Serial endosymbiosis. Plastids (pigment-bearing organelles such as chloroplasts) evolved from a cyanobacterium that was engulfed by a heterotrophic eukaryote (primary endosymbiosis).
That ancestral eukaryote diversified into red algae and green algae, some of which were subsequently engulfed by other eukaryotes (secondary endosymbiosis); these plastids may retain their own nucleus.