Unicellular C₄ photosynthesis in a marine diatom

Nearly 50 years ago, inorganic carbon was shown to be fixed in microalgae as the C₃ compound phosphoglyceric acid. The enzyme responsible for C₃ carbon fixation, ribulose-1,5-bisphosphate carboxylase (Rubisco), however, requires inorganic carbon in the form of CO₂ (ref. 2), and Rubisco enzymes from diatoms have half-saturation constants for CO₂ of 30-60 μM (ref. 3). As a result, diatoms growing in seawater that contains about 10 μM CO₂ may be CO₂ limited. Kinetic and growth studies have shown that diatoms can avoid CO₂ limitation, but the biochemistry of the underlying mechanisms remains unknown. Here we present evidence that C₄ photosynthesis supports carbon assimilation in the marine diatom Thalassiosira weissflogii, thus providing a biochemical explanation for CO₂-insensitive photosynthesis in marine diatoms. If C₄ photosynthesis is common among marine diatoms, it may account for a significant portion of carbon fixation and export in the ocean, and would explain the greater enrichment of ¹³C in diatoms compared with other classes of phytoplankton. Unicellular C₄ carbon assimilation may have predated the appearance of multicellular C₄ plants.