Photosynthesis and chlorophyll a fluorescence rhythms of marine phytoplankton

Circadian rhythms in photosynthesis were defined in field populations of phytoplankton. Measurements of carbon-dioxide fixation rates demonstrated that a diurnal periodicity of photosynthesis in samples incubated under natural light-dark (LD) cycles also were observed to continue in similar samples which had been photoadapted to constant dim light (LL) for 48 h. These changes in photosynthetic rates preceded sunset and sunrise, had daily amplitudes that ranged from 1.5 to 2.0, appeared to be independent of light-intensity, and displayed maxima about midday, while rates of dark fixation of carbon dioxide and the photosynthetic pigment content per cell were constant over the circadian cycle. Similar rhythmicity also was detected in room-temperature (22°C) chlorophyll a fluorescence yield, in both the obsence and presence of the photosynthesis inhibitor DCMU [3-(3,4-dichlorophenyl)-1, 1-dimethylurea]. However, the magnitude and timing of the fluorescence rhythm maxima seem to depend on wavelengths monitored and, in part, on the measuring technique used. Also, the circadian changes in the fluorescence intensity were abolished at low temperature (-60°C), and the shape of the emission spectra of chlorophyll fluorescence of cells in LD and LL did not change over time. The significance of the fluorescence rhythms with regard to chlorophyll a determinations and photosynthetic rates is discussed. It was concluded that there was sufficient similarity between circadian rhythms of photosynthesis in natural phytoplankton populations and in laboratory cultures of dinoflagellates to suggest that the mechanism of regulation may be the same for both of them.