Changes in Microbial Lipid Biomarkers Associated with Storage Conditions in Subsurface Shale Cores

In the current context of global interest in deep surface biogeochemistry, it is necessary to understand disturbances typically associated with subsurface sample preservation (storage). Phospholipid fatty acid (PLFA) and diglyceride fatty acid (DGFA) patterns were analyzed in deep subsurface shale cores collected and stored at different conditions. Our analyses were done using the fatty acid methyl esters (FAMEs). Core samples were collected from two Marcellus Shale wells (WV6 and MSEEL) in Monongalia County, West Virginia. Samples from WV 6 were obtained and preserved at room temperature while MSEEL samples were obtained from freshly drilled cores stored at -80°C. Samples from both cores were collected at same depth, and the PLFA and DGFA were extracted from the crushed sediments and the resulting fatty acids were methylated into FAMEs and analyzed using GC-MS. There were differences in the yield and variety of the diagnostic lipid biomarkers in the samples of the different cores. The total lipid concentration and variety of both PLFA and DGFA profiles were higher in the freshly collected MSEEL samples compared to WV 6 samples stored under room temperature conditions for several years. Some of the stress indicative biomarkers like keto-, and oxiranes were only present in the MSEEL samples and absent in the WV 6. In addition, the concentration of the trans biomarkers were also higher in the MSEEL core samples as opposed to the WV 6 core samples. The lower concentration of lipid biomarkers and the absence of stress indicative biomarkers in the WV 6 core samples compared to the freshly collected MSEEL core samples could be indicative of changes in subsurface microbial community due to exposure to surface conditions during storage. These results emphasize the importance of appropriate sample collection and storage protocols for the efficient characterization of subsurface microbial communities.