Figure A1.

Phycoerythrin to phycocyanin (PE:PC) fluorescence emission ratio as measured by spectrofluorimetry (at 572 and 651 nm, respectively) in Synechococcus sp. WH7803 cells grown under a L/D cycle with and without UV. This ratio showed significantly higher values in VL + UV than in VL-acclimated cells. In both light conditions, it increased during the day by ∼25% compared to the value at dawn then decreased during the night. The amplitude of these oscillations (ca 0.4 unit) was very low compared to those reported for Synechococcus sp. WH8102 acclimated to a large range of irradiance (2.5 units; Six et al., 2004). Further investigation is required to determine whether the diel pattern of PE:PC fluorescence emission ratio rather results from a decrease in the efficiency of energy transfer in the PBS rod and/or a progressive accumulation of free phycoerythrins during the day. These light-induced oscillations might be indirectly related with state transitions (Campbell et al., 1998b; Mullineaux and Emlyn-Jones, 2005), a light-dependent process which usually exhibits large amplitudes in marine Synechococcus (Six et al., personal communication).