Living in transparent lakes: Low food P : C ratio decreases antioxidant response to ultraviolet radiation in Daphnia

We experimentally tested the effect of food quality (phosphorus [P] : carbon [C] ratio) on the response of antioxidant enzymes to ultraviolet radiation (UVR) in Daphnia commutata fed with Chlamydomonas reinhardtii. Algal cultures were grown at different concentrations of phosphorus and light intensities, resulting in significant differences in the P : C ratios (mu mol P.[mmol C](-1); 6.05, 1.70, and 0.83). After 12 d of D. commutata growth under these three food quality treatments, we observed significant differences in individual biomass and protein content of Daphnia. Subsequently, we carried out an ultraviolet exposure experiment to determine if stoichiometric constraints imposed would limit enzymatic defenses against UVR oxidative stress. The UVR-exposure experiment consisted of a factorial design with three levels of food P : C (low, medium, and high) and two levels of UVR (exposed and protected). The activities of glutathione S-transferases (GST) and catalase (CAT), enzymes involved in protection and repair of damage caused by UVR, were determined. Enzyme activities in the animals exposed to or protected from UVR showed a direct relationship with food P : C ratio that fit exponential models. Although GST and CAT differed slightly in their response to UVR, both enzymes were significantly affected by food quality: In low P : C treatments, there was significantly lower enzyme activity in response to UVR for both enzymes. Low food quality (less P for biosynthesis) may also impose a weaker antioxidant response on the organisms, a response of considerable ecological relevance in transparent Andean lakes which combine high UVR intensities with low seston P : C ratios.