Diel Variations in the Photosynthetic Parameters of Prochlorococcus Strain PCC 9511: Combined Effects of Light and Cell Cycle

We examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light-dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2-h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation ($\phi _{\text{Cmax}}$) and the maximum rate of carbon fixation ($P_{\text{max}}^{B}$) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in $\phi _{\text{Cmax}}$ and the quantum yield of fluorescence, which was accompanied by only a small decrease (