Observations pertaining to the origin and ecology of microorganisms recovered from the deep subsurface of Taylorsville Basin, Virginia

To understand the conditions under which microorganisms exist in deep hydro carbon reservoirs, sidewall cores were collected from a natural gas-bearing formation, 2800 m below the surface in Taylorsville Basin, Virginia. Data from chemical and microbial tracers and controls indicate that the interior s of some sidewall cores contained microorganisms indigenous to the rock fo rmation. The cultured microorganisms were composed primarily of saline-tole rant, thermophilic fermenting, Fe(lll)-reducing, and sulfate-reducing bacte ria (I to 10(4) cells/g). The physiological capabilities of the cultured mi croorganisms are compatible with the temperature (76 degrees C), pressure ( 32 MPa), and salinity (approximate to 0.8 wt.% NaCl equivalent) in the samp led interval. The petrological data indicated that the strata contain inter crystalline pores of micrometer size, that occur between late diagenetic ce ment in siltstone and within cross-cutting, mineralized fractures in shale. These pores made up only 0.04% of the rock volume, were mostly gas-filled, and were interconnected by pore throats with diameters <0.04 mu m. Because the pore throats are smaller than known bacteria, the cultured microorgani sms were probably trapped within the larger pores containing alkaline, brac kish, formation water The total phospholipid fatty acid concentration of th e rock samples yielded a cellular concentration equivalent to 4 x 10(5) cel ls/g, much greater than had been determined by enumeration of the cultured bacteria. This may have resulted from either inhibition of dephosphorolatio n reactions within pores filled with reduced gases, such as methane, or the inability to culture >0.1% of the viable bacteria. The recovery of living bacteria from such an austere environment represents one of the most remark able examples of microbial survival yet reported.