Detoxifying chlorophylls was crucial for eukaryote evolution

Excited chlorophylls release reactive oxygen species, which are harmful to cells. Heterotrophic eukaryotes deal with this issue by catabolising chlorophylls into 13²,17²-cyclopheophorbide enols (CPEs). In order to establish how widespread this catabolism is, Kashiyama et al. (2019) cultured 183 strains of eukaryotes across the tree of life and tested for their ability to produce CPEs with high-performance liquid chromatography. CPEs were found in cultures of all major eukaryotic assemblages except for the Archaeplastida, which contain the red and green algae. It thus seems that primary photoautotrophs do not produce CPEs, while eukaryotes, which underwent secondary endosymbiosis, do. This in turn implies that this chlorophyll degrading pathway was inherited from the heterotrophic ancestors of secondary photoautotrophs. The authors suggest that the lack of CPEs in Archaeplastida cultures is due to the usage of a different mechanism to detoxify chlorophylls.

Kashiyama et al. (2019) provide remarkably broad evidence for the exaptation of chlorophyll catabolism, originally used in herbivory, to accommodate new cell organelles. However, CPE-production was measured in comparatively few photoautotrophic clades. Furthermore, this form of chlorophyll catabolism was not found in photosynthetic Stramenopiles. More research into the evolution of stramenopilian photosynthetic endosymbiosis is required to shed light on this knowledge gap.

Kashiyama, Y., Yokoyama, A., Shiratori, T., Hess, S., Not, F., Bachy, C., ... & Ishikawa, T. (2019). Taming chlorophylls by early eukaryotes underpinned algal interactions and the diversification of the eukaryotes on the oxygenated Earth. The ISME Journal, 13(8), 1899-1910.