ANTARCTICA CONTAINS a wealth of paleontological information that is only now beginning to be collected. Some of this is hidden in rocks as fossils of plants and animals that lived there when the Antarctic continent was more temperate, but other information is found in the ice. Cores of Antarctic ice reveal climatic records for the globe in ages past. Now the Antarctic ice also yields preserved bacterial DNA hundreds of thousands to millions of years old.
The study, published open access in PNAS, amplified community DNA and cultured viable bacteria from melted glacial ice from the Transantarctic range. The amplified DNA showed degradation over time, and live bacteria were much more difficult to culture in the older samples. The authors calculated a half-life of 1.1 My for DNA degradation, which is consistent with previous calculations. The persistence of DNA in these samples causes the authors to hypothesize that ice may act as a reservoir for genetic diversity that is made available for lateral gene transfer when ancient ice melts.
The community DNA immobilized in Antarctic ice is essentially a “gene popsicle,” which can potentially be acquired by extant organisms upon thawing (27, 28). Given the widespread influence of lateral gene transfer (LGT) within microbial populations and its putative influence on the tempo of microbial evolution (29, 30), one can envision periods in Earth’s history when large numbers of ancient genes became available as ice sheets melted. Indeed, the tempo of evolution after major global glaciations appears to have increased dramatically (31), although causal mechanisms have been poorly defined. The vast majority of the ice on the Antarctic continent is <1 Ma; release of the microbes and DNA from such ice into the environment could potentially influence microbial genome content and structure. Although the ice volume for older formations is relatively small, the DNA in 8-Ma ice contains identifiable genetic information that potentially can be incorporated and used by microbes. Our analysis suggests that melting of polar ice in the geological past may have provided a conduit for large-scale phage-independent LGT, potentially scrambling microbial phylogenies and accelerating the tempo of microbial evolution. Finally, the preservation of microbes and their genes in icy comets may have allowed transfer of genetic material among planets. However, given the extremely high cosmic radiation flux in space, our results suggest it is highly unlikely that life on Earth could have been seeded by genetic material external to this solar system.
Bidle, K. D.; Lee, S.; Marchant, D. R.; Flakowski, P. G. “Fossil genes and microbes in the oldest ice on Earth.” Proceedings of the National Academy of Sciences, USA 2007, 104, 13455-13460.