A group of ocean bacteria long considered perfectly adapted to life in nutrient-poor waters may be more vulnerable to environmental change than scientists realized. Their success is tied to genome streamlining, an evolutionary process in which organisms lose genes to reduce energy costs in nutrient-limited environments. What surprised the researchers was how SAR11 cells responded to stress. Their work will help improve our understanding of SAR11’s role in marine carbon cycling, an effort made critical by the organism’s sheer abundance. This research was supported by a Simons Foundation Early Career Investigator in Marine Microbial Ecology and Evolution Award and a Simons Foundation Investigator in Aquatic Microbial Ecology Award.