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Cotton candy ice spun by fungus

Microscopic and chemical analyses of the elusive and mysterious hair ice have finally revealed its secret: fungus.

Hair ice in a forest near Moosseedorf, Switzerland. Christian Mätzler

In frozen climes, the formation of frost flowers, created by ice extruding from plants, is a fascinating and beautiful thing. One of the most difficult kinds to find is what is known as "hair ice." In very specific conditions, it grows in long, thin threads like hair or candy floss from the rotten branches of certain trees.

It was first described almost 100 years ago in 1918 by geophysicist and meteorologist Alfred Wegener (who is famous for originating the theory of continental drift in 1912). At that time, he proposed that the unusual ice formation was probably due to fungus growing on the damp and decomposing wood.

As it turns out, he was spot on. A team of researchers from Switzerland and Germany have analysed the ice and determined that its peculiar shape is formed by fungus -- in particular, a type of fungus called Exidiopsis effusa. But it wasn't easy: hair ice grows mainly in broadleaf forests between the latitudes of 45 and 55 degrees north. It also mainly only grows at night, melting when the sun rises, and is all but invisible in the snow.

The research was led by Christian Mätzler from the Institute of Applied Physics at the University of Bern in Switzerland, who in 2005 worked with Uppsala University microbiologist Gerhart Wagner to explore the fungus theory further.

"When we saw hair ice for the first time on a forest walk, we were surprised by its beauty," Mätzler said in a statement. "Sparked by curiosity, we started investigating this phenomenon, at first using simple tests, such as letting hair ice melt in our hands until it melted completely."

They treated the wood with fungicide, or dunked it in hot water, noting that treating it inhibited the formation of the ice. This confirmed that fungus was causing the ice to form -- but the species of fungus and why it happens remains unknown.

After working with Wagner, Mätzler joined forces with chemist Diana Hofman of the Institute of Bio- and Geosciences in Jülich, Germany, and biologist Gisela Preuss of Wiedtal Gymnasium in Neustadt, Germany to figure out these missing details.

hairice2.jpg
What a cool toupee! Gisela Preuss

The first experiments were conducted by Preuss, who studied samples of the wood under a microscope. She identified 11 different species of fungus altogether -- but only one could be found on every single sample.

"One of them, Exidiopsis effusa, colonised all of our hair-ice-producing wood, and in more than half of the samples, it was the only species present," Preuss said.

Mätzler's role in the research was to study how the ice was formed. He discovered that it is formed by a process known as ice segregation (when supercooled water caught in the pores of a material, draws toward already formed ice and freezes, adding to and growing the frozen structure). In the case of hair ice, it's a little bit like material extruding through the nozzle of a 3D printer: The shape of the ice is determined by tiny pores in the surface of the wood. But this only occurs on wood where the fungus is present.

"The same amount of ice is produced on wood with or without fungal activity, but without this activity the ice forms a crust-like structure," Mätzler said. "The action of the fungus is to enable the ice to form thin hairs -- with a diameter of about 0.01 mm -- and to keep this shape over many hours at temperatures close to 0°C. Our hypothesis includes that the hairs are stabilised by a recrystallisation inhibitor that is provided by the fungus."

Chemical analyses conducted by Hoffman on the melted ice confirmed the presence of lignin and tannin -- metabolic products of fungal activity. Hofmann said that "These components may be the ones preventing the formation of large ice crystals at the wood surface."

Further work is required to find out what component in the fungus is causing the development of such small ice crystals. Meanwhile, you can read the results of the team's work, published in the journal Biogeosciences.

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