Obligator

[verb ob-li-geyt; adjective ob-li-git, -geyt] /verb ˈɒb lɪˌgeɪt; adjective ˈɒb lɪ gɪt, -ˌgeɪt/

verb (used with object), obligated, obligating.
1.
to bind or morally or legally:
to obligate oneself to purchase a building.
2.
to pledge, commit, or bind (funds, property, etc.) to meet an .
adjective
3.
morally or legally bound; ; constrained.
4.
necessary; essential.
5.
Biology. restricted to a particular condition of life, as certain organisms that can survive only in the absence of oxygen: obligate anaerobe (opposed to ).
/ˈɒblɪˌɡeɪt/
verb
1.
to compel, constrain, or oblige morally or legally
2.
(in the US) to bind (property, funds, etc) as security
adjective
3.
compelled, bound, or restricted
4.
(biology) able to exist under only one set of environmental conditions: an obligate parasite cannot live independently of its host Compare facultative (sense 4)
v.

1540s, “to bind, connect;” 1660s, “to put under moral obligation,” back-formation from obligation, or else from Latin obligatus, past participle of obligare (see oblige). Oblige, with which it has been confused since late 17c., means “to do one a favor.” Related: Obligated; obligating.

obligate ob·li·gate (ŏb’lĭ-gĭt, -gāt’)
adj.
Able to exist or survive only in a particular environment or by assuming a particular role.
obligate
(ŏb’lĭ-gĭt, -gāt’)
Capable of existing only in a particular environment or by assuming a particular role. An obligate aerobe, such as certain bacteria, can live only in the presence of oxygen. An obligate parasite cannot survive independently of its host. Compare facultative.