Contrasting diurnal patterns in antioxidant capacities, but not in expression of stress protein genes among copepod populations from clear versus glacially fed alpine and subalpine lakes

Short-term changes in environmental conditions largely influence planktonic organisms, but their responses will depend on the habitat characteristics. Here we studied diurnal patterns in antioxidative metabolites (lipophilic and hydrophilic antioxidant capacities) and in the expression of stress protein genes (heat shock proteins, hsp) of copepods to identify short-term stress responses in clear and turbid alpine lakes, as well as in less transparent subalpine ones. Cyclops abyssorum tatricus showed diurnal variation in antioxidant capacities with maxima around noon in clear, but not in glacially fed, turbid lakes. Low fluctuations of these metabolites were also observed in another copepod, Acanthodiaptomus denticornis. Although levels of hsp genes differed between populations living in clear or glacially fed lakes, there was no diurnal rhythmicity in gene expression. Our data show that when planktonic organisms may be at greatest risk of oxidative damage, such as during the daytime in high UV radiation environments, they activate antioxidant responses. Conversely, in less transparent lakes, the physiological response seems to be unnecessary. The difference in gene expression levels suggests an ecological, albeit not acute, role of these genes in copepods experiencing daily environmental fluctuations.