Obligate

Obligate could be:

An adjective meaning "by necessity" (antonym facultative) and used mostly in biology, especially in the phrases:
Obligate parasite, a parasite that cannot survive without its host
Obligate intracellular parasite, a parasitic microorganism that cannot reproduce outside their host cell
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

See also

(antonym) Facultative