In spite of their limited water availability, cold temperature, strong winds, and large variations in solar radiation input, cold deserts harbour endolithic microorganisms. Antarctica is characterized by extreme climatic conditions, with low humidity and precipitation during the winter (<10% RH; relative humidity) (Horowitz et al., 1972), which make the continent relatively inhospitable for the development of biological communities. In proper niches, however, microbial life can thrive and endolithic microbial communities have been intensively studied in the Antarctic region (Friedmann and Ocampo, 1976; Friedmann, 1982; Friedmann et al., 1988; Nienow et al., 1988a, b; Banerjee et al., 2000; Wierzchos and Ascaso, 2001, 2002; Ascaso and Wierzchos, 2002, 2003; de los Rios et al., 2003, 2004; de la Torre et al., 2003; Hughes and Lawley, 2003; Wierzchos et al., 2004, 2005; Villar et al., 2005).
Endoliths occupy habitats beneath and between porous and translucent rocks and minerals (Fig. 2). Rock porosity provides interstitial spaces for microbial colonization and translucence enables photosynthesis to take place (Friedmann, 1982).
Friedmann (1980) reported that the water content of sandstone colonized by endolithic microorganisms is represented by 0.1-0.2% by weight. Because of the low atmospheric humidity, much of the snow sublimes with little moisture penetrating the upper soil horizon (Cowan and Ah Tow, 2004). As calculated by Friedmann et al. (1987), have estimated that metabolism is possible in endolithic microbial communities for less than 1,000 h per annum, based on the assumption that the lower limit for endolithic metabolism is between -6°C and -8°C (Vestal, 1988).
Friedmann and Ocampo (1976) first reported the presence of endolithic communities in the pore space between quartz grains of Beacon Sandstone in the Dry Valley (Antarctica). Cryptoendoliths have been originally studied by microscopy and laboratory culture methods (Friedmann et al., 1988; Hirsch et al., 1988; Siebert and Hirsch, 1988; Siebert et al., 1996), and only recently their study has been combined with phylogenetic tools (de la Torre et al., 2003). Friedmann et al. (1988) identified two dominant community types of endoliths: lichens (fungal hyphae with the green algae symbiont Trebouxia) and cyanobacteria (Chroococcidiopsis or Gleocapsa species). Refractory to cultivation, most of these autotrophs have been mainly described by morphology (Friedmann et al., 1988). Studies of heterotrophic bacteria associated with lichens were instead based on laboratory cultivation (Hirsch et al., 1988; Siebert et al., 1996). Microscopy studies performed in situ have documented the presence of microbial fossils within Antarctic sandstone rocks in the McMurdo Dry Valleys Desert, where scanning electron microscope techniques enabled the identification of living and decaying endolithic communities (Ascaso and Wierzchos, 2002; Wierzchos and Ascaso, 2002). A phylogenetic study based on the analysis of 1,100 individual 16SrDNA clones of lichens and cyanobacteria (de la Torre et al., 2003), has documented that clones fell into 51 groups (phylotypes) with >98% rRNA sequence identity (46 bacterial and 5 eucaryal). In the lichen-dominated community, three phylotypes accounted for over 70% of the clones: the fungus Texosporium sancti-jacobi (29%), the green algae Trebouxia jamesii (22%) and a chloroplast related sequence (22%). In the cyanobacteria-dominated community, cyanobacterial phylotypes (mostly belonging to the Leptolyngbya-Phormidium-Plectonema group) make up over 30% of clones sequenced. Heterotrophic bacteria phylotypes represented nearly 60% of the tested clones, falling in two major groups: the a-proteobacteria and the Thermus-Deinococcus group.
In the Antarctic rocks traces of past life have been reported in form of geophysical and geochemical bioweathering patterns (Friedmann and Weed, 1987; Sun and Friedmann, 1999). The surface of the Beacon Sandstone (Beacon Supergroup, Victoria Land) shows characteristic pattern of exfoliative weathering caused by the oxalic acid secreted by microorganisms that colonized the porous rocks. This weak acid can leach the iron compounds coating the quartz crystals and produce a snow-white zone. The weathering process causes exfoliation and loss of biomass. After an exfoliation event the microorganisms grow deeper into the rock and a new siliceous crust forms on the rock surface. The alternating processes of crust formation and exfoliation produce a characteristic mosaic with several millimeters deep relief. The formation of trace fossils of microbial colonization can therefore be preserved in the geological record. Wierzchos et al. (2003) have documented that some minerals of Antarctic rocks are biologically transformed, such as the Fe-rich biogenic minerals in the form of iron oxyhydroxide nanocrystals, whereas biogenic clays are deposited around chasmoendolithic hyphae and bacterial cells.
In the Arctic region a great microbial diversity has been documented without any documented biomineralization (Omelon et al., 2006a, b). Such lack of evidence contrasts with the remarkable data collected from similar environments of the Antarctic Dry Valleys because of the warmer and wetter conditions of the Arctic summer period, with consequent longer periods of metabolic activities. Erosion rates, however, might be responsible for habitat destruction, as there may not be enough time for biomineralization (Omelon et al., 2006a, b).
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