PAR was provided by SB202190 cost two symmetrical banks of 8 dimmable, U-shaped Philips PL-L 90 daylight fluorescence tubes (Philips Lightning, Eindhoven, NL) located on each side of the 50 L glass tank containing the culture flasks, whereas UV radiation was supplied by five pairs of UVA-340 fluorescent tubes (Q-Panel

Lab products, Westlake, OH, USA) located above the cultures. PAR level was adjusted to reach a midday maximum of 100 μmol photons m-2 s-1 for LL conditions and 900 μmol photons m-2 s-1 for HL conditions. For long or short term UV experiments, HL conditions were supplemented by a 12 h/12 h L/D cycle of UV radiation reaching 7.59 W m-2 UVA (320-400 nm) and 0.57 W m-2 UVB (280-320 nm) at virtual noon (see additional file 1: Fig. S1). For preliminary growth experiments, replicate 600 mL batch cultures were maintained in 1L Erlenmeyer glass flasks (Schott Duran, Mainz, Germany) for HL only experiments or 1 L Erlenmeyer quartz flasks (Atelier Jean Premont, Bordeaux, France) for HL+UV experiments. For transcriptomic analyses, two

7 L replicate cultures were kept in exponential growth phase at cell densities of around 108 cells mL-1 by continuous dilution with fresh medium, at a rate adjusted to population growth (e.g., 4.83 L must be added per day to a 7 L culture growing at one division per day). For these large-scale experiments, we MEK inhibitor side effects used custom-made, cylindrical 8 L quartz flasks (Selleck ICG-001 Ellipse, La Chapelle-la-Reine, France). All cultures were acclimated to experimental light conditions at least

two weeks before the start of sampling. For long-term HL+UV conditions, cultures were slowly acclimated by incrementally increasing the UV dose by ca. 2 W m-2 steps with at least 2-3 days of acclimation at each step. To further reduce UV stress, the pre-cultures were diluted daily at dawn and maintained at a cell density higher than 5×105 cells ml-1. To check for the eventual occurrence of self shading, we analyzed the timing of the S phase Non-specific serine/threonine protein kinase peak and the percentage of cells in S in the peak in samples collected at different depths of the quarz flask (i.e. different distances from UV lamps) and observed that there were no significant differences (data not shown). Growth and cell cycle analyses by flow cytometry Culture samples for cell density measurements and cell cycle analyses were taken automatically at 1 h intervals using an electronic peristaltic pump (Masterflex Cartridge Pump 8; Fisher Bioblock Scientific, Illkirch, France) fitted to a custom-designed fraction collector. Aliquots were kept at 4°C in the dark and fixation of cells was done within a maximum timeframe of 9 h after sampling, a delay shown to cause only negligible changes on the DNA content in Prochlorococcus cells [92]. 400 microliter aliquots were fixed in glutaraldehyde (0.