1. Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, New Jersey 08901, USA
2. Centre de Geochimie de la Surface, Université de Strasbourg, 1 rue Blessig, 67084 Strasbourg, France
3. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA

Correspondence to: John R. Reinfelder¹ Correspondence and requests for materials should be addressed to J.R.R. (e-mail: Email: reinfelder@envsci.rutgers.edu).

Nearly 50 years ago, inorganic carbon was shown to be fixed in microalgae as the C₃ compound phosphoglyceric acid¹. 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 M (ref. 3). As a result, diatoms growing in seawater that contains about 10 M CO₂ may be CO₂ limited⁴. Kinetic and growth studies have shown that diatoms can avoid CO₂ limitation^{5, 6, 7}, 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.