A, distribution of cells in G1 (blue),

S (red) and G2 (green) phases for batch cultures of PCC9511 grown under HL. B, same for HL+UV conditions. The experiment was done in duplicates shown by filled and Inhibitor Library empty symbols. Note that only the UV radiation curve is shown in graph B since the visible light curve is the same as in graph A. White and black bars indicate light and dark periods. The dashed line indicates the irradiance level (right axis). HL, high light; PAR, photosynthetically available radiation; UV, ultraviolet radiation. Figure 1 shows the time course variations of the percentages of cells in the different phases of the cell cycle. Under HL condition, cells started to enter the S phase about 4 h before the light-to-dark transition (LDT) and the peak of S cells was reached exactly at the LDT. The first G2 cells appeared at the LDT and the peak of G2 cells was reached 4 h later. Most cells had completed division before virtual sunrise, as shown by a percentage of cells in learn more G1 close to 100% at (or 1 h after) that time (Fig. 1A). PCC9511 cultures acclimated to HL+UV conditions showed a remarkable cytological response with

regard to the timing of chromosome replication. In the presence of UV, entry into S was clearly delayed, with the onset of chromosome replication occurring about 1 h before the LDT and the maximum number of cells in S phase reached 2 h after the LDT. Entry into G2 was also delayed by 3 h, but the peak of G2 cells was reached more quickly, so that it occurred on average only 1 h after that observed under the HL condition (Fig. 1B). The faster progression of cells through S and G2 phases under HL+UV than HL only conditions in batch culture was confirmed by calculating the lengths of the S and Exoribonuclease G2 phases, which were shorter

in the former condition (Table 1). Cells grown under HL+UV exhibited a higher level of synchronization (as shown by a lower synchronization index, Sr) than those grown under HL only. However, the calculated growth rates were not significantly different between the two conditions. Therefore, the dose of UV irradiation that was used in this experiment did not prevent cells from growing at near maximal rate despite the delay of entry in S phase (Table 1). It must be noted that growth rates calculated from the percentages of cells in S and G2 (μcc) using the method described by Carpenter & Chang [30] were systematically about 10% higher than those calculated from the change in cell number (μnb). Since the latter method was used to assess the growth rate of continuous cultures (see below), these experiments in batch cultures were therefore useful to estimate the bias brought by these cell cycle-based growth rate measurements.