Microbial life beneath a high arctic glacier

Abstract

The debris-rich basal ice layers of a high Arctic glacier were shown to contain metabolically diverse microbes that could be cultured oligotrophically at low temperatures (0.3 to 4 degrees C). These organisms included aerobic chemoheterotrophs and anaerobic nitrate reducers, sulfate reducers, and methanogens. Colonies purified from subglacial samples at 4 degrees C appeared to be predominantly psychrophilic. Aerobic chemoheterotrophs were metabolically active in unfrozen basal sediments when they were cultured at 0.3 degrees C in the dark (to simulate nearly in situ conditions), producing (14)CO(2) from radiolabeled sodium acetate with minimal organic amendment (> or =38 microM C). In contrast, no activity was observed when samples were cultured at subfreezing temperatures (< or =-1.8 degrees C) for 66 days. Electron microscopy of thawed basal ice samples revealed various cell morphologies, including dividing cells. This suggests that the subglacial environment beneath a polythermal glacier provides a viable habitat for life and that microbes may be widespread where the basal ice is temperate and water is present at the base of the glacier and where organic carbon from glacially overridden soils is present. Our observations raise the possibility that in situ microbial production of CO(2) and CH(4) beneath ice masses (e.g., the Northern Hemisphere ice sheets) is an important factor in carbon cycling during glacial periods. Moreover, this terrestrial environment may provide a model for viable habitats for life on Mars, since similar conditions may exist or may have existed in the basal sediments beneath the Martian north polar ice cap.

FIG. 1

Anaerobic incubation of thawed ice samples at 4°C in the dark. Samples were amended with dilute nitrate medium (5 mM NO₃⁻) (A) or dilute sulfate medium (14 mM SO₄²⁻) (B). Symbols: ■, basal ice; □, glacier ice; ◊, uninoculated medium.

FIG. 2

Aerobic incubation of thawed ice samples in the dark. Samples collected in 1997 were supplemented with 98,000 dpm of sodium [2-¹⁴C]acetate and incubated at 8°C for 270 days with gyratory shaking at 200 rpm (■, basal ice plus dilute R2A medium; □, glacier ice plus dilute R2A medium; ◊, dilute R2A medium). Samples collected in 1998 were supplemented with 195,000 dpm of sodium [2-¹⁴C]acetate and incubated statically at 4°C for 157 days (●, basal ice without R2A medium; ○, glacier ice without R2A medium).

FIG. 3

Aerobic incubation of 1998 ice samples in the dark at increasing temperatures (−4.8, −1.8, and 0.3°C). All samples were supplemented with 195,000 dpm of sodium [2-¹⁴C]acetate. Symbols: ■, basal ice plus dilute R2A medium; □, glacier ice plus dilute R2A medium; ●, basal ice without R2A medium; ○, glacier ice without R2A medium; ⧫, basal ice sterile control; ◊, glacier ice sterile control. Incubation temperatures are also shown.

FIG. 4

TEM images of bacteria in meltwater from basal ice samples. Scale bars = 0.5 μm. (A) Coccus associated with sediment from uncultured basal ice immediately after thawing. (B and C) Long thin rod (B) and actively dividing cocci (C) from an aerobic stationary culture incubated at 0.3°C without R2A medium. (D) Short, fat rods with inclusions, associated with sediment from an aerobic culture incubated at 4°C without R2A medium.