Yellowstone National Park is home to more than 10,000 hydrothermal features. The park's hot springs, geysers, mud pots, and fumaroles are home to trillions of heat-loving microbes. For photosynthetic biofilms, the rule of thumb is that algae tend to dominate in acidic springs (pH less than 3), whereas cyanobacteria dominate in alkaline environments (pH greater than 6). However, this generalization is rooted in 50-year-old research, and most studies have overlooked the intermediate pH values between the extremes.