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Published on July 20, 2007; 10.1104/pp.107.102616

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Received May 30, 2007
Accepted July 9, 2007

C3 and C4 Pathways of Photosynthetic Carbon Assimilation in Marine Diatoms Are under Genetic, not Environmental, Control

Karen Roberts , Espen Granum , Richard C. Leegood *, and John A. Raven

Plant Research Unit, University of Dundee at SCRI, Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK; Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK

* Corresponding author; email: r.leegood{at}sheffield.ac.uk.

Marine diatoms are responsible for up to 20% of global CO2 fixation. Their photosynthetic efficiency is enhanced by concentrating CO2 around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. Diatoms have been regarded as C3 photosynthesizers, but recent metabolic labelling and genome sequencing data suggest that they perform C4 photosynthesis. We studied the pathways of photosynthetic carbon assimilation in two diatoms by short-term metabolic 14C labelling. In Thalassiosira weissflogii both C3 (glycerate-P and triose-P) and C4 (mainly malate) compounds were major initial (2-5 s) products, whereas Thalassiosira pseudonana produced mainly C3 and C6 (hexose-P) compounds. The data provide evidence of C3-C4 intermediate photosynthesis in T. weissflogii, but exclusively C3 photosynthesis in T. pseudonana. The labelling patterns were the same for cells grown at near-ambient (380 µL L-1) and low (100 µL L-1) CO2 concentrations. The lack of environmental modulation of carbon assimilatory pathways was supported in T. pseudonana by measurements of gene transcript and protein abundances of C4-metabolic enzymes (phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase) and Rubisco. This study suggests that the photosynthetic pathways of diatoms are diverse, and may involve combined CO2-concentrating mechanisms. Furthermore, it emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C4-metabolic enzymes as evidence for C4 photosynthesis.




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