Catalyzes the biosynthesis of presqualene diphosphate (PSPP). Works in combination with SSL-2 or SSL-3 to produce respectively squalene or botryococcene. In most other species, farnesyl diphosphate (FPP) is converted into squalene in a two-step reaction by a single enzyme. 2 (2E,6E)-farnesyl diphosphate = diphosphate + presqualene diphosphate kcat is 0.27 sec(-1) with farnesyl diphosphate as substrate. Not stimulated by NADPH addition. Belongs to the phytoene/squalene synthase family.