The role of C4 metabolism in the marine diatom Phaeodactylum tricornutum

Summary Diatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO2 is much higher than that of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), indicating that a CO2-concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C4, remains elusive. Although 14C labeling experiments and presence of complete sets of genes for C4 metabolism in two diatoms supported the presence of C4, 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 C4 metabolism, and examined the photosynthetic characteristics. The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K1/2 (CO2) was hardly affected, thus clearly indicating that the C4 route does not serve the purpose of raising the CO2 concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic Vmax was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation. We propose that the C4 metabolism does not function in net CO2 fixation but helps the cells to dissipate excess light energy and in pH homeostasis.