Involved in the biosynthesis of carotenoids for the production of astaxanthin. Catalyzes the condensation of two molecules of geranylgeranyl diphosphate (GGPP) to give prephytoene diphosphate (PPPP) and the subsequent rearrangement of the cyclopropylcarbinyl intermediate to yield 15-cis phytoene. 2 (2E,6E,10E)-geranylgeranyl diphosphate = 15-cis-phytoene + 2 diphosphate ATP is required for the transferase activity but it does not seem to be hydrolyzed during the reaction. Carotenoid biosynthesis; astaxanthin biosynthesis. Carotenoid biosynthesis; phytoene biosynthesis. Belongs to the phytoene/squalene synthase family.