The thermo-acidophilic, unicellular, and asexual red algae belonging to the order Cyanidiales are unique in that their environment is defined by acidic pH (0.5-3.5) and high temperature (38-56°C). These algae are a dominant component of microbial communities in the cooler reaches of acidic geothermal springs and are especially noticeable in Yellowstone National Park (YNP). The ecology of these algae, however, is poorly understood. The focus of my thesis research is aimed at better understanding cyanidial mat decline, an annual event corresponding to a seasonal period lasting from late spring to early summer. Previous monthly measurements of aqueous chemistries, temperature, and ultraviolet (UV)/visible irradiance suggests that UV radiation is a keystone environmental factor controlling the health and activity of these algae. The main objective of my graduate work is to examine in situ algal gene expression dynamics as a function of UV exposure. Using pyrosequencing and microarrays I am uncovering genome-level responses to UV irradiance, thus linking algal populations and activities with their environment.