SummaryDiatoms 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.
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