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 (mmol P?[mmol C]21; 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. The concept of ‘‘ecological stoichiometry’’ has been applied to describe the role of multiple chemical elements in controlling trophic processes and has even been proposed as a new branch of ecology (Sterner and Elser 2002; Andersen et al. 2004). It is now well established that stoichiometric constraints are important in regulating organism growth and nutrient cycling in food webs (Sterner and Elser 2002). In particular nitrogen (N) and phosphorus (P) are both structurally and functionally important in all organisms (Sterner 1995; Elser et al. 1996), often limiting primary and bacterial production (Vrede et al. 1999) and consumer growth (Gulati and DeMott 1997; Elser et al. 2000a; Ferrao-Filho et al. 2007). Moreover, it has been shown that carbon (C) : N : P stoichiometry is related to the elevated protein synthesis during rapid growth due to allocation to P-rich ribosomal ribonucleic acid (rRNA; Elser et al. 1996, 2000b). Since chemical reactions in living organisms are catalyzed by enzymes, the vast majority of which are proteins, it follows that stoichiometric constraints may be also crucial for enzymatic activities. Evolution has crafted thousands of enzymes that are efficient catalysts for a plethora of reactions. Among them, catalase (CAT) and glutathione S-transferases (GSTs) play an important role against oxidative stress caused by ultraviolet radiation (UVR; Borgeraas and Hessen 2000, 2002). Organisms are affected by UVR when key macromolecules (deoxyribonucleic acid [DNA], protein, chlorophyll) absorb specific wavelengths, altering important physiological or biochemical processes (Siebeck et al. 1994; Gonçalves et al. 2002). However, aquatic organisms can be also negatively affected by UVR through the generation of reactive oxygen species (ROS). The most long-lived ROS, hydrogen peroxide (H2O2), is of special interest because it is readily diffusible across cell membranes and functions as a signaling molecule in diverse cellular events. The generation of H2O2 is also associated with damage to DNA, proteins, and lipids and the induction of apoptosis (Martindale and Holbrook 2002). CAT, a widely distributed enzyme that reduces H2O2, is important against oxidative stress (Barata et al. 2005) and, even if it is not essential, the lack or malfunction of catalases may lead to severe defects including high mutation (Cho et al. 2000). On the other hand, GSTs, a family of cytosolic multifunctional enzymes, are detoxifying enzymes that are present in all aerobic organisms (Hayes and Pulford 1995). They catalyze the conjugation of glutathione with a variety of reactive electrophilic compounds, thereby neutralizing their active electrophilic sites and subsequently making the parent compound more water soluble. Additionally, GST has been found to be involved in the removal of reactive organic hydroperoxides, such as the products of lipid peroxidation (Bartling et al. 1993). Planktonic organisms are exposed to potentially harmful sunlight because of high intensities or damaging ultraviolet 1 Corresponding author ([email protected]).