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

Abstract

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.

Responses of alkaline phosphatase activity to phosphorus stress in Daphnia magna

“…Elemental bioreporters would allow for immediate and relatively rapid assessment of the in situ nutritional state of an animal. However, there are a range of potential complications to the development and use of metabolic indicators of nutrition including: (1) many digestive enzymes are used for the acquisition of multiple elements from diverse natural substrates and thus lack element-specificity, (2) enzyme activities may vary within a taxa due to ontogeny or other environmental variables [as we found in this study; also see Balseiro et al (Balseiro et al, 2008)], and (3) detailed knowledge of many target enzymes and their molecular regulation remains lacking. Consequently, there is a need to further study the regulation of metabolic enzymes, such as exploring the gene expression of nutritionally-relevant proteins in animals consuming foods of contrasting elemental quality under controlled laboratory and more realistic field conditions.…”

“…Sterner and Hessen, 1994;Gulati et al, 1997;Sterner and Schulz, 1998;Sterner and Elser, 2002), dietary P-stress also affects other aspects of animal metabolism (e.g. Balseiro et al, 2008). In particular, physiological processes involved in the disposal of excess carbon and the acquisition/retention of P can be especially sensitive to the P content of food (DeMott et al, 1998;Darchambeau et al, 2003;Jensen and Hessen, 2007).…”

Natural UVR Does Not Affect Decomposition by Aquatic Hyphomycetes

“…During the incubation, experimental units received 35 μW cm -2 nm -1 of the 340 nm band and this irradiance level is equivalent to surface noon summer sunlight in Andean freshwaters (BALSEIRO et al, 2008 ;BASTIDAS et al, 2009 ). Protection from UV was achieved by replacing the Petri dish lid by the same screen used in the field experiments (see details above under PAR/-UVR treatment).…”

Toward a more comprehensive theory of zooplankton diel vertical migration: Integrating ultraviolet radiation and water transparency into the biotic paradigm

“…Similarly, higher food quality was also found in deeper layers in the Southern California Bight (Napp et al 1988b). In highly transparent lakes, lower food quality (higher C : P ratios) in surface waters may also reduce antioxidant enzyme activity critical for defending against damaging UV, potentially increasing susceptibility of Daphnia to UV damage and possibly even excluding them from more transparent high C : P lakes (Balseiro et al 2008).…”