Photosynthesis: a Post-Transcriptionally Regulated Circadian Rhythm in Some Lower Eukaryotes

Summary

Although not all Eukaryotes plants display a circadian rhythm in photosynthesis, most plants do. Much effort has been devoted to the understanding of the mechanism which regulates several higher plants photosynthesis CR: it primarily results from a CR in nuclear coded mRNA level and synthesis rate of the light harvesting pigment-proteins complexes.

However, at least three unicellular eukaryotic organisms display a photosynthesis CR for which no transcription is required: Acetabularia, Euglena and Gonyaulax. Most evidently Acetabularia whose anucleate fragments maintain the CR up to 40 days. A brief summary of the evidence is given for all three species. In contrast,periodic translation on 80 S ribosomes appears to be needed for the CR expression — although the mechanism of control is unknown.

The experiments aimed at evidencing the processes which embody the photosynthetic circadian rhythm in the three investigated unicellular organisms are reviewed. The relevant reports pertain to both old and recent literature. The post-transcriptional photosynthesis CR control relies upon substructural organization changes.

In neither case could a CR in chlorophyll content or global a and b chlorophyll ratio be evidenced (or at least reliably related to the CR in photosynthesis). No change in activity rate of the two individual photosystems could be detected in Acetabularia and Euglena.

In contrast, biophysical methods evidenced circadian changes. In Acetabularia, the Hill activity changes periodically in parallel with the O₂ evolution CR. Fluorescence emission spectra at 77° K revealed differences between the times of the day, likely due to changes in the molecular structure of the thylakoid membranes -interpretation strengthened by the spectrum produced after freezethaw treatments.

In Euglena, other methods were used. Emission spectra showed a consistent but low magnitude CR in energy transfer and large periodical oscillations in coordination between the two PS. The photosystems can be poised to the so-called “state I” (in which the PS I is excited) and, similarly, to the “state II” (in which the PS II is excited): analysis of low temperature emission spectra from chloroplasts in both states and of low temperature fluorescence and fluorescence ratio in the two states can only be be explained by changes in spill over between the PS.

More recent and sophisticated methods were used in analyzing the photosynthesis CR of Gonyaulax. It was found that the supramolecular organization of the pigment-protein-complexes changes periodically.

These organisms provide a demonstration of the power of membranes to generate circadian rhythmicity (possibly together with a cytoplasmic protein).