Diel and depth profiles of DNA photodamage in bacterioplankton exposed to ambient solar ultraviolet radiation

Bacteria play a central role in the cycling of nutrients and energy flow to higher trophic levels, yet the effects of ultraviolet-B (UV-B) radiation upon bacterioplankton have been largely overlooked. Using a highly specific radioimmunoassay, measurements of solar-induced DNA photodamage (cyclobutane pyrimidine dimers) were taken in planktonic samples collected from the northern Gulf of Mexico. Diel patterns of dimer accumulation and repair were observed in both the bacterioplankton size fraction (< 0.8 μm) and in the larger eukaryotic plankton size fraction (> 0.8 μm < 120 μm) although damage induction was approximately twice as much in the bacterioplankton fraction. Depth profiles of DNA damage in the bacterioplankton size fraction during calm and moderate seas demonstrated the influence of mixing on the distribution of UV radiation effects. During calm seas, damage was greatest in surface waters, decreased with depth, and could be detected to 10 m. In moderate seas, however, no net accumulation of damage was observed, even at the surface. The results demonstrate that bacteria are more susceptible to UV-B damage and may serve as a more sensitive indicator of UV stress than other microorganisms. Wave action and mixing strongly influence the effects of UV-B in surface waters demonstrating that UV-B effects may not always be predictable from measures of UV radiation attenuation.