Organisms capable of using CO₂ as principal carbon source are called:

The correct answer is A, Autotrophs. Autotrophs take carbon dioxide from their surroundings and build their own organic food from it, so CO₂ is their main source of carbon. The word means self-feeding. Most autotrophs are photoautotrophs: green plants, algae and cyanobacteria use sunlight, water and carbon dioxide in photosynthesis to make glucose and release oxygen. A smaller group, the chemoautotrophs, such as nitrifying and sulphur bacteria, get the energy for fixing carbon from chemical reactions instead of light. Autotrophs are the producers at the base of every food chain. Option B is wrong because heterotrophs, such as animals and fungi, cannot fix carbon dioxide and get their carbon by eating ready-made organic food. Option C is wrong because parasites, like Cuscuta, tapeworm or the malaria parasite, are heterotrophs that take food from a living host. Option D is wrong because decomposers, mainly bacteria and fungi, get carbon by breaking down dead organic matter, and in doing so they return CO₂ to the air. Exam tip: autotroph = producer, fixes CO₂; heterotroph = consumer; decomposer = recycles nutrients.