Spatial and temporal constraints on the composition of microbial communities in subsurface boreholes of the Edgar Experimental Mine

Abstract

The deep biosphere hosts uniquely adapted microorganisms overcoming geochemical extremes at significant depths within the crust of the Earth. While numerous novel microbial members with unique physiological modifications remain to be identified, even greater attention is required to understand the near-subsurface and its continuity with surface systems. This raises key questions about networking of surface hydrology, geochemistry affecting near-subsurface microbial composition, and resiliency of subsurface ecosystems. Here, we apply molecular and geochemical approaches to determine temporal microbial composition and environmental conditions of filtered borehole fluid from the Edgar Experimental Mine ([~]150 meters below the surface) in Idaho Springs, CO. Samples were collected over a 4-year collection period from expandable packers deployed to accumulate fluid in previously drilled boreholes located centimeters to meters apart, revealing temporal evolution of borehole microbiology. Meteoric water feeding boreholes demonstrated variable recharge rates likely due to a complex and undefined fracture system within the host rock. 16S rRNA gene analysis determined unique microbial communities occupy the four boreholes examined. Two boreholes yielded sequences revealing the presence of Proteobacteria, Firmicutes, and Nanoarcheota associated with endemic subsurface communities. Two other boreholes presented sequences related to soil-originating microbiota, which likely indicate a direct link to surface infiltration. High concentrations of sulfate suggest sulfur-related metabolic strategies dominate within these near-subsurface boreholes. Overall, results indicate microbial community composition in the near-subsurface is highly dynamic at very fine spatial scales (<20cm) within fluid-rock equilibrated boreholes, which additionally supports the role of a relationship for surface geochemical processes infiltrating and influencing subsurface environments. ImportanceThe Edgar Experimental Mine, Idaho Springs, Colorado provides inexpensive and open access to borehole investigations for subsurface microbiology studies. Understanding how microbial processes in the near-subsurface are connected to surface hydrological influences like meteoric input is lacking. Investigating microbial communities of subsurface mine boreholes provides evidence of how geochemical processes are linked to biogeochemical processes within each borehole, and the geochemical connectedness and mobility of surface influences. This study details microbial community composition and fluid geochemistry over spatial and temporal scales from boreholes within the Edgar Mine. These findings are relevant to biogeochemistry of near-surface mines, caves and other voids across planetary terrestrial systems. In addition, this work can lead to understanding how microbial communities relating to both fluid-rock equilibration and geochemical influences may enhance our understanding of subsurface molecular biological tools that aid mining economic practices to reflect biological signals for lucrative veins in the near subsurface.