TEMPERATURE-DEPENDENCE OF UV-RADIATION EFFECTS ON ANTARCTIC CYANOBACTERIA

The mat-forming cyanobacterium Phormidium murrayi West and West isolat ed from a meltwater pond on the McMurdo Ice Shelf was grown in unialga l batch cultures to evaluate the temperature dependence of ultraviolet radiation (UVR) effects on pigment composition, growth rate, and phot osynthetic characteristics. Chlorophyll a concentrations per unit biom ass were generally reduced in cells grown under UVR (low UV-A plus UV- B). In vivo absorbance spectra showed that the carotenoid/chlorophyll a ratio increased as a function of photosynthetically available radiat ion (PAR) and UVR exposure and varied inversely with temperature. Ultr aviolet inhibition of growth (percentage reduction of (mu)(max) at eac h temperature) increased linearly with decreasing temperature, consist ent with the hypothesis that net inhibition represents the balance bet ween temperature-independent photochemical damage and temperature-depe ndent biosynthetic repair. There was no significant effect of UVR on p hotosynthesis over the first hour of exposure, but significant UV inhi bition was observed after 5 days. Unlike growth, however, there was no apparent effect of temperature on the magnitude of UV inhibition of p hotosynthesis. These results imply that assays of UVR effects on photo synthesis are not an accurate guide to growth responses and that low a mbient temperatures can have a major influence on the UV sensitivity o f polar organisms. In a set of assays at 20 degrees C (preacclimation under 300 mu mol photons.m(-2).s(-1) and 20 degrees C), growth was str ongly depressed by UVR over the first day of exposure but then gradual ly increased over the subsequent 4 days, approaching the growth rates in the minus UVR control. This evidence of acquired tolerance indicate s that the damaging effects of UVR will be most severe in environments where there is a mismatch between the timescale of change in exposure and the timescale of UV acclimation.