SYNERGISTIC EFFECTS OF LIGHT, TEMPERATURE, AND NITROGEN SOURCE ON TRANSCRIPTION OF GENES FOR CARBON AND NITROGEN METABOLISM IN THE CENTRIC DIATOM THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)

ABSTRACT

The effects of nitrogen source, temperature, and light level on transcription of five genes from three pathways of carbon and nitrogen metabolism were determined for the centric diatom Thalassiosira pseudonana (Hustedt) Hasle et Heimdal. The targeted genes were NR (nitrate reductase) and GSII (glutamine synthetase II), both required for nitrate utilization; PGP (phosphoglycolate phosphatase) and GDCT (glycine decarboxylase T-protein), both required for photorespiration; and SBP (sedoheptulose 1,7-bisphosphatase), required for carbon fixation (Calvin cycle). Quantitative reverse transcriptase PCR was used to compare transcript abundances for these genes from T. pseudonana cells grown at two light intensities (50 or 300 μmol photons·m⁻²·s⁻¹), at two temperatures (12 or 22°C), and on two different nitrogen sources (nitrate or ammonium). Transcript abundance (normalized to 18S rRNA) was higher for all five genes at the higher light intensity. At 12°C, fewer transcripts were detected for the Calvin cycle gene, whereas more transcripts were detected for the photorespiration and nitrate utilization genes. Nitrogen source affected transcript accumulation of the photorespiration and nitrate utilization genes but not the Calvin cycle gene. Extracellular concentrations of glycolate, a photorespiratory-specific product, were determined under the high light conditions and were found to decrease at 12°C and when cells were grown on ammonium. A hypothetical model is presented to explain the patterns of transcript accumulation and glycolate release in terms of energy balance within the cell.