Unicellular C4 photosynthesis in a marine diatom

Nearly 50 years ago, inorganic carbon was shown to be fixed in microalgae as the C-3 compound phosphoglyceric acid(1). The enzyme responsible for C-3 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 muM (ref. 3). As a result, diatoms growing in seawater that contains about 10 muM CO2 may be CO2 limited(4). Kinetic and growth studies have shown that diatoms can avoid CO2 limitation 5-7, but the biochemistry of the underlying mechanisms remains unknown. Here we present evidence that C-4 photosynthesis supports carbon assimilation in the marine diatom Thalassiosira weissflogii, thus providing a biochemical explanation for CO2-insensitive photosynthesis in marine diatoms. If C-4 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 C-13 in diatoms compared with other classes of phytoplankton. Unicellular C-4 carbon assimilation may have predated the appearance of multicellular C-4 plants.