Photosynthetic eukaryotes from marine and terrestrial environments demonstrate a schism of light-harvesting pigment composition. Land plants employ chlorophyll (Chl) a and Chl b for light energy capture. In oceans, instead, the most abundant eukaryotic algal groups (diatoms, haptophytes, and dinoflagellates) use Chl c to replace Chl b. Furthermore, many of the algae contain a large amount of fucoxanthin in their light-harvesting antennae. The biosynthetic paths of Chl c and fucoxanthin had long remained elusive.
We recently employed genetics to reveal biosynthetic enzymes of Chl c and fucoxanthin (collaboratively) in the diatom Phaeodactylum tricornutum (Bai et al. 2022 PNAS; Cao et al. 2023 Plant Cell; Jiang et al. 2023 Science). Intriguingly, whereas the enzymes are conserved among unicellular algae, the multicellular brown algae (kelps) accumulate Chl c and fucoxanthin but lack the homologs of the diatom enzymes found. Identification of the counterpart enzymes for the biosynthesis of Chl c and fucoxanthin in brown algae is in progress.
In addition to light harvesting, diatoms and green plants diverge in light signal detection. Our recent findings in P. tricornutum reveal a photoreceptor with a LOV domain that detects high light and a bZIP domain that directly activates the transcription of photoprotective subunits of the light-harvesting complex (unpublished); contrastingly, green plants typically have LOV domains paired with non-transcription factor domains. This research will be presented alongside our studies on pigment biosynthesis pathways.