Role of mycosporine-like compounds in alpine lakes

In several parts of the world, stratospheric ozone depletion is changing the spectral characteristics of solar ultraviolet radiation (UVR, 290-400 nm) due to increasing levels of UVB radiation (290-320 nm). Ultraviolet solar radiation penetrates to significant depths in alpine lakes and several studies have indicated its ecological importance. In addition, alpine lakes have generally a very high water UV transparency due to their low concentration of colored dissolved organic matter. Consequently, highly transparent lakes deserve special attention, particularly, considering a scenario of increasing UVB fluxes. Organisms living in such ecosystems, however, must have developed adaptive mechanisms to cope with high incident UVR. Understanding these mechanisms and strategies is a crucial step in assessing the potential negative effects of increased UVB fluxes. Significant effort has been devoted in the last years to understand the role played by a family of intracellular sunscreens compounds widespread in marine organisms and known as mycosporine-like amino acids (MAAs). Information on the ecological importance of MAAs in freshwater organisms and lakes, however, is not available, mainly because of their recent discovery. The main goal of this project is to fill the gap of information concerning the ecological role of MAAs in alpine lakes. Particularly, the following questions will be addressed: how widespread are MAAs distributed among phytoplankton living in lakes of different UV transparency?; how light climate influences the synthesis of MAAs in phytoplankton?; how these sunscreens are obtained by freshwater crustacean zooplankton and which role do they play for the known UV resistance of certain species?; are MAAs in natural lakes a significant component of the dissolved organic matter? These questions will be answered using a combination of experimental and field studies during a 3-years project, including also an important component on methodological comparison. This study will increase our understanding of the many potential roles played by MAAs in alpine lakes, as well as their dynamics, fate, and efficiency as sunscreens for UVR.

Ultraviolet radiation (UVR) plays an important ecological role in many alpine lakes because of the high water transparency of these ecosystems and the natural increase of the UVR flux with elevation. In response to UVR, aquatic organisms have evolved different strategies to either avoid high levels of solar radiation (e.g., deep distribution) or to protect themselves (e.g., photoprotective pigments). Knowledge about these strategies is crucial to predict potential negative effects of increasing levels of UVB radiation (destruction of the ozone layer) on aquatic ecosystems. One of the main findings in this project was the widespread distribution of natural UV- absorbing compounds, the so-called mycosporine-like amino acids or MAAs in microalgae and small aquatic animals (zooplankton). However, microalgae from mountain lakes located at different elevations had a wide range of MAA concentrations with the highest values observed in the clearest and shallowest lakes. In the lakes, the concentration of MAAs decreased with depth supporting their role as sunscreens. Also in zooplankton, which obtain their MAAs from the diet, these compounds were detected, but not in all species. These differences reflect the wide range of UV sensitivity observed among distinct species of this group of aquatic animals. The concentration of MAAs in the most frequently found crustacean species (Cyclops) showed a positive relationship with the altitude and UV transparency of the lakes. In both microalgae and zooplankton, MAA concentrations showed a clear seasonal trend with the highest values found during the ice-free period, and the lowest during the ice-covered one. Among the life stages of Cyclops, the highest MAA concentrations were measured in eggs and in the first larval stages which thrive in the lake during the time of highest UV intensity. Concentrations of MAAs in this copepod species were surprisingly constant for several months even when kept in the laboratory without further addition of MAA-rich diet, suggesting an efficient adaptation strategy. MAAs in Cyclops were mainly concentrated in the dorsal region and antenna. Overall, in cold aquatic ecosystems such as alpine and Antarctic lakes, accumulation of MAAs appears to be the most important adaptation to high levels of UVR. In future, the exceptional characteristics (e.g., chemical stability) of these natural photoprotective compounds might serve as a model for the production of synthetic sunscreens.