Bacterial Growth Dynamics, Limiting Factors, and Community Diversity in a Proposed Geological Nuclear Waste Repository Environment

Abstract

Microbiological growth parameters, including limiting factors, kinetics, and minimal cell densities were assessed for subsurface microbiological communities collected with rock from an area proposed for a nuclear waste repository at Yucca Mountain, Nevada. Phospholipid fatty acid analysis revealed that approximately 104–105 viable cells per gram of dry rock are extant, and water availability was shown to be the primary factor limiting microbial growth in situ. Phosphate and carbon limitation, however, also suppress final cell densities by at least one order of magnitude under saturated conditions. Despite these limiting factors, significant growth of aerobic chemoheterotrophic microorganisms was shown to occur in unconcentrated simulated groundwater with or without addition of a reduced carbon source (7 × 107 and 8 × 106 planktonic cells/mL, respectively), indicating that when water becomes available in the repository environment, microbial growth will ensue. Organisms that were isolated from stationary cultures grown from Yucca Mountain rock in concentrated and unconcentrated simulated groundwaters showed significant 16S rDNA sequence divergence from reference organisms. Different (but related) organisms were isolated from concentrated and unconcentrated groundwater-grown cultures. Generally, as experimental conditions approached those expected to be encountered in situ, the organisms isolated were more divergent from reference organisms. Organisms that were isolated have metabolic properties that could allow them to be active and grow within the repository environment if water availability is sufficient.