POST-SAMPLING CHANGES IN MICROBIAL COMMUNITY COMPOSITION AND ACTIVITYIN A SUBSURFACE PALEOSOL

Laboratory storage of deep vadose zone sediments has previously result ed in an increase in the abundance of cultured microorganisms by as mu ch as 10,000-fold, without concomitant increases in total microscopic counts. In the present study, factors contributing to the time-depende nt stimulation of various microbiological parameters were examined dur ing a 224 d post-sampling period, using a factorial-design experiment that partitioned the effects of storage time, sediment condition (inta ct blocks or homogenized) during storage, and O-2 concentration (0.5, 4.5, and 21%) during storage at 15 degrees C. Stored samples were anal yzed at selected intervals, to determine direct microscopic counts, vi able biomass, lipid biomarker profiles, cultured aerobic heterotrophic microorganisms, and microbial activity. Time of storage prior to anal ysis of the samples was the most important factor affecting the microb iological response. Sediment condition influenced the stimulation resp onse: microbial activity and the population of cultured microorganisms increased faster, and reached slightly higher values, in the homogeni zed samples, although maximum values were reached at similar times in the homogenized and intact samples. O-2 concentration also influenced the response, but was the least important of the factors evaluated. To tal cells and viable biomass, measured as total phospholipid fatty aci ds, changed little during storage. Maximum cultured populations and ac tivity were attained at 63 to 112 d, with culture counts approximating the total numbers of microscopically counted cells. At approximately the same time, unbalanced growth (evidenced by high ratios of polyhydr oxybutyrate to phospholipid fatty acid) indicated that inorganic nutri ents became limiting. Lipid biomarkers indicative of Gram-positive bac teria, including actinomycetes, became dominant components of the comm unity profiles in samples maintained at 0.5% and 4.5% O-2. The shift i n the microbial community from relatively inactive, predominantly uncu ltured microorganisms to metabolically active populations that were ne arly all cultured highlights the need for rapid initiation of analyses after sample acquisition, ii measurement of in situ microbiological p roperties is desired. The fact that these processes also occur in inta ct sediment blocks suggests that minor perturbations in the chemical o r physical properties of subsurface sediments can result in major chan ges in the activity and composition of the microbial community.