RESPONSES TO SOLAR UV RADIATION OF THE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE) GROWN AT DIFFERENT Zn2+ CONCENTRATIONS

Zn availability in the ocean has been suggested to limit primary production by affecting CO2 acquisition processes for photosynthesis, therefore influencing the global carbon cycle. Also, UV radiation (UVR, 280-400 nm) is known to affect primary production in different ways. It remains to be ascertained whether Zn availability and UVR can act synergistically, antagonistically, or independently on oceanic primary production. We cultured the cosmopolitan diatom Skeletonema costatum (Grev.) Cleve under different radiation treatments with or without UVR (only photosynthetically active radiation), at 0, 3, and 10 pmol . L-1 Zn2+. Specific growth rate, photosynthetic carbon assimilation, external carbonic anhydrase (eCA) activity, and estimated cell abundance increased with increasing concentrations of Zn2+ from 0 to 3 and 10 pmol . L-1, irrespective of the radiation treatment. Higher eCA activity was observed in the cells grown at the high level of Zn2+ in the presence of UVR. An approximately linear relationship between mu and the daily dose of PAR was observed at 3 and 10 pmol . L-1 Zn2+ concentrations. However, the dependency of mu on the daily PAR dose disappeared when the cells were grown in the presence of UVR, which overall depressed both mu and photosynthetic carbon assimilation. The inhibitory effect of UVR was inversely related to Zn2+ concentrations. The ultraviolet-B (UVB)-related inhibition of growth and photosynthesis decreased with time, reflecting a faster acclimation of the cells to UVR at replete Zn2+ levels. Overall, growth in the presence of higher Zn2+ concentrations reduced the sensitivity to UV radiation in Skeletonema costatum.