PORE-SIZE CONSTRAINTS ON THE ACTIVITY AND SURVIVAL OF SUBSURFACE BACTERIA IN A LATE CRETACEOUS SHALE-SANDSTONE SEQUENCE, NORTHWESTERN NEW-MEXICO

To investigate the distribution of microbial biomass and activities to gain insights into the physical controls on microbial activity and po tential long-term survival in the subsurface, 24 shale and sandstone c ores were collected from a site in northwestern New Mexico. Bacterial biomass in the core samples ranged from below detection to 31.9 pmol t otal phospholipid fatty acid (PLFA) g(-1) of rock with no apparent rel ationship between lithology and PLFA abundance. No metabolic activitie s, as determined by anaerobic mineralization of [C-14]acetate and [C-1 4]glucose and (SO42-)-S-35 reduction, were detected in core samples wi th pore throats <0.2 mu m in diameter, smaller than the size of known bacteria. However enrichments revealed the presence of sulfate-reducin g bacteria, and (SO42-)-S-35 reduction was detected upon extended (14 days) incubation in some small-pore-throat samples. In contrast, relat ively rapid rates of metabolic activity were more common in core sampl es containing a significant fraction of pore throats >0.2 mu m in diam eter. These results suggest that subsurface bacteria require interconn ected pore throats greater than 0.2 mu m diameter for sustained activi ty but that viable bacteria can be maintained and stimulated in poorly permeable rocks, such as shales, with restrictive pore throat diamete rs. In addition, the detrital organic matter in the small-pore-diamete r shales is not subject to direct microbial attack. Rather, bacteria i n adjacent sandstones with a more open pore structure are probably sus tained by endogenous nutrients that are slowly released from the shale . These results have implications for the long-term maintenance of ano xia and the impact of anaerobic biogeochemical processes on groundwate r chemistry.