A chlorophyll <i>c</i> synthase widely co-opted by phytoplankton

Abstract

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