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Oxidative stress in the copepod Boeckella gracilipes in Andean lakes: intra-specific differences
Congreso; Global Change and the World’s Mountains; 2010
In freshwater systems, ultraviolet radiation (UVR) is recognised as an important biological stressor. UVR can penetrate the euphotic zone, affecting physiology of aquatic organisms to different degrees, and shorter wavelengths (UV-B, 280–315 nm) are more deleterious than longer ones (UV-A, 320–400nm). Planktonic organisms have developed a variety of photoprotection strategies, including behavioural avoidance of photodamage, production or incorporation of UV-absorbing compounds such as carotenoids, melanin and mycosporine-like amino acids (MAAs), and enzymes involved in DNA repair and in antioxidant defences. Among the latter, glutathione S-transferase (GST) is a detoxifying enzyme involved in the removal of reactive organic hydroperoxides, such as the products of lipid peroxidation under oxidative stress. Antioxidant mechanisms are energetically demanding and may require additional elemental supplies of phosphorus (P) for antioxidant enzymes. Thus, the relative importance and efficiency of UVR defences would depend on the availability of dietary factors, such as P and N (nitrogen). Among the deleterious effect on proteins, UVR oxidant injuries might affect specific enzymatic endpoints, such as that of acetylcholinesterase (AChE). AChE catalyses the hydrolysis of acetylcholine (ACh), the primary neurotransmitter in sensory and neuromuscular systems in most species. The interaction between ACh and AChE is vital for normal behaviour and muscular function. Natural factors such as UVR may also influence this enzyme activity but there is little information on the direct effects of UVR on enzyme activity in natural populations of planktonic organisms. Boeckella gracilipes is the dominant copepod species in many Andean north Patagonian lakes. In these lake ecosystems, high C:P ratios are associated with high light:P ratios, thus organisms living in these transparent lakes would be constrained by potentially hazardous ultraviolet radiation (UVR). We analysed natural populations that showed intra-specific differences in photoprotective pigments (carotenoids and MAAs), elemental ratios (C:N:P) and antioxidant enzyme activities. From field and laboratory exposure experiments, we determined the response to UVR in populations inhabiting different lakes (medium- and high-altitude lakes). In particular, we analysed the response of antioxidant enzymes (GST) and the detrimental effects of UVR on AChE. We were able to determine that natural populations of the same species exhibited differences in the response to UVR. Although carotenoid concentrations were higher in organisms inhabiting fish-free high-altitude lakes, GST activity was also higher in these copepods than in low-altitude and less transparent lakes. However, in the former there was no UVR effect on AChE whereas a reduction in activity was observed in the colourless copepods, confirming that AChE activity depends more on the antioxidant capacity (GST) for enzyme synthesis than on the presence of photoprotective compounds. Nevertheless, during laboratory incubations, up to 30% of the pigmented copepod individuals exhibited damage on antenna 1, suggesting that these copepods were indeed affected by UVR. Because calanoid copepods depend on swimming as an escape reaction and on feeding currents, dysfunction of AChE would affect their fitness by affecting their motility patterns. Motion is not a singular event; it results from the integrated function of several reaction components that imply multiple nerve impulses that are accurately coordinated. Thus, any alteration in neurotransmitter mechanisms, such as AChE, will have strong negative effects on this vital function. Our results indicated that the level of GST activity was key in the protection of AChE, and that GST activity level depends on the food stoichiometry (C:P ratio); thus high C:nutrient ratios will increase the risk of UVR damage to the copepod neurotransmitter system.