obligate

As an adjective, 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 term used in fish physiology to describe those that respire entirely from the atmosphere
• Obligage biped, Bipedalism designed to walk on two legs
• Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
• Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.
• 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 that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions
• Obligate symbionts, organisms that can only live together in a symbiosis