Alpine streams are among the most sensitive ecosystems to climate change. As temperatures rise and glaciers shrink, these streams face increasing stress from warmer water and changing flow patterns. Benthic biofilms, the layer of microorganisms dominating streambed rocks and sediments, are essential for keeping these ecosystems functioning. Recent advances have improved our understanding of the freshwater biofilm responses to climate change, yet few studies have focused on high-mountain ecosystems, especially glacier-fed streams. This thesis explores how glacier-fed stream biofilms are affected by climate-induced environmental stressors, especially streamwater warming and drought. Using a combination of controlled field and laboratory experiments, we investigated changes in microbial community composition, their metabolic activity, and how individual bacteria cope with environmental stress. By studying both natural biofilm communities and isolated bacterial strains, this thesis provides new insights into how glacier-fed stream biofilms may shift as the cryosphere continues to vanish. Particularly, we demonstrated substantial drought-induced shifts in microbial composition and phototrophic metabolism, as well as revealed strain-level drought priming potential in taxa isolated from glacier-fed stream biofilms. Our findings contribute to a better understanding of the impacts of warming and drought on key alpine freshwater environments in a changing world.