UV-B-induced damage and photoreactivation in three species of Boeckella (Copepoda, Calanoida)

Solar ultraviolet (UV) radiation poses a threat to most living organisms. Aquatic organisms have evolved three basic mechanisms to cope with harmful levels of radiation. Two mechanisms, avoidance (e.g. vertical migration) and photoprotection (e.g. production of photoprotective compounds that act as filters, antioxidants, etc.), serve to minimize the dose of UV radiation that reaches the organism's vital structures (DNA, membranes, etc.). The third mechanism, repair (e.g. dark repair mechanisms, such as nucleotide excision repair; or photoreactivation mechanisms, such as photoenzymatic repair), serves to repair the damage following UV exposure. Here, we compare the vulnerability to UV-B radiation of three copepod species (Boeckella brevicaudata, Boeckella gibbosa, and Boeckella gracilipes) that occur in lakes that differ in UV-B penetration and depth. Our aim was to gain insight into the significance of each of the three mechanisms in different UV-B environments. Results from a 3-day ‘in situ’ incubation in ultra-oligotrophic Lake Toncek showed that B.gracilipes is highly vulnerable to UV-B and UV-A radiation. In contrast, virtually no mortality was observed in B.gibbosa and B.brevicaudata during the same period. In order to discriminate the contribution of photoprotection and photoreactivation, the three species were subsequently exposed in the laboratory to an artificial source of UV-B radiation, both in the presence and absence of visible radiation (recovery radiation). The photoprotection potential (i.e. resistance to UV-B in the absence of recovery radiation) of B.gracilipes and B.gibbosa was lower than that of B.brevicaudata. On the other hand, photoreactivation (higher resistance to UV-B in the presence of recovery radiation) was observed in B.brevicaudata and B.gibbosa, but not in B.gracilipes. To cope with damaging UV-B levels in nature, B.gracilipes depends exclusively on the attenuation by the external media (i.e. avoidance). Although B.gibbosa tends to avoid the surface waters of lakes, it also occurs in shallow transparent pools. Most likely its ability to survive in these shallow, high UV environments is due to its photoreactivation potential. Finally, despite its occurrence in highly turbid lakes, B.brevicaudata seems extremely well suited to cope with UV-B radiation thanks to a combination of photoreactivation and photoprotection.