PRIMARY CARBON AND NITROGEN METABOLIC GENE EXPRESSION IN THE DIATOM THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE): DIEL PERIODICITY AND EFFECTS OF INORGANIC CARBON AND NITROGEN

Diel periodicity and effects of inorganic carbon (C-i) and NO3- on the expression of 11 key genes for primary carbon and nitrogen metabolism, including potential C-4 photosynthesis, in the marine diatom Thalassiosira pseudonana Hasle et Heimdal were investigated. Target gene transcripts were measured by quantitative reverse transcriptase PCR, and some of the gene-encoded proteins were analyzed by Western blotting. The diatom was grown with a 12 h photoperiod at two different C-i concentrations maintained by air-equilibration with either 380 mu L . L-1 (near-ambient) or 100 mu L . L-1 (low) CO2. Transcripts of the principal C-i and NO3- assimilatory genes RUBISCO LSU (rbcL) and nitrate reductase displayed very strong diel oscillations with peaks at the end of the scotophase. Considerable diel periodicities were also exhibited by the beta-carboxylase genes phosphoenolpyruvate carboxylase (PEPC1 and PEPC2) and phosphoenolpyruvate carboxykinase (PEPCK), and the Benson Calvin cycle gene sedoheptulose-bisphosphatase (SBPase), with peaks during mid- to late scotophase. In accordance with the transcripts, there were substantial diel periodicities in PEPC1, PEPC2, PEPCK, and especially rbcL proteins, although they peaked during early to mid-photophase. Inorganic carbon had some transient effects on the beta-carboxylase transcripts, and glycine decarboxylase P subunit was highly up-regulated by low C-i concentration, indicating increased capacity for photorespiration. Nitrogen-starved cells had reduced amounts of carbon metabolic gene transcripts, but the PEPC1, PEPC2, PEPCK, and rbcL transcripts increased rapidly when NO3- was replenished. The results suggest that the beta-carboxylases in T. pseudonana play key anaplerotic roles but show no clear support for C-4 photosynthesis.