Survival strategies of planktonic organisms in alpine lakes and beyond: Baldi Memorial Award Lecture presented at the 37th Congress of the International Society of Limnology.

Recent research on alpine lakes has revealed both novel adaptive strategies and established ones, now examined from a new perspective, that planktonic organisms use to persist under the harsh environmental conditions characteristic of these ecosystems. The 2024 Edgardo Baldi lecture addresses 3 major topics, namely, the multifactorial drivers of photoprotection, emerging mechanisms of microbial solar energy utilization, and virus-mediated facilitation of host survival. Photoprotective phenotypes, including the accumulation of mycosporine-like amino acids and carotenoids, are increasingly recognized as outcomes of complex interactions among ultraviolet radiation exposure, nutrient (particularly nitrogen) availability, temperature fluctuations, and predation pressure. Deciphering this interplay is critical for predicting planktonic responses to ongoing global environmental change. A significant advance in the understanding of microbial photoenergetics has been the recent discovery of dual phototrophy in certain bacteria that combine aerobic anoxygenic photosynthesis with rhodopsin-based proton pumping. This dual mechanism enhances energy acquisition and biomass production under conditions of fluctuating light intensity and limited dissolved organic carbon concentrations, with potential parallels in eukaryotic phytoplankton. Finally, the identification of novel viral groups in alpine lakes, such as Polinton-like viruses and virophages, suggests a protective role against host mortality induced by giant viruses. These interactions seem to stabilize microeukaryotic populations and may significantly influence ecosystem productivity and dynamics. Together these findings emphasize the ecological and evolutionary significance of such adaptations, extending beyond alpine lakes to other aquatic ecosystems, and improving our understanding of how plankton respond to environmental changes.