Microcolony formation from single-cell Synechococcus Strains: a protective strategy against UVR

UVR have different effects on prokaryotic cells as, for instance, filamentation and aggregation in bacteria. We studied the effect of UVR on microcolony formation in two freshwater picocyanobacterial strains from different ribotypes (Group B and Group I) and pigment composition (phycoerythrin, PE-rich and phycocyanin, PC-rich). The cultures were photoacclimated at low (LL, 10 µmol m-2 s-1) and medium (ML, 100 µmol m-2 s-1) light for two months and then exposed to UVR (Q-Panel) for 7 days. We observed the higher percentage of cells in microcolonies in PE-rich strain acclimated to LL and then exposed to UVR. Conversely, PE acclimated to ML and PC did not aggregate significantly. We identified and measured the specific carotenoids by HPLC at the different times of the experiment. We suggest that the induction of protective pigments during ML acclimation enables single-cell survival during UVR irradiation and that the strategy of microcolony formation is a protective response of LL acclimated cells. The extent to which the microcolony formation from single cell Synechococcus is related to pigment type, phylogenetic position or photoacclimation is discussed.