A chlorophyll c synthase widely co-opted by phytoplankton.

Marine and terrestrial photosynthesis exhibit a schism in the accessory chlorophyll (Chl) that complements the function of Chl a : Chl b for green plants versus Chl c for most eukaryotic phytoplankton. The enzymes that mediate Chl c biosynthesis have long remained elusive. In this work, we identified the CHLC dioxygenase (Phatr3_J43737) from the marine diatom Phaeodactylum tricornutum as the Chl c synthase. The chlc mutants lacked Chl c , instead accumulating its precursors, and exhibited growth defects. In vitro, recombinant CHLC protein converted these precursors into Chl c , thereby confirming its identity. Phylogenetic evidence demonstrates conserved use of CHLC across phyla but also the existence of distinct Chl c synthases in different algal groups. Our study addresses a long-outstanding question with implications for both contemporary and ancient marine photosynthesis.