OBLIGATE

'Obligate' could be:

★ A verb (see obligation)

★ An adjective meaning "by necessity" (antonym ''facultative'') and used mostly in biology, especially in the phrases:

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★ Obligate parasite, a parasite that cannot survive without its host

★
★ Obligate aerobe, an organism that cannot survive without oxygen

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★ Obligate anaerobe, an organism that cannot survive in the presence of oxygen

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★ 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