Obligate

Obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:

• Obligate aerobe, an organism that cannot survive without oxygen
• Obligate anaerobe, an organism that cannot survive in the presence of oxygen
• Obligate air-breather, a complex organism which obtains required oxygen directly from the atmosphere through respiration rather than extracting dissolved oxygen from water, as, for example, the South American lungfish
• Obligate carnivore, a true carnivore solely dependent for survival on the nutrients found in animal flesh.
• Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
• Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host
• Obligate photoperiodic plant, a plant requiring long or short nights before flowering
• Obligate symbionts, organisms which can only live together in a symbiosis
• Obligate hibernation, a state used by some mammals to survive conditions where food is scarce.

See also

• (antonym) Facultative