Involved in the biosynthesis of the hopanoid precursor squalene (SQ) from farnesyl diphosphate (FPP). Catalyzes the first step, the formation of presqualene diphosphate (PSPP) from two molecules of FPP. 2 (2E,6E)-farnesyl diphosphate = diphosphate + presqualene diphosphate Secondary metabolite biosynthesis; hopanoid biosynthesis. Belongs to the phytoene/squalene synthase family. HpnD subfamily.