Microbes in Marcellus Shale: Extremophiles living more than two kilometers inside the Earth?

Marcellus Shale drilling is a relatively new activity that has expanded natural gas resources in the United States. Although many assumed Marcellus Shale was sterile due to extreme conditions endured since deposition, conditions present in the shale today are within range where microorganisms can occur. Here, we consider whether some microbes seen in produced fluids from Marcellus Shale drilling may be native by comparing organisms found in core samples and produced fluids to those found in injected fluids from active wells in Pennsylvania. Illumina sequencing was used to identify organisms based on 16s rRNA barcoding regions of DNA extracts from Marcellus Shale samples. Identified microbial communities were then compared using principal components analysis and SourceTracker tools. Frequency of homology between sequences showed that produced fluids contained more microorganisms in common with shale cores than with injected fluids. Principal components analysis and SourceTracker corroborated these results. Possible native organisms found in these samples included radiotolerant extremophiles such as Deinococcus radiodurans. This work suggests that deep subsurface Marcellus Shale, previously believed sterile, may contain native microorganisms, which may have been deposited during the original deposition or migrated into the deep subsurface Marcellus during a more recent water influx. Additional investigation is recommended.