7 Endosymbiosis and the Diversification of Eukaryotic Algae

With a Focus on Glaucophytes and Chlorarachniophytes

If you could travel back in time 3 billion years or so, prokaryotes would likely be the only life forms you would find on Earth. You would have to point the time machine ahead 1.5 billion years to find the earliest eukaryotes. But if you could fast-forward about 500 million years—to 1 billion years or so prior to the present day—you could find diverse phyla of eukaryotic algae, which are known from fossils. This evolutionary blossoming of eukaryotes was fostered by the development of a stratospheric layer of ozone that protected life in shallow waters from UV damage. The early ozone layer, generated from oxygen released from cyanobacterial photosynthesis, helped the first eukaryotes to thrive and diversify, just as today’s ozone layer protects modern life on Earth. An equally important role was played by endosymbiosis, a state in which organisms live within the cells or bodies of others. For example, cyanobacteria living symbiotically within the cells of early, non-photosynthetic eukaryotes endowed them with the power of photosynthesis and became the first plastids. Since then, endosymbiosis has continued to be an important ecological and evolutionary force.

This chapter begins by briefly considering the origin of eukaryotic algae and defining several types of endosymbiosis that have influenced algal plastid evolution. We then focus on the widespread occurrence of endosymbiosis in the modern world and the roles of endosymbiosis in the diversification of algal phyla, highlighting the Glaucophyta and Chlorarachniophyta.