| Summary: | Photosynthesis allows for the formation of biomass from inorganic carbon and therefore greatly enhances the amount of organic material on planet Earth. Especially, oxygenic photosynthesis removed a major bottleneck in the formation of biomass by utilising ubiquitous water (H<sub>2</sub>O) and CO<sub>2</sub> molecules as raw materials for organic molecules. This, over billions of years, shaped the world into the form we know today, with an oxygen-containing atmosphere, largely oxygenated water bodies and landmasses consisting of sediment rocks. Oxygenic photosynthesis furthermore enabled the evolution of aerobic energy metabolism, and it would be very difficult to imagine animal (including human) life in the absence of molecular oxygen as an electron acceptor. Oxygenic photosynthesis first, and exclusively, evolved in cyanobacteria. However, eukaryotes also learned to photosynthesise, albeit with a trick, which is the integration of formerly free-living cyanobacteria into the eukaryotic cell. There, the former bacteria became endosymbionts, and from these endosymbionts, the photosynthetic organelles (termed plastids) evolved. In almost all major groups of eukaryotes, plastid-containing members are found. At the same time, plastid-related features also indicate that these plastids form a monophyletic group. This can be explained by the transfer of plastids between the eukaryotic super-groups, leading to plastids being found in groups that are otherwise non-photosynthetic. In this chapter, we discuss the evolutionary origin of plastids, with a special emphasis on the evolution of plankton algae, such as diatoms or dinoflagellates, who acquired their plastids from other photosynthetic eukaryotes.
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