Protection of photosystem II against UV-A and UV-B radiation in the cyanobacterium Plectonema boryanum: The role of growth temperature and growth irradiance

Photochemistry and Photobiology, Dec 2000 by Ivanov, Alexander G, Miskiewicz, Ewa, Clarke, Adrian K, Greenberg, Bruce M, Huner, Norman P A

Absorption spectra of acetone and methanol extracts of P. boryanum cells exhibited a prominent absorption in the UV region of the spectrum, especially at 384 nm and around 315 nm (data not shown). This is likely due to the presence of Scy and OS-MAA (11,12). Quantitative data presented in Table I demonstrate that cultures grown at 15/150 resulted in a 1.7-fold enhancement of Scy and a 2.2-fold increase in OS-MAA compared to control cultures grown at 29/150. Although cultures grown at 15/6 did not show any changes in the amount of Scy, the level of OS-MAA increased by about 30%.

The experimental data derived from modulated fluorescence measurements revealed that growth of P. boryanum cells at 15/150 resulted in a 43% reduction of the maximal photochemical efficiency of PSII measured as Fv/Fm (0.295 0.013) compared to 29/150 cells (0.521 /- 0.013). In contrast, cells grown at 15/6 exhibited Fv/Fm values (0.603 0.010) about 16% greater than those observed in control cells grown at 29/150. Despite the lower PSII photochemical efficiency in 15/150 cells, qP was higher in cells grown at 15/150 (0.690 0.014) than in those grown at 29/150 (0.552

0.030). Although the qP values exhibited by cells acclimated to either 15/150 or 15/6 were similar, both cultures acclimated to low temperature exhibited qN values (0.101 0.016 and 0.119 0.028, respectively) that were two-fold lower than control cells acclimated to 29/150 (0.249 0.023).

The results summarized in Table 2 illustrate the effect of growth regime on the 77 K fluorescence emission spectra of P. boryanum cells. The emission maxima at 658 (F658),684 (F684) and 722 nm (F722) represent fluorescence emission associated with phycobilisomes, PSII and PSI, respectively (22). The ratio of F658/F684 in cells grown at 15/150 was almost double that of cells grown at either 29/150 or 15/6. In contrast, growth regime appeared to have smaller effects on the ratio of F6841F722 (Table 2). These results are consistent with recently published reports (23).

Effects of UV irradiation

The results of Fig. 3 illustrate the wavelength dependence of the spectral fluence rates to which P. boryanum cells grown at one of 29/150, 15/150 or 15/6 were exposed. The UV-A fluence rate was maintained constant at 2.37% of PAR whereas the LTV-B fluence was varied from either 0.1% (PAR UV-A) or 1.1% of PAR (PAR UV-A UV-B).

Exposure of control (29/150) P. boryanum cells to PAR and UV-A at 15'C resulted in a gradual decrease of the maximal PSII photochemical efficiency to about 50% of the Fv/ Fm values in control, nontreated cells after 90 min of UV-A treatment (Fig. 4A). Exposure of 29/150 cells to PAR and UV-A UV-B caused only a 30% reduction of PSII efficiency during the same time interval. In contrast, cells acclimated to low temperature and the same irradiance (15/ 150) did not exhibit any significant changes of PSII efficiency regardless of whether the cells were exposed to either PAR and LTV-A or PAR and UV-A UV-B (Fig. 4B). Although the F,/Fm of the cultures grown at 15/6 was sensitive to both PAR and UV-A as well as PAR and LTV-A LTVB (Fig. 4C), the PAR and UV-A UV-B treatment was more effective in reducing the PSII efficiency than UV-A treatment alone (Fig. 4C). Treatment with PAR (150 (mu)mol m-2 s-1) alone at 15'C for 2 h did not cause any significant changes in PSII photochemistry in 29/150 cells and only a 14% decrease of Fv/Fm in 15/6 cells (data not shown). These data clearly indicate that the observed strong inhibition of PSII could not be ascribed to exposure to lower temperature (29/150 cells), or higher visible light irradiance (15/6 cells), but rather reflect the impact of the UV exposure.