Microbes that live deep in ocean sediments help drive Earth's elemental cycles, influencing key aspects of habitability on the planet, such as the amount of oxygen in the atmosphere. Microbes in the deep subsurface have a remarkable lifestyle. They turn over their biomass on timescales of decades, centuries, or even millennia, while microbes in most other environments do so every few hours or days. The driving hypothesis for this project is the unusually high stability of the deep subsurface microbes' enzymes is key to these microbes' survival. The scientific results of this project will help understand the function of a vast and mysterious ecosystem at the bottom of the ocean and may lead to new discoveries about enzyme structure and function that could have valuable applications in the food or pharmaceutical industries. The project supports the training of undergraduate and graduate students in research and teaching. In addition, K12 students participating in a summer school program called East Tennessee Freedom Schools help generate data and design experiments, giving them early input into the scientific process. The educational program allows a set of young scientists to gain experience with authentic scientific research. The central hypothesis that unusually stable enzymes help subsurface microbes to persist over long timescales is tested via a field program to collect subsurface sediments in the Gulf of Alaska, experimental manipulations of those sediments, labor