pessimal vs pessimum

pessimal and pessimum both are adjectives.

Word	Noun	Adjective	Verb	Adverb
pessimal	No	Yes	No	No
pessimum	No	Yes	No	No

As adjectives, pessimal and pessimum are synonyms defined as:

pessimal and pessimum: of an organism's environment; least favorable for survival

pessimal (adjective)	pessimum (adjective)
of an organism's environment; least favorable for survival	of an organism's environment; least favorable for survival