Diatoms are unicellular algae and important primary producers. The process of carbon fixation in diatoms is very efficient even though the availability of dissolved CO2 in sea water is very low. The operation of a carbon concentrating mechanism (CCM) also makes the more abundant bicarbonate accessible for photosynthetic carbon fixation. Diatoms possess carbonic anhydrases as well as metabolic enzymes potentially involved in C4 pathways; however, the question as to whether a C4 pathway plays a general role in diatoms is not yet solved. While genome analyses indicate that the diatom Phaeodactylum tricornutum possesses all the enzymes required to operate a C4 pathway, silencing of the pyruvate orthophosphate dikinase (PPDK) in a genetically transformed cell line does not lead to reduced photosynthetic carbon fixation. In this study, we have determined the intracellular location of all enzymes potentially involved in C4-like carbon fixing pathways in P. tricornutum by expression of the respective proteins fused to green fluorescent protein (GFP), followed by fluorescence microscopy. Furthermore, we compared the results to known pathways and locations of enzymes in higher plants performing C3 or C4 photosynthesis. This approach revealed that the intracellular distribution of the investigated enzymes is quite different from the one observed in higher plants. In particular, the apparent lack of a plastidic decarboxylase in P. tricornutum indicates that this diatom does not perform a C4-like CCM.

Centre Algatech Institute of Microbiology of the Czech Academy of Sciences Třeboň Czech Republic

Department of Bioscience School of Science and Technology Kwansei Gakuin University Sanda Hyogo 669 1337 Japan

Department of Bioscience School of Science and Technology Kwansei Gakuin University Sanda Hyogo 669 1337 Japan Lion Corporation Pharmaceutical Laboratories No 1 Odawara Kanagawa 256 0811 Japan

Department of Bioscience School of Science and Technology Kwansei Gakuin University Sanda Hyogo 669 1337 Japan Tech Manage Corp Tokyo 160 0023 Japan

Department of Plant and Environmental Sciences Edmond J Safra Campus Givat Ram Hebrew University of Jerusalem 91904 Jerusalem Israel

Fachbereich Biologie Universität Konstanz 78457 Konstanz Germany

Fachbereich Biologie Universität Konstanz 78457 Konstanz Germany Biology Centre Institute of Parasitology Czech Academy of Sciences České Budějovice Czech Republic

Fachbereich Biologie Universität Konstanz 78457 Konstanz Germany Centre Algatech Institute of Microbiology of the Czech Academy of Sciences Třeboň Czech Republic

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