Many biological processes are under the control of an endogenous circadianclock, a prominent example within the plant kingdom being photosynthesis.Here we report a robust circadian rhythm of photosynthetic oxygen evolutionin the red marine macroalga Grateloupia turuturu. The rhythm was obviousin continuous white light at irradiances ranging from 50 to 500 µmol photonsm-2 s-1. To further assess whether this physiological rhythm was correlatedwith changes at the level of gene transcription in G. turuturu, abundance ofthe transcripts encoding the phycoerythrin a and b subunits and theribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) subunits was analysed under light-dark cycles (LD) and constant illumination (LL).Abundance of both transcripts exhibited diurnal and circadian changes inG. turuturu. In LD conditions, abundance of the phycoerythrin transcriptpeaked between midday and late afternoon. This pattern was maintainedunder constant conditions (LL) for up to three cycles. In case of the Rubiscotranscript, mRNA abundance started to increase in the morning and peakedaround midday in LD cycles. In LL, Rubisco transcript abundance continuedto oscillate, but the increase of mRNA started earlier, already duringsubjective night. Our results suggest control of both transcripts by thecircadian clock in G. turuturu.