The Evolution of Algae by Secondary and Tertiary Endosymbiosis

The endosymbiotic origin of plastids (chloroplasts) from cyanobacteria was a pivotal event in eukaryotic evolution. By giving certain eukaryotes the ability to carry out photosynthesis it contributed to the oxygenation of the biosphere and provided the impetus for subsequent endosymbiotic events. On multiple occasions, so-called 'primary' plastids have been passed from one eukaryotic lineage to another by the process of secondary endosymbiosis, that is the assimilation of a primary plastid-bearing alga by a eukaryotic heterotroph. Despite the prominence of primary plastid-bearing organisms such as green algae and their land plant descendents, the bulk of the algal biodiversity present on Earth today is known to have acquired photosynthesis via eukaryote-eukaryote endosymbioses. Such organisms include bloom-forming haptophytes, diatoms and giant kelp. In this chapter, I summarize the known diversity of secondary plastid-bearing algae and provide an overview of the current state of knowledge of how their plastids have evolved. Despite numerous advances made possible by the comparative genomics revolution, many fundamental questions about the tempo and mode of secondary plastid diversification remain. This includes the extent to which cryptic tertiary plastid acquisitions have contributed to the spread of plastids.