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BIOENERGETICS AND PHOTOSYNTHESIS

C₃ and C₄ Pathways of Photosynthetic Carbon Assimilation in Marine Diatoms Are under Genetic, Not Environmental, Control^1,[W],[OA]

Plant Research Unit, University of Dundee at Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom (K.R., J.A.R.); and Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom (E.G., R.C.L.)

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 labeling 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 labeling. 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 labeling 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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Richard C. Leegood (r.leegood{at}sheffield.ac.uk).

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.102616

^* Corresponding author; e-mail r.leegood{at}sheffield.ac.uk.

Received May 30, 2007; accepted July 9, 2007; published July 20, 2007.

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