RESPONSES TO SOLAR UV RADIATION OF THE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE) GROWN AT DIFFERENT Zn2+ CONCENTRATIONS1

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 μ and the daily dose of PAR was observed at 3 and 10 pmol · L^?1 Zn²⁺ concentrations. However, the dependency of μ on the daily PAR dose disappeared when the cells were grown in the presence of UVR, which overall depressed both μ 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.