IDENTIFICATION OF A GLYCINE DECARBOXYLASE GENE IN THALASSIOSIRA WEISSFLOGII: A PROBE FOR PHOTORESPIRATION IN DIATOMS

Abstract

Diatoms are responsible for a large percentage of CO2 fixation in the world's oceans. The export of diatom‐fixed carbon from the euphotic zone represents a major sink of CO2 in the global C cycle. Under conditions of high light and high O2/CO2, carbon fixation may be reduced due to photorespiration. The fixation of O2 in photorespiration produces low molecular weight metabolites that cannot be used in the Calvin cycle and are either excreted from the cell or inefficiently salvaged in the photorespiratory pathway. One of the key enzymes of photorespiration is glycine decarboxylase (GDC). We have cloned and sequenced a cDNA corresponding to the gene encoding the T‐protein of GDC from the centric diatom Thalassiosira weissflogii. This cDNA is 1100 bp and its predicted amino acid sequence shows a 45&‘ercnt; identity and a 62% homology with the T‐protein from higher plants. The first 20 amino acids display characteristics of mitochondrial transit peptides, consistent with the localization of GDC in the mitochondria of higher plants. Using competitive RT‐PCR we have demonstrated that transcription of this gene is light‐dependent. Cells maintained at constant growth‐saturating light conditions have much higher levels of T‐protein message than cells kept in the dark for 25 hours. We are currently investigating the effect of other environmental parameters, such as O2 concentration, on T‐protein gene transcription. Ultimately, the goal is to use this gene as a probe for diatom photorespiration in the field. To this end, we are developing degenerate primers and sequencing this gene from other diatom species.