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
 * Obligate biped, Bipedalism designed to walk on two legs
 * Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
 * Obligate chimerism, a kind of organism with two distinct sets of DNA, always
 * 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