Fossil genes and microbes in the oldest ice on Earth

Abstract

Although the vast majority of ice that formed on the Antarctic continent over the past 34 million years has been lost to the oceans, pockets of ancient ice persist in the Dry Valleys of the Transantarctic Mountains. Here we report on the potential metabolic activity of microbes and the state of community DNA in ice derived from Mullins and upper Beacon Valleys. The minimum age of the former is 100 ka, whereas that of the latter is Ϸ8 Ma, making it the oldest known ice on Earth. In both samples, radiolabeled substrates were incorporated into macromolecules, and microbes grew in nutrient-enriched meltwaters, but metabolic activity and cell viability were critically compromised with age. Although a 16S rDNA-based community reconstruction suggested relatively low bacterial sequence diversity in both ice samples, metagenomic analyses of community DNA revealed many diverse orthologs to extant metabolic genes. Analyses of five ice samples, spanning the last 8 million years in this region, demonstrated an exponential decline in the average community DNA size with a half-life of Ϸ1.1 million years, thereby constraining the geological preservation of microbes in icy environments and the possible exchange of genetic material to the oceans.ancient ice ͉ community DNA ͉ metabolism ͉ metagenomic analysis ͉ cosmic radiation A ntarctica offers unique environments for understanding the limits of biological metabolism and geological preservation of life and genetic material. Analyses of microorganisms in subglacial lakes, including Lake Vostok (1) and Lake Bonney (2), as well as the Taylor Dome region of the Transantarctic Mountains (3), have revealed the potential viability of microorganisms preserved in ice up to Ϸ300 ka. In the Dry Valleys of the Transantarctic Mountains, however, patches of much older ice persist, yet little is known about the viability of microbes or the state of genetic material in these regions.Here we report on the microbial activity and state of the community DNA in samples from a debris-covered alpine glacier that heads in Mullins Valley (sample no. DLE-98-12) and terminates along a diffuse boundary in Beacon Valley (sample no. EME-98-03) ( Fig. 1 A and B). Synthetic aperture-radar interferometry analysis indicated that ice within Mullins Valley ranges from modern at the valley head to Ϸ300 ka near the tributary mouth, whereas buried ice on the floor of upper and central Beacon Valley could be as much as 10 Ma (4). DLE-98-12, which is assigned an age of Ϸ100 ka, lies halfway down Mullins Valley, whereas EME-98-03 lies in the zone estimated to be up to 8 Ma (Fig. 1 A). These ice chronologies are supported by 3 He and 21 Ne cosmogenic dating of surface boulders (5) and laser-fusion 40 Ar/ 39 Ar radiometric dating of surface ash-fall deposits (6, 7) [supporting information (SI) Text]. Analyses of the ice crystal structure and stable-isotope composition (␦ 18 O, ␦D) indicate that the ice samples have remained frozen since transformation from snow to glacier ice (e.g., refs. 6 and 7). There were also no dete...

The Interplanetary Exchange of Photosynthesis

“…Nevertheless, the models reported to date suggest the interstellar transfer of oxygenic photosynthesis, let alone any organisms, to be a very low probability event. In addition, the integrated exposure to cosmic radiation over such long durations of time will likely be sterilizing, at least extrapolating evidence on the viability of terrestrial microorganisms preserved in ice (Bidle et al 2007). …”

Phylogenetic Diversity and Metabolic Potential Revealed in a Glacier Ice Metagenome

“…Hodson et al (22) reported the presence of inorganic and organic forms of nitrogen and phosphorus for two glaciers in Arctic Svalbard. These authors proposed the occurrence of NH 4 ϩ assimilation and nitrification on the glacier surface and denitrification and sulfate reduction in subglacial environments. In the present study, no evidence for nitrification or respiratory sulfate reduction was detected, but dissimilatory and assimilatory nitrate/ nitrite reduction was found.…”

“…To date, a pyrosequencing-based metagenomic analysis of a permanently frozen habitat has not been conducted. In several studies, the prokaryotic diversity of glacial and subglacial habitats in America (9,44), Asia (10,55), Antarctica (4,37), Greenland (32,42), and New Zealand (19) has been analyzed based on cultivation and analysis of 16S rRNA genes. However, studies of the microbial composition of European glaciers are rare.…”

DNA Double-Strand Break Repair at −15°C

“…Genomic DNA and viable bacteria are preserved in ancient ice and permafrost for hundreds of thousands to millions of years (4)(5)(6)(7)(8). The presence of a solute-rich liquid phase within the ice matrix has been argued to be a habitat suitable for microorganisms (9)(10)(11).…”