Among the microorganisms that have been recovered and isolated from Siberian permafrost samples, Psychrobacter species have remarkable capabilities at subzero temperatures which identify them as potential model organisms for the study of low-temperature adaptations relevant to inhabiting permafrost (Vishnivetskaya et al. 2000; Bakermans et al. 2003). These Psychrobacter species grow quickly at low temperatures, actively reproduce at -10°C, easily survive freeze-thaw cycles, and are tolerant to 12% NaCl (Bakermans and Nealson 2004; Ponder et al. 2005; Bakermans et al. 2006, 2007). Psychrobacter species are commonly isolated from a variety of low-temperature environments, including: Antarctic sea ice, ornitho-genic soil, and sediments; the stomach contents of the Antarctic krill Euphausia; sea water (NW Pacific Ocean, 300 m depth); the deep sea; and the internal tissues of a marine ascidian (Bowman et al. 1997; Maruyama et al. 2000; Romanenko et al. 2002; Yumoto et al. 2003). In addition, quantitative PCR analyses have revealed that Psychrobacter species are widespread in polar regions and have been found throughout Antarctica and Siberia at 16S ribosomal RNA gene copy numbers ranging from 103 to 107 per |g of total community DNA (Rodrigues 2007).
To date, genomic (and post-genomic) studies have focused on two species that were isolated from the Kolyma Lowland region of Siberia where the permafrost is continuous, approximately 800 m thick, and remains stable at -9 to -11°C (Gilichinsky et al. 1992; Shi et al. 1997). Psychrobacter arcticus 273-4 was recovered from a depth of 12.5 m within a 20,000-30,000-year-old sandy loam that froze as it was deposited and has remained frozen to modern times (Sher et al. 1977; Vishnivetskaya et al. 2000). Psychrobacter cryohalolentis K5 was recovered from a cryopeg (a highly saline, 13%, lens of water) at a depth of 11 m, within a marine layer that was deposited beneath shallow lagoons at temperatures slightly above 0°C and froze sub-aerially as the polar ocean regressed some 110,000-112,000 years ago (Bakermans et al. 2003; Gilichinsky et al. 2003, 2005). The complete genomes for both of these organisms have been sequenced in collaboration with the Joint Genome Institute, and are available at http://genome.jgi-psf.org/mic_home. html (comparative genomic studies are ongoing and will not be discussed here).
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