Sunburn is a serious problem in the Southern Hemisphere where depleted ozone provides less protection from UVB. Tadpoles are at particular risk because they are growing rapidly, making them vulnerable to UVB DNA damage. Niclas Lundsgaard, of The University of Queensland (UQ), Australia says that the seriousness of sunburn was thought to be directly linked to the degree of exposure, so that a long weak dose of UVB would yield the same DNA damage as a short intense dose that resulted in the same overall exposure. But Lundsgaard, Craig Franklin and Rebecca Cramp (also from UQ), weren't sure. A short intense UVB dose can be 3 times more lethal for striped marsh frog tadpoles (Limnodynastes peronii) than a long weak UVB dose amounting to the same overall exposure. The team publishes their discovery in Journal of Experimental Biology that sunburned tadpoles suffer significantly more DNA damage from short intense doses of UVB than longer weaker doses, which could dramatically increase mortality on sunny days.
Collecting recently laid frogspawn from a creek in Brisbane, Lundsgaard took them to the UV-free lab where they developed for 5 weeks. Then, he turned on the UVB, making sure that different tadpoles got the same total exposure of UVB, no matter how strong the dose, by adjusting how long the UV lights were on for.
Some tadpoles received one intense (80μW/cm2) dose of UVB – simulating the light levels in a clear pond – for an hour, while others received a second strong hour-long dose a day later. Meanwhile, he turned down the sun lamps for other tadpoles, so the UVB was less intense (40μW/cm2), but kept the lamps on for 2 h so the tadpoles received the same overall exposure, with a fourth group of tadpoles receiving a second weak dose of UVB a day later.
Finally, Lundsgaard checked for evidence of DNA damage.
Alarmingly, the strength of UVB that the developing tadpoles experienced had a dramatic impact.
The tadpoles that experienced the most intense doses of UVB accumulated damage almost 3 times faster, resulting in 47% more DNA damage than the tadpoles exposed to mild sunburn.
Lundsgaard then checked the tadpoles that were exposed to UVB on consecutive days, and the tadpoles that had experienced the strongest sunburn were in worse shape than the tadpoles that had only received mild UVB doses.
The intensely sunburned tadpoles carried over twice as much DNA damage into the second day, and the tadpoles that experienced 2 days of sunburn had 20% more DNA damage than the tadpoles that only experienced sunburn on a single day.
Surprisingly, Lundsgaard noticed that the smallest tadpoles seemed to suffer the most DNA damage, experiencing 50% more than their larger siblings, thanks possibly to their larger surface area relative to their mass.
Even though the striped marsh frog tadpoles had experienced the same amount of UVB, the strength of the sunburn made an enormous difference.
The tadpoles exposed to the high UVB dose accumulated more dangerous DNA damage than those that had received the same total UVB exposure over longer periods, leaving them at greater risk of mortality.
Lundsgaard warns also that fish and corals could be in similar danger from the sun's rays. 'The larvae of other aquatic animals might struggle with more frequent exposure to short, intense UVB', he says.
However, Lundsgaard adds that we cannot draw any conclusions about whether stronger sunburn is more dangerous for people. 'Frogs and humans have different mechanisms for repairing DNA damage caused by UV radiation', he says, but adds, 'Our study has highlighted that the risk of sun exposure is not necessarily measured simply by how much radiation you are exposed to'.
