Monday, June 16, 2014

Microbes Shoulder the Blame for Melting Glaciers

No one ever really thinks of glaciers as a really happenin' place; in fact, the recognition of glaciers (and other portions of the cryosphere) as a biome emerged only recently.  Although glacial surfaces are lower in cell number than more temperate environments, the microbes found on glaciers have a significant impact on the rate of glacier melting.
Illustration only.

Productivity among glacier-inhabiting microbes is limited, often because of light and nutrient availability.  But algae, which are equipped with pigments (like chlorophyll) allowing them to photosynthesize, are able to thrive in frozen, apparently barren environments.  Ice algae differ from snow algae, with a less complex life cycle as immotile cells that rarely form spores.  Snow algae may be green or red, while ice algae has a brownish pigment, making the ice look gray-brown.  Meanwhile, water-filled holes in glaciers tend to be dominated by cyanobacteria.  Most studies to-date have looked at the microbiome in individual cryosphere habitats.

A Ph.D. candidate out of the University of Leeds spent three weeks studying the Mittivakkat glacier, in the southeast of Greenland, during the high-melt season in July 2012.  Her work integrated the three types of glacier environments discussed above, and the effects of the various microbes on the physical characteristics of the Mittivakkat glacier.

What the researchers found was that over the course of the three week study, as ice and snow began melting under the higher-than-average temperatures, algae began creeping up the glacier:

When the ice started melting, a number of small water pools began to form, and became rich in a brownish biofilm.  This biofilm was characterized by high microbial content and photosynthetic activity, and also by an accumulation of cryconite - dust, black carbon, soot, and microbes.  Cryconite is composed of residues from natural occurrences (fires and volcanic eruptions) and from human activities (burning of fossil fuels).  Normally, snow is able to reflect most sunlight, but the accumulation of black cryconite on snow and ice absorbs sunlight and speeds up melting.  At the end of three weeks, the researchers observed that 70-90% of the glacier was snow-free, and was dominated by a grey-looking ice interspersed with cryconite holes (as shown in the diagram above).

The level of light being reflected (technical term: albedo) changed from one habitat to another, with clean white snow reflecting 75% of light, red snow (covered by red algae) reflecting 49% of light, and biofilm reflecting 20% of light, similar to the cryconite holes.  This is attributed to the pigmentation and mineral content of each habitat.  Like the melanin in your skin, pigments like chlorophyll and carotenoids also absorb light.  The increasing levels of pigments in each habitat of the glacier, along with cryconite, contributes to faster warming and melting of the glacier.

Dr. Liane Benning, the principal investigator of the study, proposes that future climate change scenarios need to take the microbiome into consideration, as well as cryconite, when predicting cryosphere melting.

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