Microbial Communities in High and Low Recharge Environments: Implications for Microbial Transport in the Vadose Zone |
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Authors: | DL Balkwill EM Murphy DM Fair DB Ringelberg DC White |
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Institution: | (1) Department of Biological Science, Florida State University, Tallahassee, FL 32306-3043, USA, US;(2) Environmental and Energy Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA, US;(3) Center for Environmental Biotechnology, University of Tennessee, Knoxville, TN 37932-2567, USA, US;(4) Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA, US |
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Abstract: | Abstract
Microbial communities along vertical transects in the unsaturated zone were evaluated at five sites in the Pasco Basin, in
southeastern Washington State. Sites with contrasting recharge rates were chosen to maximize or minimize the potential for
microbial transport. Pore water ages along the vertical transects were established using natural chloride tracers, and ranged
from modern to either ∼15,000 yBP (years before present) or ∼30,000 yBP at the two low-recharge sites. Unsaturated flow processes
were short-circuited by preferential flow at two of the three high-recharge sites, resulting in rapid movement of water through
the vertical transects. Microbial numbers and biomass, based on plate counts, and phospholipid fatty acid (PLFA) concentrations
decreased with depth at all sites. The majority (55–90%) of the culturable chemoheterotrophs recovered from most samples were
streptomycete bacteria. 16S rRNA gene sequence and MIDI analyses indicated that 75% of the remaining isolates were Gram-positive
bacteria (most likely species of Arthrobacter and Bacillus) 25% were Gram-negative bacteria (probably members of several genera in the alpha- and gamma-Proteobacteria). Comparison of microbial communities at low-recharge sites vs. high-recharge sites, where preferential flow occurs, revealed
several differences that might be attributed to vertical transport of microbial cells at the high-recharge sites. Plate counts
and PLFA analyses indicated that the proportion of streptomycetes, which were abundant at the surface but present in the subsurface
as spores, decreased, or remained constant, with depth at the low-recharge sites, but increased with depth at the high-recharge
sites. PLFA analyses also indicated that Gram-negative bacteria displayed increased nutrient stress with depth at the high-recharge
sites characterized by preferential flow, but not at the low recharge site. This may be a result of advective transport of
microbes to depths where it was difficult for them to compete effectively with the established community. Moreover, PLFA community
structure profiles fluctuated considerably with depth at the low-recharge sites, but not at the high-recharge sites. This
might be expected if transport were distributing the microbial community along the vertical profile at the high-recharge sites.
In contrast to the high-recharge sites at which preferential flow occurs, filtration likely prevented vertical transport of
microorganisms at the high-recharge site that was characterized by unsaturated flow.
Received: 6 November 1996; Accepted: 9 May 1997 |
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