Although controversial, chromalveolates are proposed as a clade whose common ancestor engulfed a photosynthetic red algal cell in a secondary endosymbiosis event. This secondary endosymbiosis led to the presence of red alga-derived plastids in chromalveolates, which include diverse groups such as diatoms, brown algae, and some important parasitic and photosynthetic protists. However, some chromalveolates appear to have lost these plastids or their genes, and the idea of chromalveolates as a monophyletic group remains debated and considered a working hypothesis subject to revision.