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 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 K_1/2 (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 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 C₄ metabolism does not function in net CO₂ fixation but helps the cells to dissipate excess light energy and in pH homeostasis.