Wavelength-dependent induction of thymine dimers and growth rate reduction in the marine diatom Cyclotella sp. exposed to ultraviolet radiation

Cultures of the marine diatom Cyclotella sp. were subjected to various polychromatic exposures of UVB radiation (280-320 nm), UVA radiation (320-400 nm) and photosynthetically active radiation, PAR (400-700 nm). Changes in growth rate and residual thymine dimer content (a measure for DNA damage) were measured during prolonged exposure (6 to 7 d) to these conditions. Also, changes in mean cell size were studied as an indication of UV radiation induced cell cycle arrest in Cyclotella sp. Growth rate reduction was strongly related with residual thymine dimer content in treatments including wavelengths below 302 nm. Additionally, significant increases in mean cell size were found in these cultures. This suggests that UVB-induced residual DNA damage is followed by cell cycle arrest and growth rate reduction in Cyclotella sp. We discuss how these results can be interpreted in relation to changes in the solar spectrum as a result of stratospheric ozone reduction.