DIFFERENTIAL RESPONSES OF ANABAENA SP. PCC 7120 (CYANOPHYCEAE) CULTURED IN NITROGEN‐DEFICIENT AND NITROGEN‐ENRICHED MEDIA TO ULTRAVIOLET‐B RADIATION1

Stratospheric ozone depletion increases the amount of ultraviolet‐B radiation (UVBR) (280–320 nm) reaching the surface of the earth, potentially affecting phytoplankton. In this work, Anabaena sp. PCC 7120, a typically nitrogen (N)‐fixing filamentous bloom‐forming cyanobacterium in freshwater, was individually cultured in N‐deficient and N‐enriched media for long‐term acclimation before being subjected to ultraviolet‐B (UVB) exposure experiments. Results suggested that the extent of breakage in the filaments induced by UVBR increases with increasing intensity of UVB stress. In general, except for the 0.1 W · m^?2 treatment, which showed a mild increase, UVB exposure inhibits photosynthesis as evidenced by the decrease in the chl fluorescence parameters maximum photochemical efficiency of PSII (F_v/F_m) and maximum relative electron transport rate. Complementary chromatic acclimation was also observed in Anabaena under different intensities of UVB stress. Increased total carbohydrate and soluble protein may provide some protection for the culture against damaging UVB exposure. In addition, N‐deficient cultures with higher recovery capacity showed overcompensatory growth under low UVB (0.1 W · m^?2) exposure during the recovery period. Significantly increased (～830%) ATPase activity may provide enough energy to repair the damage caused by exposure to UVB.