Many algae and plant species contain photosynthetic membrane-bound organelles called plastids that are actually remnants of a free-living cyanobacterium. At some point in evolutionary history, a cyanobacterium was engulfed by an ancestral alga, trapping it forever as a host-controlled endosymbiont in a process called organellogenesis. All modern algae and plants are the descendants of this ancestral alga containing the first plastid. But as if by karmic intervention, some of these algae were themselves engulfed during secondary endosymbiotic events, generating what are known as complex algae.
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Source: Phys.org