Responses in pigmentation and anti-oxidant expression in Arctic Daphnia along gradients of DOC and UV exposure

Abstract

Responses in carapace melanization and expression of the major anti-oxidant catalase (CAT) and glutathione transferase (GST) in Arctic Daphnia were assessed in enclosures along a gradient of dissolved organic carbon (DOC). This gradient was created by adding freeze-dried humic matter to 2 m3 UV-transparent enclosures, yielding final nominal concentrations of 1, 2.5, 5 and 10 mg C l-1. The UV attenuation was strongly affected by additions of DOC, and attenuation coefficients at 320 nm increased from 3.0 in the control to approximately 3.5 and 11.0 m-1 in the 1 and 10 mg DOC treatments respectively. Most Daphnia showed pronounced carapace melanization, and the absorbance of short-wave radiation through the carapace was strongly related to the degree of melanization. Nevertheless, the different UV climate in the enclosures did not cause any short-term adaptation in Daphnia pigmentation over a 3 week period. The levels of CAT and GST were assessed over time in the control and at 10 mg DOC. These enzymes displayed opposite patterns, with somewhat lower activities of CAT at low DOC (control) relative to 10 mg DOC, while the opposite was found for GST. There was also a significant negative correlation between CAT and solar irradiation for GST in both bags, while no effects were found for GST.