Induction of photorespiration by light in the centric diatom Thalassiosira weissflogii (Bacillariophyceae):: Molecular characterization and physiological consequences

Glycine decarboxylase (GDC) is a multi-subunit enzyme that plays a key role in the photorespiratory pathway. A cDNA (GDCT) with a deduced amino acid sequence sharing up to 64% homology and 50% identity with the T-protein subunit of GDC from land plants was identified from the centric diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle. The effect of light on transcript abundance was determined using competitive RT-PCR. GDCT mRNA per cell decreased during prolonged dark incubations and was higher in cells grown at higher light intensities, independent of growth rate. Cells acclimated to a 12:12-h light:dark cycle maintained the same amount of GDCT transcripts before and after the dark phase, but rapidly increased transcript levels in the light with a peak mid-day. Rapid increases in GDCT message abundance also occurred in response to an increase in irradiance. Transcript abundance of twca1, the gene for carbonic anhydrase, was also measured and displayed a slower and much less pronounced response to changes in light intensity. An HPLC-based method was used to measure the concentration of the photorespiratory-specific compound glycolate in the culturemedia. Only cultures shifted from low subsaturating irradiances to the highest light intensity released significant amounts of glycolate. Our results suggest that quantifying GDCT mRNA in conjunction with measurements of glycolate will allow us to monitor both the initiation of photorespiration and its physiological consequences in the marine environment.