Comparative Sensitivity of Seven Marine Microalgae to Cumulative Exposure to Ultraviolet-B Radiation with Daily Increasing Doses

Abstract

A comparative study of the relative sensitivity to ultraviolet radiation (UVR) among seven marine microalgae from the classes Cyanophyceae (Synechococcus sp.), Rhodophyceae (Porphyridium cruentum), Cryptophyceae (Rhodomonas salina), Prymnesiophyceae (Isochrysis galbana), Bacillariophyceae (Chaetoceros sp.), Chlorophyceae (Dunaliella salina), and Prasinophyceae (Tetraselmis suecica), was carried out. Daily exposure for 4 h over a 4-day period with daily increasing doses of UVR (mainly ultraviolet-B, UV-B) led to a continuous decrease in the effective quantum yield (φPSII) in Rhodomonas salina, Chaetoceros sp. and Isochrysis galbana. In contrast, Tetraselmis suecica did not show a significant inhibition over the 4-day period of exposure. Synechococcus sp., Dunaliella salina and Porphyridium cruentum were able to restore, at least partially, the values of φPSII during the recovery period (white light only, 20 h), even though a significant decrease in φPSII was also measured in these algae after each exposure. Relative inhibition, defined here as the decrease in φPSII after exposure as compared to the value before exposure and normalised to the latter value (RI = ∆φ PSII /φ PSII ), was fitted against UV-B irradiance according to an equation of the type y = 1 ‐ (a x) c e -bx , where y is RI, x is the UV-B irradiance (W m ‐2 ), and a, b and c are constants determined from the fit, using unweighted and weighted irradiances. From the fit, the UV-B irradiance for 50% relative inhibition of φ PSII (RI = 0.5, E 50 [W m ‐2 ]), was calculated for each alga. Rhodomonas salina and Isochrysis galbana exhibited the lowest values of E50 (0.21 and 0.25, respectively, for unweighted irradiance), being the most sensitive algae to UV-B, while the highest value was calculated for Dunaliella salina (0.47 for unweighted irradiance). The E50 could not be calculated for Tetraselmis suecica since this alga showed an RI maximum of 0.1, and was therefore considered as a species which was highly tolerant of UV-B. A rise in optical density of each algal cell suspension at 680 and 750 nm, as well as in chlorophyll a content determined from methanolic extracts, was evident over the experimental period in all of the species except for Isochrysis galbana and Rhodomonas salina. In these two algae, in contrast to the other species, there was a notable increase in the carotenoid/chlorophyll a ratio, although this had negligible effects for the protection against deleterious UV-B. The phycobilin/chlorophyll a ratio showed a gradual decrease with time in the three phycobilin-containing algae, but it was particularly evident in R. salina after the third day of exposure.