Diel cycles of DNA damage and repair in eggs and larvae of northern anchovy, Engraulis mordax, exposed to solar ultraviolet radiation

Damage from UVB radiation (280-320 nm) in the form of cyclobutane pyrimidine dimers (CPD) in DNA and the capacity for their repair were measured in newly spawned eggs and yolk-sac larvae of northern anchovy, Engraulis mordax, exposed to natural diel cycles of sunlight, The CPD were measured by a newly developed chemiluminescent immunoblot assay capable of measuring CPD in samples as small as 50 ng DNA, Eggs and yolk-sac larvae exposed to full irradiance levels died. At lower dose rates, equivalent to deeper more natural locations in the water column, there was a diel cycle of dimer concentration that tracked solar intensity. This diel cycle was due to the interaction of damage and repair processes. Repair of CPD in anchovy eggs and larvae could be attributed to true photodependent repair that could be stopped by moving samples into the dark. The CPD present at sunset remained until the following morning. The diel cycles of damage and repair were maintained over at least 4 days without a long-term upward or downward trend in dimer concentration. This indicates that at the UVB doses used for these experiments, there was no long-term accumulation of CPD nor an induction of increased repair capacity. Unhatched embryos spawned in the dark also exhibited a strong photorepair response, suggesting that photolyase expression was innate and not dependent on previous light exposure. The diel cycle observed here indicates that, at least for northern anchovy, the CPD concentration at the time of sampling is a good indicator of dose rate but a poor indicator of cumulative dose (i.e. late afternoon samples have the highest cumulative dose but relatively low CPD concentrations). The CPD immunoassay described here has the required sensitivity for measuring DNA damage in wild populations of ichthyoplankton exposed to natural sunlight. These results will guide the collection and interpretation of field data on natural levels of CPD in wild larvae collected at different depths and times of the day.