The effects of UV radiation on photosynthesis in an Antarctic diatom (Thalassiosira sp.): Does vertical mixing matter?

HERNANDO, M.P. AND G.A. FERREYRA

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

Elsevier

Año: 2005 vol. 325 p. 35 - 45

0022-0981

The reduction of Antarctic stratospheric ozone results in significant increases in ultraviolet B radiation (UVB-R, 280–320 nm) reaching ocean’s surface, potentially damaging phytoplankton. Several studies refer to the negative direct and indirect effects of UVB-R and ultraviolet A (UVA-R, 320–400 nm, which is not modified by ozone concentration) on different targets within algal cells. There are, however, internal and external processes, like vertical mixing, which can in part counteract such effects. The hypothesis that vertical mixing is a significant factor reducing the negative effects of ultraviolet radiation (UV-R, 280–400 nm) on planktonic algae photosynthesis was tested at Potter Cove (South Shetland Is., Antarctica). Three laboratory (solar simulator, SOLSI) and two field (natural Sun exposure) experiments were conducted. Vertical mixing was studied exposing cells of Thalassiosira sp., a typically bloom forming diatom in Antarctic waters, to variable light conditions simulating 6 h cycles (Mix treatment), whereas incubations at two fixed depths were used as controls (0.5 and 5 m, Sfix and Dfix treatments, respectively). Light effects were studied for each of the previous exposure conditions considering three treatments: PAR-T (exposure to PAR, photosynthetic active radiation, 400–700 nm), UVA-T (exposure to PAR and UVA-R) and UVB-T (exposure to PAR, UVA-R and UVB-R). During SOLSI experiments no significant differences were found between the different light treatments under simulated normal and medium ozone concentrations. Under low ozone conditions, 40% reduction in photosynthesis was observed in the UVB-T for surface incubations. In contrast, no significant differences were observed among the light treatments under mixing conditions. Field and laboratory experiments showed similar results. However, during one of the field experiments when ozone was low, not only Sfix but Mix incubations presented a significant reduction in photosynthesis, suggesting that vertical mixing under such conditions was not efficient enough to prevent harmful UVB-R effects. On the other hand, during a day with high insulation and normal ozone, but with elevated absorption of light in the water column, no significant effects of any of the studied factors were detected.Thalassiosira sp., a typically bloom forming diatom in Antarctic waters, to variable light conditions simulating 6 h cycles (Mix treatment), whereas incubations at two fixed depths were used as controls (0.5 and 5 m, Sfix and Dfix treatments, respectively). Light effects were studied for each of the previous exposure conditions considering three treatments: PAR-T (exposure to PAR, photosynthetic active radiation, 400–700 nm), UVA-T (exposure to PAR and UVA-R) and UVB-T (exposure to PAR, UVA-R and UVB-R). During SOLSI experiments no significant differences were found between the different light treatments under simulated normal and medium ozone concentrations. Under low ozone conditions, 40% reduction in photosynthesis was observed in the UVB-T for surface incubations. In contrast, no significant differences were observed among the light treatments under mixing conditions. Field and laboratory experiments showed similar results. However, during one of the field experiments when ozone was low, not only Sfix but Mix incubations presented a significant reduction in photosynthesis, suggesting that vertical mixing under such conditions was not efficient enough to prevent harmful UVB-R effects. On the other hand, during a day with high insulation and normal ozone, but with elevated absorption of light in the water column, no significant effects of any of the studied factors were detected.fix and Dfix treatments, respectively). Light effects were studied for each of the previous exposure conditions considering three treatments: PAR-T (exposure to PAR, photosynthetic active radiation, 400–700 nm), UVA-T (exposure to PAR and UVA-R) and UVB-T (exposure to PAR, UVA-R and UVB-R). During SOLSI experiments no significant differences were found between the different light treatments under simulated normal and medium ozone concentrations. Under low ozone conditions, 40% reduction in photosynthesis was observed in the UVB-T for surface incubations. In contrast, no significant differences were observed among the light treatments under mixing conditions. Field and laboratory experiments showed similar results. However, during one of the field experiments when ozone was low, not only Sfix but Mix incubations presented a significant reduction in photosynthesis, suggesting that vertical mixing under such conditions was not efficient enough to prevent harmful UVB-R effects. On the other hand, during a day with high insulation and normal ozone, but with elevated absorption of light in the water column, no significant effects of any of the studied factors were detected.fix but Mix incubations presented a significant reduction in photosynthesis, suggesting that vertical mixing under such conditions was not efficient enough to prevent harmful UVB-R effects. On the other hand, during a day with high insulation and normal ozone, but with elevated absorption of light in the water column, no significant effects of any of the studied factors were detected.