Diversity and Evolution of Algae: Primary Endosymbiosis

Oxygenic photosynthesis, the chemical process whereby light energy powers the conversion of carbon dioxide into organic compounds and oxygen is released as a waste product, evolved in the anoxygenic ancestors of Cyanobacteria. Although there is still uncertainty about when precisely and how this came about, the gradual oxygenation of the Proterozoic oceans and atmosphere opened the path for aerobic organisms and ultimately eukaryotic cells to evolve. There is a general consensus that photosynthesis was acquired by eukaryotes through endosymbiosis, resulting in the enslavement of a cyanobacterium to become a plastid. Here, we give an update of the current understanding of the primary endosymbiotic event that gave rise to the Archaeplastida. In addition, we provide an overview of the diversity in the Rhodophyta, Glaucophyta and the Viridiplantae (excluding the Embryophyta) and highlight how genomic data are enabling us to understand the relationships and characteristics of algae emerging from this primary endosymbiotic event.