Permafrost Soils

Permafrost, which is defined as a subsurface frozen layer that remains frozen for more than two years, makes up more than 20% of the land surface of the earth, including 82% of Alaska, 50% of Russia and Canada, 20% of China, and most of the surface of Antarctica (Williams and Smith 1989; Storad 1990). Permafrost poses unique challenges to its resident biota because of the permanently cold temperature of the soils, averaging 10-12°C, and the length of time over which the soils were frozen, which ranges from a few thousand to even 2-3 million years. Antarctica has an area of 14 million km2; however, exposed permafrost soils cover a mere 49,000 km2, or about 0.35% of the entire continent (Campbell and Claridge 2008). In Antarctica, the soil climate and permafrost properties are strongly influenced by the surface radiation balance, since the thermal regime of the soil is dependent upon the gains and

Soil Biota Classification
Fig. 1.1 Classification of soil biota in relation to soil pore and particle size, as used in soil biology (modified from Buscot and Varma 2005)

losses of radiation from the soil surface. Soils with dark-colored surfaces have low albedo values (approximately 5% at Scott Base), while soils with light-colored surfaces have much higher albedo values (26% at Northwind Valley) (Balks et al. 1995; Macculloch 1996). These soils are formed mainly from Precambrian to Lower Paleozoic basement rocks, intruded by granites and peneplained by weathering and glacial erosion, with overlying sediments of sandstones, siltstones, coal measures and tillites. Biodiversity is extremely low, and diminishes with increasing severity of climatic conditions. Primary producers are bryophytes, lichens, cyanobacteria and algae, and terrestrial fauna include collembolans, mites, and groups of microscopic organisms. Two important pedological processes that operate in Antarctica soils are oxidation and salinization. Coarse particle reduction takes place mainly at the soil surface, with particle size decreasing through granular disintegration and abrasion. Within the soil, coarse particles are nearly always angular and unstained, indicating low cryoturbic activity. The organic regime is significant everywhere in Antarctica, owing to the paucity of biological communities. For soil morphological properties, see Campbell and Claridge (2008). The soils of Antarctica are mostly formed in the absence of biological processes and, as a consequence of the prevailing low temperatures, are underlain everywhere by permafrost, with the active layer varying in thickness from about one meter in northern areas to a few centimeters or less in the soils of the inland edge of the Transantarctic Mountains. The permafrost is generally ice-cemented, but in aged and dehydrated soils may be loose. Because of extreme aridity, chemical weathering processes are assisted by salts, which allow unfrozen saline solutions to be present on grain surfaces and cracks in rock particles, even at very low temperatures. Weathering comprises the breakdown of ferromagnesian minerals, releasing iron and cations into the soil matrix. The iron oxidizes and is precipitated on grain surfaces, giving rise to the red coloring of aged sols. The cations, especially calcium and magnesium, combine with nitric and sulfuric acids that arrive in precipitation to form part of the thick salt horizons found in older soils. The concentrated salt solutions react with silica, which is also released by weathering to form secondary clay minerals and in some cases zeolites.

Culture-dependent and culture-independent methods have revealed that permafrost harbors diverse and novel microbial communities. The future challenge for studies of permafrost microbiology is to begin to address the ecology of these unique microbial ecosystems. The knowledge gained from culture-independent surveys of microbial diversity can be used to design targeted strategies in order to determine if phylogenetic groups detected by molecular strategies are part of the viable microbial community. The application of technologies such as stable isotope probing and FISH-microautoradiography could identify active microorganisms and better define the functioning and maintenance of permafrost microbial ecosystems at ambient subzero temperatures (Jeewon and Hyde 2008; Solaiman 2008; Solaiman and Marschner 2008; Marschner 2008). As microbial activities in situ are expected to be minute and extremely slow, new methods and techniques specific to the permafrost environment will be required. Developing methods for detecting and characterizing the active bacteria and archaea in permafrost will lead to the differentiation of the active microbial populations that are presumed to exist in permafrost from cryopre-served microbial fossils that may have remained frozen for geological timescales.

A database on non-lichenized fungi from Antarctica has been created in the United Kingdom (see http://www.antarctica.ac.uk/bas_research/data/access/fungi/, as well as Gilichinsky et al. 2007; Ruisi et al. 2007; Somjak et al. 2007; Ozerskaya et al. 2008). The mycobiota of arctic permafrost have been studied over the last decade (Panikov and Sizova 2007). The most common fungi belong to the genera Acremonium, Alternaria, Arthrinium, Aspergillus, Aureobasidium, Bispora, Botrytis, Chaetophoma, Chrysosporium, Cladosporium, Fusarium, Geomyces, Geotrichum, Gliocladium, Lecythophora, Malbranchea, Monodictys, Mucor, Paecilomyces, Penicillium, Phoma, Rhinocladiella, Scopulariopsis, Stachybotrys, Sphaeronaemella, Sporotrichum, Thysanophora, Trichoderma, Ulocladium, Valsa, Verticillium, Xylohypha, as well as sterile mycelia with sclerotia. Permafrost fungal microfauna provide evidence of the existence of extremotolerant organisms that are capable of retaining their viability and developing under the conditions present in extreme ecological niches and that show high adoptive potential.

Microorganisms are usually found in a dormant state under frozen and permafrost conditions (endo-and exospores, cysts, non-spore antibiotic cells, etc.). High numbers of viable microorganisms have been counted. The detected phylotypes form eleven established lines of descent for bacteria and one entirely new sequence that was not assigned to any of the known groups. Most of the clones belonged to the alpha (20%) and delta (25.6%) subdivisions of the Proteobacteria, with fewer from the beta (9.3%) and gamma (4.7%) subdivisions, groups that are typically isolated from soil by culture methods. Most of the permafrost-derived clones (77%) had sequence similarities of less than 95-80% with those in the database, indicating a predominance of new genera and families (Panikov 2008). It is true that the anammox process has not been investigated in permafrost soils, however, "marine" anammox 16 rRNA sequences have been identified in Siberian frozen alluvial sandy loam from the Middle Pleistocene epoch 300,000-400,000 years ago (Penton and Tiedje 2006). The anammox process was found to be responsible for up to 19% of the total nitrogen production in Greenland sea ice, but was not detectable in annual sea ice, perhaps due to increased stability (Rysgard and Glud 2004). A novel cold-adapted nitrite-oxidizing bacterium was isolated from a Siberian permafrost sample (Alawi et al. 2007). The detection of anam-mox activity in sea ice suggests that this may be an active process in permafrost, where anammox bacteria have also been identified. In the context of current warming trends, a thorough characterization of the nitrogen cycle in permafrost soils is needed in order to quantify effects on organic matter mineralization and ultimately carbon dioxide release as a positive feedback mechanism for global warning.

Long-term survival strategies in permafrost are thought to fall into two main categories. In the first, microbes maintain viability by entering a dormant state in which they can resist damage to cellular insults; in the second, microbes maintain viability by metabolizing and repairing damage at rates sufficient to equal or exceed the rate of death due to environmentally induced damage. In situ permafrost bacteria, which are further characterized by thickened cell walls, altered structure of cytoplasm, and compact nucleoids, showed similarities to cyst-like resting forms of non-spore-forming bacteria. The survival mechanisms may include reducing the polar polysac-charide capsular layer, decreasing the fractional volume of cellular water, increasing the fraction of ordered cellular water, or extracting energy by catalyzing the redox reactions of ions in thin aqueous films in the permafrost (Gilichinsky 2002). Those that fall into latter category, such as the observed changes in the genome and in gene expression, are primarily directed toward the maintenance of molecular motion and resource efficiency for continued growth in frozen conditions. Long-term survival is closely tied to cellular metabolic activity and DNA repair, which over time proves to be superior to dormancy as a mechanism for sustaining bacterial viability (Johnson et al. 2007). Specific sets of cold-induced proteins (CIPs) are considered to facilitate and allow cell growth at low temperatures. CIPs can be classified into cold-shock proteins (CSPs) and cold-acclimation proteins (CAPs). Bacteria that contain these proteins include Psychrobacter and Exiguobacterium (Bakermans et al. 2007). The adaptive nature of permafrost bacteria at near-freezing temperatures is governed by cellular physiological processes through the regulation of certain cellular proteins. It is possible that proteins synthesized at low temperatures may support temperature homeostasis, protect other proteins from denaturation and damage, and enable the cells to adapt to near- or below-freezing temperatures.

Most planets of the solar system, as well as their moons, asteroids and comets, are cryogenic in nature, and so the cryosphere is a common phenomenon in the cosmos. This is why the cells found in the Earth's cryosphere, as well as their metabolic by-products and biosignatures (biominerals, biomolecules and biogases), provide a range of analogs that could be used in the search for possible ecosystems and potential inhabitants of extraterrestrial cryogenic bodies. If life ever existed on other planets during their early stages of development, then it may have consisted of primitive cell forms. Similar to life on Earth, such primitive life may have been preserved on other cosmic bodies deep within their ice or permafrost layers. The orbits of both Earth and Mars lie between those of Mercury and Venus (which are close to the Sun and therefore dehydrated) and the bodies of the Jupiter system (which mostly consist of volatile hydrogen, methane, and water). Biota from the Greenland ice sheet (120,000 years old) and the Antarctic ice sheet (<400,000 years old) have been widely studied to depths of more than 3 km (Miteva et al. 2004; Miteva and Brenchley 2005; Mitrofanov et al. 2007).

The age of the oldest glacial ice, as well as immured bacteria, is still under discussion: >500,000 years old in the Guliya ice cap on the Tibetan Plateau; >2 million years old at the bottom of the Vostok ice core; or even >8.1 million years old (Bidle et al. 2007). The surface conditions in the Antarctic desert - intense levels of solar radiation, an absence of snow and vegetation cover, and ultralow temperatures, which can be as low as -60°C - share similarities with those on Mars.

On Earth, most volcanoes are located in areas where oceanic and continental plates are colliding. Despite active volcanism, permafrost often exists on slopes of high-elevation or high-latitude volcanoes (Palacios et al. 2007). The fundamental question is: do ecological niches such as volcanoes and associated environments contain microbial communities? The task is to find thermophilic microorganisms associated with volcanoes that were deposited with the products of eruption and then survived in permafrost after the scoria and ash froze. Cores extracted from a borehole into young volcanic deposits contained biogenic CH4 and viable bacteria, including thermophilic anaerobes. Among these were methanogens growing on CO2 plus H2. Thermophiles may survive in permafrost and even produce biogenic gases.

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