Glacial Ice as Potential Ecological Niches for Xerotolerant Halotolerant Fungi

Due to the adaptive fungal behaviour at low aw, we have assumed that coastal Arctic environments, in particular diverse type of ice, could represent a potential ecological habitat for xerotolerant/ halotolerant fungi. High concentrations of NaCl create both ionic and osmotic stress, while high concentrations of sugars and drought cause osmotic stress. There is great similarity between osmotic stress and matric water stress since in both cases the water activity is low. As water becomes ice it is not biologically available, and diverse types of ice can therefore be characterized as well as low aw environments. Additionally, freezing leads to cellular dehydration due to reduced water absorption, while high salinity causes the same effects due to osmotic imbalances.

Atmospheric circulation over polar regions provides air-mass exchange with lower latitudes. As a result, microorganisms from air-borne terrestrial dust may become embedded in ice formed from snow. Glacial microbial diversity is thus represented by taxa that are probably endemic to the polar regions as well as exotic species from temperate and tropical regions, which can originate from ocean mist, wind-borne pollen and soil particles, infected plant surfaces, and many other sources. They may have been transported and deposited by the action of waves, wind, rain, snow, animals, or by other means [Abyzov, 1993; Ma et al., 1999, 2000]. Glacial ice thus provides a unique global source of microorganisms, enabling the study of both contemporary and ancient microbial diversity. Viable microorganisms, randomly entrapped in ice even for thousands of years, are destined to be released during glacial melts or after the calving of icebergs into the ocean [Ma et al., 1999, 2000].

Glacial ice is known as an extremely stable, frigid and static environment. However, recent investigations have shown that glaciers are much more dynamic than previously assumed on the micro scale as well as on the geomorphological level. Ice in temperate glaciers is permeated by a continuous network of aqueous veins, formed at the linear junctions of three ice crystals. They are formed due to sea salts deposited as aerosols, that are essentially insoluble in ice crystals. These liquid veins, with diameters range from ~1 pm at -50 °C to ~10 pm at -4 °C [Rohde and Price, 2007], can have high ionic strength. Due to the percolation of salts from the top of the glacier to its bottom, salts can be accumulated to relatively high concentrations in the bottom parts of polythermal glaciers [Price, 2000]. Besides, due to quick seismic shifts [Ekstrom et al., 2003; Fahnestock, 2003] and cryokarst phenomena in connection with massive surface ablations, liquid water can temporarily appear as ponds or streamlets on the surface of the glacier and as caves or interglacial lakes, artesian fountains and moulins within the glaciers [Christner et al., 2000]. These supraglacial waters can also reach the glacier bed and mix with groundwater and basal meltwater generated by frictional and geothermal melting of ice at the glacier base. These liquid waters interact with rocks and sediments, and hence contain high solute and suspended sediment concentrations. When frozen onto the basal glacier ice, they can be transported to the glacier margins, where subglacial ice can be aseptically sampled.

These processes create subglacial environments, until recently considered abiotic. However, recent studies have revealed prokaryotic microbial communities dominated by aerobic heterotrophic Betaproteobacteria [Foght et al., 2004]. They have been mainly associated with sediment particles [Foght et al., 2004; Gaidos et al., 2004; Skidmore et al., 2005; Siegert et al., 2001; Skidmore et al., 2000], since thin films of liquid water around embedded mineral grains act as potential microniches [Rohde and Price, 2007]. In all these cases, there were no reports on the presence of fungi.

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