| Popis: |
Glacial ice currently occupies roughly 11% of Earths surface and contains approximately 70% of the planets freshwater. Once thought to be inhospitable due to the physiochemical challenges presented by freezing temperatures, the basal zones of glaciers and ice sheets have recently been identified as a potential habitat for psychrophilic microorganisms with the ability to mediate biogeochemical cycles on a global scale. Basal ice is found in the deepest layers of a glacier and has distinct chemical and physical characteristics as a result of its proximity to the glacier bed. Basal ice is generally the warmest ice found in a glacier and often contains entrained debris and sediment from the underlying subglacial substrate which may provide nutrients and redox couples for microorganisms immured in the basal ice matrix. ATP/ADP concentrations and ratios, enrichment culturing, 16S rRNA surveys, and cell counts were combined with nutrient, major ion, and gas chemistry analyses to evaluate the microbial assemblages immured in both sediment-rich and sediment-poor basal ice environments. The primary material for this study was a 4 m profile of basal ice collected from Taylor Glacier (Antarctica) but also included basal ice samples from the Matanuska Glacier (Alaska) and Støre Landgletscher Glacier (Greenland). Microbial community abundance varied significantly between the different basal ice samples and was correlated with the presence of sediment in the ice. Sediment-rich banded basal ice from Taylor Glacier contained elevated concentrations of CO2 (60,000 to 325,000 ppmv) occurring simultaneously with decreased O2 concentrations (4 to 18% of total gas volume) suggesting the resident microbial assemblages may be respiring in situ and modifying the gas composition of the basal ice. Molecular surveys of 16S rDNA and rRNA sequences revealed species of the genus Paenisporosarcina to be numerically abundant and active members of the microbial assemblages inhabiting these same basal ice horizons. Members of this genus were readily culturable from the basal ice samples and radiolabeled [3H]-leucine and [3H]-thymidine assays with these isolates revealed their ability to conduct macromolecular synthesis while frozen in basal ice melt-water at -15°C. These results support the hypothesis that basal ice environments are microbial habitats harboring bacteria with the physiological capacity to remain metabolically active and cycle elements within the cryosphere. |
