Anaerobic hydrocarbon biodegradation in deep subsurface oil reservoirs

Biodegradation of crude oil in subsurface petroleum reservoirs is an important alteration process with major economic consequences1. Aerobic degradation of petroleum hydrocarbons at the surface is well documented2 and it has long been thought that the flow of oxygen- and nutrient-bearing meteoric waters into reservoirs was necessary for in-reservoir petroleum biodegradation3. The occurrence of biodegraded oils in reservoirs where aerobic conditions are unlikely4, together with the identification of several anaerobic microorganisms in oil fields5 and the discovery of anaerobic hydrocarbon biodegradation mechanisms6,7, suggests that anaerobic degradation processes could also be responsible. The extent of anaerobic hydrocarbon degradation processes in the world's deep petroleum reservoirs, however, remains strongly contested. Moreover, no organism has yet been isolated that has been shown to degrade hydrocarbons under the conditions found in deep petroleum reservoirs8. Here we report the isolation of metabolites indicative of anaerobic hydrocarbon degradation from a large fraction of 77 degraded oil samples from both marine and lacustrine sources from around the world, including the volumetrically important Canadian tar sands. Our results therefore suggest that anaerobic hydrocarbon degradation is a common process in biodegraded subsurface oil reservoirs.