The effects of microbial life on glaciers and ice sheets
Around 10% of the Earth’s surface is covered by glacial ice (glaciers and ice sheets), some of which melt as part of a natural cycle for a few months each year. Although the physical aspect of glacier and ice sheet melting at both polar regions has been extensively studied, biological processes and their role in the physical behaviour of glaciers and the global carbon cycle have received less attention up until now.
Reporting this week in the journal Nature Geoscience, an international team of scientists review our current understanding of biological processes on glacier surfaces and how these affect the physical and chemical properties of glaciers.
Globally, around 5% of glacier surfaces melt as part of a natural cycle for a few months each year. The surface melt water harbours microbial life – such as cyanobacteria and algae – carried there by the wind. These tiny microbes become active during the melting season and concentrate in patches on the ice. As part of their life cycle, the micro-organisms produce dark pigments which subsequently decrease the reflectivity of the ice surface promoting the rate of melting.
The photosynthetic micro-organisms on the surface of glaciers act as a carbon sink by capturing carbon dioxide from the atmosphere and converting this into organic carbon. However, the carbon is then processed by other microbial communities on the ice and released as carbon dioxide back into the atmosphere, thus acting as a carbon dioxide source. Therefore it is unclear whether glacier ecosystems act as a global carbon sink or source.
It is thought however that the more stable parts of Arctic and Antarctic glaciers and ice sheets are likely to act as a sink for carbon dioxide, while small glaciers, mountain glaciers, and the edges of ice sheets, which are more likely to melt quickly, release more carbon into the atmosphere than they fix.
As glaciers melt in response to climate change, carbon not consumed by the microbes on the ice is exported by streams and rivers, which may increase productivity of downstream soil, lake communities and coastal waters. Besides micro-organisms and nutrients, the melt water may also carry toxins and harmful substances that were temporarily stored within the glacier ice.
Biological processes on glacier surfaces clearly play an important role in the physical behaviour of glaciers, particularly in the melting of the ice. Therefore, it is important to include these when new global carbon budgets and climate models are created.
Most data compiled for this study came from field observations from relatively easily accessible sites such as mountain glaciers, Svalbard and Greenland due to the inaccessibility of many parts of the Southern Hemisphere.
The review paper – Biological processes on glacier and ice sheet surfaces – by Marek Stibal of the Geological Survey of Denmark and Greenland; Marie Šabacká of British Antarctic Survey; and Jakub Žárský of Institute of Ecology at University of Innsbruck is published in Nature Geoscience (online) on 28 October 2012.
Glacier – a ‘river of ice’ fed by the accumulation of snow. Glaciers drain ice from mountains to lower levels, where the ice either melts, breaks away into the sea as icebergs, or feeds into an ice shelf.
Ice sheet – the huge mass of ice, up to 4 km thick that covers bedrock in Antarctica or Greenland. It flows from the centre of the continent towards the coast where it feeds ice shelves.