Cell cycle regulation by light in Prochlorococcus strains

The effect of light on the synchronization of cell cycling was investigated in several strains of the oceanic photosynthetic prokaryote Prochlorococcu s using flow cytometry. When exposed to a light-dark (L-D) cycle with an ir radiance of 25 mu mol of quanta m(-2) s(-1), the low-light-adapted strain S S 120 appeared to be better synchronized than the high-light-adapted strain PCC 9511. Submitting LD-entrained populations to shifts (advances or delay s) in the timing of the "light on" signal translated to corresponding shift s in the initiation of the S phase, suggesting that this signal is a key pa rameter for the synchronization of population cell cycles. Cultures that we re shifted from an L-D cycle to continuous irradiance showed persistent die l oscillations of flow-cytometric signals (light scatter and chlorophyll fl uorescence) but with significantly reduced amplitudes and a phase shift. Co mplete darkness arrested most of the cells in the G(1), phase of the cell c ycle, indicating that light is required to trigger the initiation of DNA re plication and cell division. However, some cells also arrested in the S pha se, suggesting that cell cycle controls in Prochlorococcus spp. are not as strict as in marine Synechococcus spp. Shifting Prochlorococcus cells from low to high irradiance translated quasi-instantaneously into an increase of cells in both the S and G(2) phases of the cell cycle and then into faster growth, whereas the inverse shift induced rapid slowing of the population growth rate. These data suggest a close coupling between irradiance levels and cell cycling in Prochloroeoccus spp.