Fungi in Polar Regions

Cold polar regions are extreme environments, in which the majority of studies have been oriented towards psychrotolerant/ psychrophilic bacteria, while the occurrence and diversity of psychrotolerant/ psychrophilic fungi remained largely unknown.

So far, fungi have been reported primarily in connection with sub-Arctic vegetation and soil in polar regions. Mainly basidiomycetous yeasts were isolated from berries, flowers, vegetation of the littoral zone, soils, forest trees, grasses [Babjeva and Reshetova, 1998] and Antarctic mosses [Tosil et al., 2002]. Recently, fungi belonging to the Ascomycota and Basidiomycota, many of them new, were discovered in abundance below snow-covered tundra [Pennisi, 2003; Schadt et al., 2003]. Although much of the water in tundra regions for most of the year is not biologically available, the peak in fungal activity was detected during winter, while in spring and summer bacteria prevailed [Hodkinson et al., 1999; Schadt et al., 2003].

Few studies exist on the biodiversity of fungi in Antarctic soils [Vishniac and Onofri,

2003]. Such soils represent an interesting habitat for xerophilic fungi, since they exhibit extraordinary aridity, with a correspondingly low aw, as well as a relatively high salt content [Vishniac and Onofri, 2003]. The major soluble salts in Antarctic soils are sulfates, chlorides, nitrates of Na, K, Mg and Ca. Besides, microbial life is exposed to low temperatures, low nutrient availability, seasonally increased UV radiation, and geographic isolation [Onofri et al., 2004]. In contrast to the mycobiota present in mesophilic soils, dominated primarily by diverse ascomycetous filamentous fungi [Domsch et al., 1980], in Antarctic soils diverse basidiomycetous yeasts prevail. The dominant yeast genera were Candida, Cryptococcus and Leucosporidium [Vishniac and Klinger, 1986; Vishniac and Onofri, 2003]. In all cases their diversity was low and dominated by a few highly specialised and often endemic taxa [Abyzov, 1993; De Wit et al., 2003]. The highest halotolerance for yeasts isolated from Antarctic soil was recorded for Cr. albidus and Cr. himalayensis (9% NaCl) [Onofri et al.,

2004], although basidiomycetous yeasts in general show low salt tolerance and inability to grow on media with low aw.

Viable yeast and fungi were isolated sporadically also from Siberian permafrost sediments, firmly fixed by ice. They were maintained in a frozen state for extended periods, but upon thawing they were nevertheless able to resume their metabolic activity [Takano, 2004]. The most common yeast genera were again Cryptococcus with the most frequently encountered species being halotolerant Cr. albidus, followed by Sporobolomyces, Rhodotorula and Cystofilobasidium. They were found with the highest frequency of occurrence in the youngest layers, less than 10,000 years old, although they were also detected in three millions years old Pliocene samples. In all cases the share of the yeasts represented 25% of all aerobic heterotrophs, independent of the organic matter content [Dmitriev et al., 1997; Rivkina et al., 2000].

The occurrence of fungi in polar aquatic habitats has been even less investigated. Yeasts and fungi were isolated from fresh water samples, benthic microbial mats and biofilms on pebbles beneath the ice of Antarctic lakes [Baublis et al., 1991; De Wit et al., 2003].

From polar offshore sea waters mainly basidiomycetous yeasts of the genera Leucosporidium, Rhodosporidium and Sporobolomyces were isolated [Jones, 1976]. Sequences belonging to Eumycota were detected in up to 3,000 m deep Antarctic polar front waters [Lopez-Garcia et al., 2001]. There is no report on the isolation of fungi from sea ice, although their characteristic small subunit rRNA gene sequences were present in DNA extracted from diverse Antarctic and one Arctic sea ice sample [Brown and Bowman, 2001]. Fungi were detected as well in the hypersaline Antarctic Lake Wanda [Kriss et al., 1976].

The presence of fungi was least investigated in polar glaciers. Filamentous fungi and yeasts were found in the microbial cryoconite holes that probably serve as biological refuges during extreme cold [Margesin et al., 2002; Reeve et al., 2002]. Viable filamentous fungi and yeasts have been isolated from 10,000-13,000 years old Greenland ice [Ma et al., 1999, 2000], 12,000 years old Antarctic Vostok ice core sections [Christner et al., 2000, 2002], and even from Antarctic ice layers up to 38,600 years old [Abyzov, 1993]. In all these cases the isolated fungi were filamentous and their numbers were low, while viable yeasts of the genera Cryptococcus and Rhodotorula have been found only in the upper, younger ice-sheet horizons and surface layers of ice and snow. The oldest yeasts were isolated from horizons 700-3,250 years old [Abyzov, 1993].

By PCR amplification of fragments of the eukaryotic 18S rRNA gene, a diversity of fungi was identified in 2,000-4,000 years old ice-core samples from North Greenland. They were not tested for viability [Price, 2000]. All findings of fungi in glacier ice were interpreted as the result of coincidental Aeolian deposits of spores or mycelium into the ice during its geological history.

+1 0

Post a comment