Abstract
A thermophilic photosynthetic purple bacterium was isolated from the waters of a hot spring in Yellowstone National Park, Wyoming. The organism differs from all known purple bacteria in that it grows optimally at a temperature of about 50° C. The isolate contains bacteriochlorophyll a and grows autotrophically, oxidizing sulfide to elemental sulfur which is then stored as globules inside the cell. These properties indicate that the phototroph is a member of the Chromatiaceae (purple sulfur bacteria).