The 9th International Symposium on Subsurface Microbiology in Pacific Grove, California, attracted 228 scientists, engineers, and students from 25 countries and 5 continents. The symposium offered four plenary sessions, ten regular sessions and two poster sessions. Since the last meeting in 2011, a great deal of new knowledge was adopted and for this reason the organizers decided to combine contributions into the following topics: Biomes, Chemistry, Contaminants, Ecology, Fate and Methods. A special session within Biomes was also dedicated to Cave, Karst, and Fractured Rock Microbiology. Microbial metabolism was the focus of several oral and poster presentations. Abundant polysaccharide biofilms in the form of reduced sugars can act as an electron sink. Water quality and pollution is a global urgent issue, with the respect of water usage increase of 1% annually.