The concentrations of extracellular glycolate and intracellular free pools of serine and glycine were monitored in nitrogen-limited continuous cultures of Dunaliella tertiolecta (Butcher) UTEX LB999, grown at two different irradiances on a light:dark cycle. Under steady-state conditions, this microalga excreted into the medium a large amount of glycolate 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 disappeared from the dissolved phase in the dark. Cells grown at 70 and those grown at 430 mu mol photons m(-2).s(-1) differed in maximal glycolate 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 extracellular glycolate and intracellular serine and glycine pools. These results suggest that photorespiratory metabolism in D. tertiolecta could be approximately quantified by measuring the changes in dissolved glycolate and intracellular serine and glycine concentrations, extending previous results from cultured phytoplankton and suggesting methods for field studies.