"of or pertaining to the algae," 1846, from Latin alga (see algae) + -al (1).
Entries linking to algal
algae n.
(plural), 1794, from alga (singular), 1550s, from Latin alga "seaweed," which is of uncertain origin. Perhaps connected to Latin ulva "grass-like or rush-like aquatic plant," or perhaps from a PIE root meaning "to putrefy, rot," but de Vaan considers this unlikely and suggests it might be a foreign loan-word.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).