PHOTORESPIRATION IN CONTINUOUS-CULTURE OF DUNALIELLA-TERTIOLECTA (CHLOROPHYTA) - RELATIONSHIPS BETWEEN SERINE, GLYCINE, AND EXTRACELLULAR GLYCOLATE

The concentrations of extracellular glycolate and intracellular free p ools of serine and glycine were monitored in nitrogen-limited continuo us cultures of Dunaliella tertiolecta (Butcher) UTEX LB999, grown at t wo different irradiances on a light:dark cycle. Under steady-state con ditions, this microalga excreted into the medium a large amount of gly colate during the light phase, up to 100 nmol.(10(6) cells)(-1) for a cell concentration of around 1.5 10(8) cells.L-1, but glycolate disapp eared from the dissolved phase in the dark. Cells grown at 70 and thos e grown at 430 mu mol photons m(-2).s(-1) differed in maximal glycolat e concentration, intracellular serine and glycine concentrations, and serine:glycine ratio. Reversal of these photon flux densities to which the cultures were exposed caused rapid modification of the extracellu lar glycolate and intracellular serine and glycine pools. These result s suggest that photorespiratory metabolism in D. tertiolecta could be approximately quantified by measuring the changes in dissolved glycola te and intracellular serine and glycine concentrations, extending prev ious results from cultured phytoplankton and suggesting methods for fi eld studies.