UV stress in alpine lakes: hsp gene expression in copepods

Ultraviolet radiation is a key parameter in alpine lakes because its intensity increases with elevation and due to the high water transparency of these generally small and shallow aquatic systems. Planktonic organisms in alpine lakes are exposed to high solar radiation intensities, but they have adapted to the harsh environmental conditions through strategies that reduce the risk of UV-induced damage. For example, zooplankton avoid the upper few meters of the water column during the day, and accumulate UV-absorbing or antioxidant compounds that protect against UVR. The copepod Cyclops abyssorum tatricus, a dominant zooplankton species of many high elevation lakes from the Central Eastern Alps, is highly resistant to UVR when residing in UV-transparent habitats, while populations from glacially turbid lakes show low UV protection. Compared to the relatively well known direct and indirect effects of UVR on freshwater zooplankton, responses at the molecular level have been scarcely investigated in these organisms. Studies based on gene expression analyses provide detailed insight into how organisms respond to genotoxic stressors such as UVR. Upregulation of heat shock protein (hsp) genes may increase the capacity of C. abyssorum tatricus populations to cope with high UV levels encountered in their natural environment. The major aim of this project is to provide insight into the role of UV-induced molecular response mechanisms in freshwater zooplankton using nucleic acid-based techniques. I will test how stress protein gene expression in copepod populations changes in response to different levels of UV stress in laboratory and in situ experiments. In detail, dose- and depth-dependent variations, as well as diurnal and seasonal changes in stress protein gene expression will be studied in copepod populations from alpine lakes of different UV transparency. Expression of four principal heat shock protein genes (hsp20, hsp60, hsp70 and hsp90) will be quantified using real-time RT-PCR. Photo- protective compounds (mycosporine-like amino acids, carotenoids) and the antioxidant capacity of the copepods will be measured to evaluate the level of UV protection in the diverse C. abyssorum tatricus populations. The anticipated results will shed new light onto how organisms living in these sensitive environments will respond to expected changes in water transparency by glacial retreat and other climate-driven changes.

UV radiation penetrates deep into the water column of clear alpine lakes. Planktonic organisms evolved several strategies to reduce the risk of UV-induced damage. For example, zooplankton avoids the upper water layers during the day and accumulates high levels of photoprotective compounds to protect themselves from this damaging radiation. In glacier-fed turbid lakes, however, UV radiation is largely excluded and zooplankton is more evenly distributed in the water column having only low concentrations of photoprotective compounds. The overall objective of this project was to investigate how populations of the dominant zooplankton species, Cyclops abyssorum tatricus, from clear and glacially turbid lakes respond to the effects of UVR on the molecular level. The central focus was on the expression of heat shock proteins (hsps) genes that have an important protective function under stress conditions. When these small crustaceans were exposed to high levels of solar UV radiation at the lake surface, upregulation of hsp70, the most commonly expressed hsp, was observed within hours. Under highly variable environmental conditions, this seems to be an important strategy compared to physiological defenses, which have a slower response. Furthermore, laboratory experiments showed that, depending on the level of UV protection, these animals reacted differently during the course of a year. Similar to the findings in the lake, the hsp70 gene expression increased after exposure to UV radiation during the ice-free summer season. During the long season with ice cover (~8 months), when the copepods are probably more UV sensitive because of their low photoprotection, the extent of hsp (hsp70) upregulation increased with decreasing levels of photoprotective compounds and peaked at the time of lowest photoprotection. This plasticity in these small crustaceans with the potential to rapidly respond to stress is essential for survival in highly variable ecosystems such as alpine lakes. When comparing the cellular stress response in clear or glacially turbid lakes, these organisms show different strategies: copepods from turbid ecosystems have a lower baseline expression of hsps and induce a stronger stress response than the ones from the clear lake, which are better adapted to UV exposure with high levels of photoprotective compounds. Diurnal changes in hsp gene expression and antioxidative compounds were also observed for the first time. In the turbid lake, no diurnal patterns were found, whereas copepods from clear lakes showed diurnal variation in antioxidant capacities, with maxima during midday, but not in hsp gene expression. This indicates that diurnal changes in antioxidant capacities help to reduce potential oxidative stress, whereas the costly production of hsps only occurs under acute stress. Alpine lakes are sensitive ecosystems, experiencing profound changes caused by glacier retreat and other signs of climate change. The results obtained in this project contributed to better understand how aquatic organisms living in these ecosystems will respond to these changes.