The effect of light on the biosynthesis of beta-carotene and superoxide dismutase activity in the photosynthetic alga Gonyaulax polyedra

Daily oscillations of both beta-carotene and superoxide dismutase (SOD) activity are related to the intracellular control of reactive oxygen species (ROS). It is well established that ROS are present in all aerobic cells. We studied the marine dinoflagellate Gonyaulax polyedra which has been extensively used as a model to understand the biological clock at the molecular level. beta-Carotene, besides suppressing singlet molecular oxygen (1O2), may act as a photoreceptor pigment in many photosynthetic cells. The levels of beta-carotene during the day phase were shown to be twice as high as during the night phase. The dose-response curve for light-induced carotenoid synthesis was linear for up to 45 min of light exposure, after which night phase cells contained the same levels of beta-carotene as day phase cells. Cells exposed to light pulses at different times during the dark period displayed the highest beta-carotene induction in the middle of the night. SOD activity of cell-free extracts of G. polyedra was three to four times higher during the day. This rhythm continued in cells kept in constant light, indicating that the regulation can be attributed to the cellular circadian clock. Non-denaturing polyacrylamide gels revealed the presence of several SOD isoenzymes in G. polyedra, including CuZnSOD and MnSOD. Furthermore, G. polyedra SOD cross-reacts with a polyclonal antibody raised against SOD. In addition to being gene regulated by ROS concentration, G. polyedra SOD expression seems also to be under the control of the biological clock.

Imaging oscillations in Gonyzaulax: a chloroplast rhythm of nitrate reductase visualized by immunocytochemistry

Gonyaulax polyedra is a unicellular marine photosynthetic dinoflagellate known to display numerous circadian rhythms, including bioluminescence, motility, cell division and several chloroplast-related rhythms. Due to this, Gonyaulax has become a widely used model organism for studying the cellular biological clock. In this work we describe another rhythm for Gonyaulax cells also associated with the cell's chloroplasts, a rhythm in localization of the enzyme nitrate reductase (NR). A polyclonal antibody was raised against NR purified from G. polyedra cells and used as a probe in immunogold labelling experiments on cell thin sections, comparing day- and night-phase cells. The enzyme localizes to chloroplasts in day-phase cells, while the enzyme is active, and is largely absent in night-phase cells. Counts of gold particle distribution in day- versus night-phase cells show an approximate three-fold increase in enzyme labelling in day-phase plastids. These results closely approximate the four-fold differences shown for NR activity between day and night Gonyaulax cells by biochemical studies. We conclude from the diurnal difference in labelling that NR is localized in Gonyaulax chloroplasts during the day phase and is absent (broken down) in night-phase cells. Thus NR in Gonyaulax is compartmentalized in the chloroplasts and is therefore subject to similar circadian control mechanisms exhibited for other plastid rhythms.