The role of C4 metabolism in the marine diatom Phaeodactylum tricornutum.
Diatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO(2) is much higher than that of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), indicating that a CO(2)-concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C(4), remains elusive. Although (14)C labeling experiments and presence of complete sets of genes for C(4) metabolism in two diatoms supported the presence of C(4), other data and predicted localization of the decarboxylating enzymes, away from Rubisco, makes this unlikely. We used RNA-interference to silence the single gene encoding pyruvate-orthophosphate dikinase (PPDK) in Phaeodactylum tricornutum, essential for C(4) metabolism, and examined the photosynthetic characteristics. The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K(1/2) (CO(2)) was hardly affected, thus clearly indicating that the C(4) route does not serve the purpose of raising the CO(2) concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic V(max) was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation. We propose that the C(4) metabolism does not function in net CO(2) fixation but helps the cells to dissipate excess light energy and in pH homeostasis.
Department of Plant and Environmental Sciences, Edmond J. Safra Campus - Givat Ram, Hebrew University of Jerusalem, Jerusalem, 91904, Israel.
Maya
M
Haimovich-Dayan
Department of Plant and Environmental Sciences, Edmond J. Safra Campus - Givat Ram, Hebrew University of Jerusalem, Jerusalem, 91904, Israel.
Nitsan
N
Garfinkel
Fachbereich Biologie, Universität Konstanz, Konstanz, 78457, Germany.
Daniela
D
Ewe
Department of Molecular Biology and Ecology of Plants, Tel Aviv University, Tel Aviv, 69978, Israel.
Yehouda
Y
Marcus
Fachbereich Biologie, Universität Konstanz, Konstanz, 78457, Germany.
Ansgar
A
Gruber
Institut für Biologie, Abteilung Pflanzenphysiologie, Universität Leipzig, Leipzig, 04103, Germany.
Heiko
H
Wagner
Fachbereich Biologie, Universität Konstanz, Konstanz, 78457, Germany.
Peter G
PG
Kroth
Department of Plant and Environmental Sciences, Edmond J. Safra Campus - Givat Ram, Hebrew University of Jerusalem, Jerusalem, 91904, Israel.
Aaron
A
Kaplan
D002244:Carbon / Q000378:metabolism*; D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000378:metabolism*; D002250:Carbon Radioisotopes / Q000378:metabolism; D003001:Cloning, Molecular; D017377:Diatoms / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development; D004789:Enzyme Activation; D057075:Enzyme Assays; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D008027:Light; D050356:Lipid Metabolism; D010788:Photosynthesis; D010957:Plasmids / Q000235:genetics / Q000378:metabolism; D011772:Pyruvate, Orthophosphate Dikinase / Q000235:genetics* / Q000378:metabolism; D034622:RNA Interference; D017550:Spectroscopy, Fourier Transform Infrared; D014158:Transcription, Genetic
D002250:Carbon Radioisotopes; D002245:Carbon Dioxide; D002244:Carbon; D011772:Pyruvate, Orthophosphate Dikinase