A major biological event contributing to the biodiversity of present-day organisms was the enslavement of a photosynthetic eubacterium by a primitive eukaryote and its entrapment as an integral part of the cell: the plastid. Since the first acquisition by eukaryotes of photosynthetic capability by primary endosymbiosis, two major lineages (red and green) appear to have arisen. Subsequently, an amazing variety of secondary endosymbioses took place, resulting in a complex assemblage of organisms containing photosynthetic organelles surrounded by multiple membranes. This assemblage includes the extant cryptomonads, chlorarachniophytes, chromophytes, haptophytes, euglenoids, and