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.

Cyanobacteria possess CO2 -concentrating mechanisms (CCM) that functionally compensate for the poor affinity of their ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to CO2 . It was proposed that 2-phosphoglycolate (2PG), produced by the oxygenase activity of Rubisco and metabolized via photorespiratory routes, serves as a signal molecule for the induction of CCM-related genes under limiting CO2 level (LC) conditions. However, in vivo evidence is still missing. Since 2PG does not permeate the cells, we manipulated its internal concentration. Four putative phosphoglycolate phosphatases (PGPases) encoding genes (slr0458, sll1349, slr0586 and slr1762) were identified in the cyanobacterium Synechocystis PCC 6803. Expression of slr0458 in Escherichia coli led to a significant rise in PGPase activity. A Synechocystis mutant overexpressing (OE) slr0458 was constructed. Compared with the wild type (WT), the mutant grew slower under limiting CO2 concentration and the intracellular 2PG level was considerably smaller than in the wild type, the transcript abundance of LC-induced genes including cmpA, sbtA and ndhF3 was reduced, and the OE cells acclimated slower to LC - indicated by the delayed rise in the apparent photosynthetic affinity to inorganic carbon. Data obtained here implicated 2PG in the acclimation of this cyanobacterium to LC but also indicated that other, yet to be identified components, are involved.

Simultaneous catabolic and anabolic glucose metabolism occurs in the same compartment during photomixotrophic growth of the model cyanobacterium Synechocystis sp. PCC 6803. The presence of glucose is stressful to the cells; it is reflected in the high frequency of suppression mutations in glucose-sensitive mutants. We show that glucose affects many cellular processes. It stimulates respiration and the rate of photosynthesis and quantum yield in low- but not high-CO(2) -grown cells. Fluorescence and thermoluminescence parameters of photosystem II are also affected but the results did not lend support to sustained glucose driven over reduction in the light. Glucose-sensitive mutants such as ΔpmgA (impaired in photomixotrophic growth) and Δhik31 (lacking histidine kinase 31) are far more susceptible under high than low air level of CO(2) . A glycine to tryptophan mutation in position 354 in NdhF3, involved in the high-affinity CO(2) uptake, rescued ΔpmgA. A rise in the apparent photosynthetic affinity to external inorganic carbon is observed in high-CO(2) -grown wild-type cells after the addition of glucose, but not in mutant ΔpmgA. This is attributed to upregulation of certain low-CO(2) -induced genes, involved in inorganic carbon uptake, in the wild type but not in ΔpmgA. These data uncovered a new level of interaction between CO(2) fixation (and the CO(2) -concentrating mechanism) and photomixotrophic growth in cyanobacteria.