When glaciers disappear, the bugs move in
Earth 27 August 2008
By Catherine Brahic
Seemingly barren soil can be seen at the foot of the Puca glacier, which is retreating by 20 metres per year
(Image: Sasha C Reed)
We've all been stunned by images showing the dramatic retreat of mountain glaciers. Yet few of us have given much thought to what happens next.
Now the first study to look at how life invades soil immediately after mountain glaciers melt has an answer. Primitive bacteria step in to colonise the area, enrich the soil with nutrients, and even cement the ground, preventing landslides, say researchers who have studied the process in the Peruvian Andes.
A few studies have looked at the types of plants that colonise mountain valleys that were previously covered in ice. But before plants move in there is usually a period, which at high latitudes and altitudes can last several years, during which the newly uncovered soil appears totally barren (see picture, right).
To investigate what is happening during this period, Steve Schmidt of the University of Colorado and colleagues examined the soil at the retreating edge of the Puca glacier in the Peruvian Andes. Between 2000 and 2005, they sampled the top 10 centimetres of ground that was revealed as the glacier moved uphill at a rate of 20 metres per year.
Nutrient boost
They analysed the chemical structure of the samples and screened for bacteria. They found that over the years, the "oldest" soil — the dirt taken from the point that was revealed at the glacier edge in 2000 — changed rapidly.
The first organisms to appear in the soil were cyanobacteria. These primitive bacteria are found in many marine ecosystems and some land-based ecosystems. It is these bacteria we have to thank for pumping oxygen into Earth's atmosphere 3.4 billion years ago, allowing land life to evolve.
By running DNA analyses on the soil, Schmidt and his colleagues show how the bacteria population changed over the first five years. Whereas "young" soil contained just three distinct genetic strains of cyanobacteria, four-year-old soil harboured up to 20.
The cyanobacteria increased the amount of carbon available in the soil, through photosynthesis and, along with other types of bacteria, they also boosted nitrogen levels in the soil, an essential nutrient for plant life.
Ground glue
Another, perhaps more surprising, function of the cyanobacteria seems to be to hold the ground together.
Previous studies have shown that they secrete sugary chemicals that help hold the soil particles together and prevent erosion. At Puca glacier, the researchers found that soil shear strength was nearly double in the oldest soil relative to the youngest.
"An important role of cyanobacteria in extreme environments may be to hold the soil in place," say Schmidt and colleagues.
Previously, it had been suggested that newly uncovered soil might draw its life from nutrients and bacteria deposited by wind, or from ancient carbon "pools" that were trapped beneath the glacier. Instead, the team say that though ancient carbon may help fuel the very early stages of new life, the cyanobacteria and nitrogen-fixing bacteria rapidly take the leading role.
Journal reference: Proceedings of the Royal Society B: Biological Sciences (DOI: 10.1098/rspb.2008.0808)
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