Microbial origin of excess methane in glacial ice and implications for life on Mars

Abstract

Methane trapped in the 3,053-m-deep Greenland Ice Sheet Project 2 ice core provides an important record of millennial-scale climate change over the last 110,000 yr. However, at several depths in the lowest 90 m of the ice core, the methane concentration is up to an order of magnitude higher than at other depths. At those depths we have discovered methanogenic archaea, the in situ metabolism of which accounts for the excess methane. The total concentration of all types of microbes we measured with direct counts of Syto-23-stained cells tracks the excess of methanogens that we identified by their F420 autofluorescence and provides independent evidence for anomalous layers. The metabolic rate we estimated for microbes at those depths is consistent with the Arrhenius relation for rates found earlier for microbes imprisoned in rock, sediment, and ice. It is roughly the same as the rate of spontaneous macromolecular damage inferred from laboratory data, suggesting that microbes imprisoned in ice expend metabolic energy mainly to repair damage to DNA and amino acids rather than to grow. Equating the loss rate of methane recently discovered in the Martian atmosphere to the production rate by possible methanogens, we estimate that a possible Martian habitat would be at a temperature of Ϸ0°C and that the concentration, if uniformly distributed in a 10-m-thick layer, would be Ϸ1 cell per ml.metabolism by methanogenic archaea ͉ methane in glacial ice ͉ methanogens on Mars ͉ origin of microbes in glacial ice T he record of atmospheric methane (CH 4 ) concentration trapped in the Greenland Ice Sheet Project 2 (GISP2) ice core serves as a climate proxy, showing that climate during the last glacial period oscillated rapidly between cold and warm states that lasted for several thousand years (1). The source is believed to be wetland methane emissions that depend on temperature, precipitation, net ecosystem production, and oxidation by tropospheric OH. Because of its short atmospheric mixing time relative to its lifetime, variations of methane recorded in ice cores are believed to reflect global changes in the methane budget. Fig. 1 shows measurements of methane in the GISP2 ice core as a function of depth by Ed Brook [National Oceanic and Atmospheric Administration Geophysical Data Center (www. ngdc.noaa.gov͞paleo͞paleo.html); and E. Brook, additional data for depths of 2,806-3,038 m, personal communication] down to 3,038 m, just 3 m above the silt-laden basal ice. All but four of his methane values range between 337 and 880 parts per billion by volume (ppbV) and correlate with other climate proxies such as ␦ 18 O and CO 2 (1). The intervals between his samples were Ͼ10 m at depths Ͻ2,000 m and 2-6 m at depths of Ϸ2,000-3,038 m. Our study focuses on the origin of the four values marked with arrows in Fig. 1, which stand out above the 99% that are related to climate. We report here our discovery that the excess methane values are produced by methanogens (microbes that metabolize with emission of methane) that were metabolizing wh...

Subglacial methanogenesis: A potential climatic amplifier?

“…These viable microbes include methanogens, found in basal ice from Greenland (via GISP2 [Tung et al, 2005]) and John Evans Glacier, Canada [Skidmore et al, 2000]. In the latter case, incubation experiments confirm their ability to release considerable concentrations of methane from subglacial debris.…”

“…We do not consider either of these values to be truly representative of methane produced from SOC during ice advance over North America and Europe. GRIP values partially or wholly reflect methane production within the vein system of glacial ice [Tung et al, 2005[Tung et al, , 2006 where closed system conditions will quickly lead to SOC substrate exhaustion. The methane in glacial till is also produced in an environment where labile SOC has been exhausted (by degradation to carbon dioxide and methane during 75 ka of glaciation).…”

Diffusion-controlled metabolism for long-term survival of single isolated microorganisms trapped within ice crystals

“…Fig. 3 (Left) shows TUCS intensity at 300-m depth intervals at a depth of 2,953.9 m in a 1-m-long GISP2 ice core chosen for calibration purposes because Tung et al (50) had found a huge excess of microbial cells including methanogens in a 20-ml sample from that depth. The full spectra for every point are stored on disk.…”

“…With the TUCS we did ground truth calibrations at depths 2,953.6, 2,953.95, 3,000, 3,018, and 3,035.88 m in GISP2 by comparing fluorescence intensity, I TUCS , with counts of stained cells (50). We obtained the following conversion factors: Assuming an average cell mass of 38 fg (5), the background level for the TUCS fluorescence signal corresponds to Ϸ0.5 to Ϸ1.4 cell; the number of cells detected in the active cylindrical volume of depth 0.5 cm is 243 ϫ I TUCS ; and the concentration is C cells ϭ 1.52 ϫ 10 6 cm Ϫ3 ϫ I TUCS .…”

“…It uses a 404-nm laser to excite F420 fluorescence in a volume 1.9 ϫ 10 Ϫ3 cm 3 . To calibrate this instrument, we did scans along the same regions of GISP2 ice cores with very high microbial concentrations where Tung et al (50) counted methanogens with epifluorescence microscopy. We found that the background level corresponds to 0.15 to 0.3 methanogenic cell of mass 38 fg.…”