UVB radiation modifies protein and photosynthetic pigment content, volume and ultrastructure of marine diatoms

Three marine diatom species (Cyclotella sp., Nitzschia closterium and Thalassiosira nordenskioldii) were exposed to a range of daily doses of ultraviolet B radiation (UVBR: 280-320 nm). The lowest UVBR treatments (<2000 J m(-2) d(-1), DNA weighted biologically effective dose, normalised at 300 nm: daily BED(DNA 300 nm) resulted in decreased division rates, volume enlargement and elevated cellular protein and pigment content levels. The highest UVBR treatments (between 2000 and 3800 J m(-2) d(-1) daily BED(DNA 300 nm)) resulted in complete growth inhibition, accompanied by only minor changes in protein, pigments and cell volume. Recovery of cell division after UVBR exposure was decreasingly successful with increasing UVBR dose rates. Ultrastructural examination of exposed Cyclotella cells indicated that high UVBR levels induced plasmolysis and disorientation of cell organelles. Lower levels (<2000 J m(-2) d(-1) daily BED(DNA 300 nm)) seemed to cause an increase in volume and the amount of chloroplasts. The results support the notion conceived earlier that UVBR causes DNA damage, an arrest in the S or G2 phase of the cell cycle, and consequently growth without cell division.