energy to drive the synthesis of organic compounds from carbon dioxide. The specialized metabolic machinery of these organisms include internal membranes with light-harvesting pigment systems. Among the diverse groups of photosynthetic eukaryotes - plants and certain protests - also fit this nutritional category.

2. Chemoautotrophs need only CO2 as a carbon source, but instead of using light for energy, these bacteria obtain energy by oxidizing inorganic substances. Chemical energy is extracted from hydrogen sulfide (H2s), ammonia (NH2), ferrous ions (Fe2+), or some other chemical, depending on the species. This mode of nutrition is unique to certain prokaryotes. For instance, archaebacteria of the genus Sulfobolus oxidize sulfur.

3. Photoheterotrophs can use light to generate ATP but must obtain their carbon in organic form. This mode of nutrition is restricted to certain prokaryotes.

4. Chemoheterotrophs must consume organic molecules for both energy and carbon. This nutritional mode is found widely among prokaryotes, protests, fungi, animals, and even some plants.