The possibility that extreme life forms might exist in the lakes beneath the Antarctic ice sheet has fascinated scientists for decades. However, direct sampling of these lakes in the interior of Antarctica continues to present major technological challenges especially some of the them are covered with thick ice sheets. Recognising this, scientists from the British Antarctic Survey, and the Universities of Northumbria and Edinburgh have been searching around the retreating margins of the ice sheet for subglacial lakes that are becoming exposed for the first time since they were buried more than 100,000 years ago. This is because parts of the ice sheet are melting and retreating at unprecedented rates as the temperature rises at the poles due to global warming.
|Credit: British Antarctic Survey|
The researchers targeted Lake Hodgson on the Antarctic Peninsula which was covered by more than 400 m of ice at the end of the last Ice Age, but is now considered to be an emerging subglacial lake, with a thin layer of just 3-4 metres of ice. By using clean coring technologies they drilled through the ice in order to reach the sediments at the bottom of the lake. Samples were analysed with a variety of methods such as light microscopy, Fluorescence in Situ Hybridization (FISH), SEM, direct culture, and massive parallel sequencing.
The lake was thought to be a harsh environment for any form of life but the layers of mud at the bottom of the lake represent a time capsule storing the DNA of the microbes which have lived there throughout the millennia. The top few centimetres of the core contained current and recent organisms which inhabit the lake but once the core reached 3.2 m deep the microbes found most likely date back nearly 100,000 years. Some of the life discovered was in the form of fossil DNA showing that many different types of bacteria live there, including a range of extremophiles which are species adapted to the most extreme environments.
...the most numerous closest sequence matches were to both marine and soil derived organisms, many of which come from thermophilic environments. Although all were a relatively low fraction of the total, the two most abundant sequence matches were to marine bacteria that are found all over the world: Pirellula staleyi and Rhodopirellula baltica are globally distributed marine bacteria (the former a planctomycete which can also be found in terrestrial habitats). The second group of frequent matches were to the soil bacteria. Spirochaeta aurantia is an aerobe isolated from mud and Conexibacter woesei is a member of the Actinobacteria isolated from forest soil. The third group could be classified as an extremophile: Spirochaeta thermophila which is extremely thermophilic and marine, Thermobaculum terrenum from an extreme thermal soil and Leptospirillum ferrodiazotrophum was isolated from a subsurface acid mine drainage biofilm and is involved in iron oxidation. Further clues about the ecology of the environment could be derived from frequent sequence matches to Frankia—a genus of nitrogen fixing bacteria, Dehalococcoides ethenogenes—which is anaerobic and cannot use inorganic electron acceptors and Methylococcus capsulatus, a thermotolerant obligate methanotroph that is able to oxidize some organic hydrogen containing compounds...
However, only 77% of the DNA sequences identified could be matched to a known sequence, species or type strain, suggesting that a vast amount of biodiversity remains to be discovered. Many of the species are likely to be new to science making clean exploration of the remote lakes isolated under the deeper parts of the ice sheet even more pressing.