The impact of ultraviolet-B radiation on the motility of the freshwater epipelic diatom Nitzschia lineariz

Depletion of the stratospheric ozone layer has been of increased concern du e to correlated increases in ambient ultraviolet radiation. Our research in vestigated the diel response of the motile behaviour of Nitzschia lineariz in both natural (freshwater stream) and experimental conditions. We classif ied the behaviour of individual diatom cells as gliding, immobile and oscil lating. The experimental conditions were of simultaneous exposure either to net ambient solar radiation (control) or to ambient plus enhanced UV-B lev els (treatment). UV-B for the control condition was filtered out using poly ester filters; cells exposed to UV-B were irradiated through cellulose diac etate filters. UVB-313 levels were augmented to 33% (290 mW m(-2)) and 66% (365 mW m(-2)) above the local average ambient levels (220 mW m(-2)) for 10 h. The mobile behaviour of cells was examined and quantified every 2 conse cutive hours from dawn to dusk in subsampled populations. The number of gli ding cells in subsample populations declined on overcast days at both the 3 3 and 66% UV-B enhancement levels, whereas the number of immobile and oscil lating cells increased with increased UV-B exposure. On sunny days, mobile behaviour was not affected at either enhancement level, which suggests that cellular repair mechanisms may be activated during sunny conditions.