Organisms on Glacier Surfaces May Function as Carbon Sinks

A new study shows that life processes of microbes living on the surface of glacier ice–organisms known as supraglacial microbes–may have an impact on the melting of glacial ice and on global greenhouse gas levels. It documents a previously unrecorded process by which these microbes produce compounds which retain carbon on the glacier surface, rather than releasing it into the atmosphere.

Forni and other glaciers in the Italian Alps (source: Viola Sonans)
Forni and other glaciers in the Italian Alps (source: Viola Sonans)

Since 2009, Dr. Andrea Franzetti, an environmental microbiologist at the University of Milan, and a team of Italian scientists have used DNA sequencing to determine the taxonomic characteristics of bacteria and algae from glaciers in several regions of the world, and to infer their metabolic processes.  Their latest work, Light-dependant Microbial Metabolisms Drive Carbon Fluxes on Glacier Surfaces, was published in The ISME Journal, a multidisciplinary journal of microbial ecology,  in April of 2016.

Dr. Franzetti and his colleagues who studied microbes dwelling on the surface of glacier ice hypothesize that the supraglacial microbes determine whether glaciers can on average absorb or release atmospheric carbon dioxide (CO2), a major greenhouse gas. Since roughly 10% of the Earth’s surface is covered by glaciers, ice sheets and sea ice, the cumulative impact of supraglacial microbes on global CO2 levels could have a significant effect on global climate.

The key issue whether the supraglacial microbes are predominantly CO2 consumers, like plants, or producers, like animals. The balance of these two types of microbes determines whether the world’s ice surfaces produce more CO2 than they absorb–or vice versa.

Although “it is still an open question,” said Dr. Franzetti during an interview with GlacierHub, he stated there is a trend that implies that marginal glaciers at the edge of ice sheets and mountain glaciers are dominated by CO2 producers and tend to act as carbon sources, while the interior regions of glaciers and ice sheets have mostly CO2 consumers and act as carbon sinks. The rates of production and absorption, multiplied by the areas where these activities are found, will determine the net effect of these organisms.

Schematic of psbD photosynthesizing gene, found in cryoconite algae (source: Curtis Neveu)
Schematic of psbD photosynthesizing gene, found in cryoconite algae (source: Curtis Neveu)

Their research in two sites–Forni in the Italian Alps of Italy and Baltoro in the Pakistani Karakoram–shows a number of biochemical processes that contribute to the production of organic molecules, removing carbon from the atmosphere. In particular, they find that organisms can process the carbon monoxide (CO) that is formed as sunlight breaks down organic matter in cryoconite (a mixture of dark sediment and microbes found on ice surfaces), turning it into compounds that remain on the glacier surface. Their sequencing techniques have documented the presence of a number of genes that support photosynthesis. The discovery of this carbon sink is a key contribution of their research. 

Since the type of microbes found on glaciers are predominantly the same as those found on ice sheets, Dr. Franzetti hypothesizes the most common metabolism is determined by the area of the ice and the availability of nutrients.  Mountain glaciers and marginal glaciers have a more confined surface area and tend to have more organic compounds from windblown sediment and upstream melt water.  Thus, marginal and mountain glaciers can support a greater number of CO2 producers than other areas of the glacier or ice sheet.

In addition to influencing the trapping or releasing of atmospheric CO2, microbial activity may lead to the darkening of the glacial surface and the reduction of the glacier’s albedo, or solar reflectivity, which leads to increased melting.  Glacial darkening can originate from various microbial activities.  These include the natural pigmentation, or color, of algae on bare surface ice, and the buildup of cryoconite.

Many supraglacial microbes produce an adhesive substance that trap sediment carried by wind and melt water.  Over time, the fine sediment and microbes coalesce into larger cryoconite granules, which are more resistant to displacement.  The dark cryoconite is able to absorb more heat from solar radiation than bare ice and causes more melting around the granule.  This process commonly creates what is known as cryoconite holes.  As the ice around the granules continues to melt, an impression is made in the ice surface, which allows for a greater accumulation of sediment in the growing hole and further melting. 

Air bubbles found in ice within cryoconite holes (source: Alean, Hambrey/swisseduc)
Air bubbles found in ice within cryoconite holes in the Italian Alps (source: Alean, Hambrey/swisseduc)

The accumulation of cryoconite is not the only way microbial activity can lead to the darkening of glacier ice. Past studies of supraglacial microbes found several species of algae that exist on bare ice, outside of the cryoconite deposits.  In order to combat the often lethal amounts of solar radiation that the algae are exposed to on bare ice, they release dark colored pigments.  Not only does this dark pigment allow the algae to withstand the high level of solar radiation, it also promotes surface melting.  A larger amount of melt water increases the available habitat of the algae and can lead to greater glacial darkening and melting. 

Although several studies show microbial activity does lead to glacial darkening and melting, Dr. Franzetti stated, “[The] assessment of the relative contribution [to darkening the glacier] of biological processes, chemical processes [normal sediments and geologic processes] and anthropogenic processes is controversial.”  It is unclear how much glacial melt is actually attributed to the microbial darkening.

Glacial ice is a major component of global climate.  As studies on supraglacial microbes continue to reach publication, it is becoming apparent that bacterial and algae activity has an influence well beyond the surface of the ice.

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