PNNL

Abstract

This study was aimed at understanding how the population structure of bacteria within terrestrial deep-subsurface environments correlates with the physical and chemical structure of their environment. Phylogenetic analysis was performed on strains of Arthrobacter collected from various depths. This included a number of different sedimentary units from the Yakima Barricade borehole at the U.S. Department of Energy's Hanford site, Washington, in August 1992. At the same time bacteria were isolated, detailed information on the physical, chemical, and microbiological characteristics of the sediments was collected. Phylogenetic trees were prepared from the 39 deep-subsurface Arthrobacter isolates (as well as 17 related type strains) based on 16S rRNA and recA gene sequences. Analyses based on each gene independently were in general agreement. They showed that, for all but one of the strata (sedimentary layers characterized by their own unifying lithologic composition), the deep-subsurface isolates from the same stratum are largely monophyletic. Notably, the layers where this is true were composed of impermeable sediments. This suggests populations within each of these strata have remained isolated under constant, uniform conditions, selected for a particular dominant genotype in each stratum. Conversely, few strains isolated from a gravel-rich layer appeared along several lineages. This suggests that the higher-permeability gravel decreases the degree of isolation of this population (through greater groundwater flow), creating fluctuations in environmental conditions or allowing migration, such that a dominant population has not been established. No correlation was seen between the relationship of the strains and any particular chemical or physical characteristics of the sediments. This suggests that within sedimentary deep-subsurface environments, permeability of the deposits plays a major role in determining the genetic structure of resident bacterial populations.