A COMPARISON OF MICROBIAL COMMUNITY CHARACTERISTICS AMONG PETROLEUM-CONTAMINATED AND UNCONTAMINATED SUBSURFACE SOIL SAMPLES

Measurements of microbial community size, including total cell counts and specific degrader enumerations, were conducted on subsurface soil samples from both petroleum-contaminated and pristine aquifers. Sample s were collected from both uncontaminated and contaminated areas of th e petroleum-contaminated sites. In pristine and uncontaminated samples , total cell counts (acridine orange direct counts) were related to de pth. The deeper samples contained smaller total microbial populations. However, indices of microbial activity varied considerably from sampl e to sample and probably reflect soil and site heterogeneity. Exposure to petroleum contamination apparently altered the microbial community structure. In samples exposed to low levels of contaminants as vapors and/or dissolved phases (ppb concentrations), and not free product, t he toluene-specific degrader populations were larger at greater depths , and the numbers of amino acid-specific degraders were highly correla ted to the numbers of decane-specific degraders, indicating that petro leum-adapted microbial communities were present in the contaminated sa mples. In highly contaminated samples, total microbial population dens ities decreased with increasing depth; however, microbial activity ten ded to increase with depth. These results indicate that petroleum cont aminants exert toxic effects on the active microbial community at high exposures and enrich specific degraders at ppb levels of dissolved co ntaminants.