Ultraviolet radiation and the snow alga Chlamydomonas nivalis (Bauer) Wille[para]

Photochemistry and Photobiology, Jun 2003 by Gorton, Holly L, Vogelmann, Thomas C

The extent of UV damage to an organism depends not only on the amount of UV absorbed by susceptible cellular components but also on the ability of cells to repair the damage. The ratio of VIS to UV radiation can be important for repair. For example, an important pathway for DNA repair is photoreactivation by a photolyase enzyme that requires UV-A or blue light. Although we do not know which DNA repair mechanisms function in C. nivalis, it is clear that photolyase is important in both chloroplastic and nuclear DNA repair in congeneric C. reinhardtii (38). Although penetration of UV-B, UV-A and blue into C. nivalis cells is low, it is similar for all three wavebands (Fig. 5), so if UV-B can penetrate to cause damage, UV-A and blue can penetrate to aid repair. In addition, the D1 protein of PSII is continually replaced as it becomes damaged, and UV can increase the rate of D1 turnover (39,40). If repair keeps pace with UV-induced damage, there may be no change in the electron transport rate, but the cells could still incur an increased metabolic cost because of the increased rate of D1 turnover (41). An increased PAR-UV ratio means an increased rate of photosynthesis relative to damage and hence, perhaps, increased energy available for repair (42). We suggest that if C. nivalis responds to the balance of UV and VIS radiation as do other photosynthetic organisms, repair to DNA and the photosynthetic apparatus would be more likely with increasing depth because the ratio of PAR and UV-A to UV-B would be increased.

Although attenuation of UV by the snow should provide protection to subsurface cells, that protection would be inconsistent because of the mutability of summer snowfields. The location of algae within the snow can change dramatically during summer days when warm temperatures and intense sun can melt the snowpack by 10 cm or more each day. During these times, the algae move laterally and vertically with meltwater, and they can experience widely varying photon fluxes. Given these dynamic mixing conditions, there is little opportunity for cells to adapt to a local light environment within the snow. Strong protection against UV would be required as the snow surface melts downward, exposing some algae to surface conditions while allowing others to ride the meltwater to a temporary refuge deeper within the snowpack, where repair might be more likely to prevail over damage.

UV optics of C. nivalis cells

In algae, two of the main protective mechanisms to prevent UV-induced damage are synthesis of screening compounds and movement of organelles or entire cells out of the high-UV area (43,44). In C. nivalis aplanospores, both the nucleus and chloroplast are centrally located, and the radiation load comes from all directions, so organelle movement would not provide any additional protection. In addition, C. nivalis cells lack flagellae and cannot move to escape a high-UV environment. Although cells may sometimes wash down into the snow with meltwater and thus find a temporary low-UV respite, screening would be more reliable than motility and therefore likely to be important in avoiding UV damage.