One of the longstanding theories of microbial biogeography is the paradigm that "everything is everywhere, but the environment selects" (Baas-Becking 1934, cited in O'Malley 2007). However, various studies have started to challenge this traditional theory with research showing divergence of microbial types due to geographical constraints on microbial migration, and environmental factors driving spatial and temporal distributions (Hughes Martiny et al. 2006). Comprehensive descriptions of permafrost environments encompassing both molecular and culture-based approaches are only starting to emerge in the literature (Vishnivetskaya et al. 2006; Gilichinsky et al. 2007; Hansen et al. 2007; Steven et al. 2007a, 2008a); therefore, it may be premature to put these into a biogeography context. Nevertheless, trends are beginning to appear including the dominance of high G + C Gram-positive organisms within permafrost as revealed by culture-dependent and culture-independent methods (Tables 5.2 and 5.3). The high similarity between 16S rRNA gene sequences and isolates recovered from permafrost samples (Gilichinsky et al. 2007; Hansen et al. 2007; Steven et al. 2007a, 2008a) and those from other similar cryoenvironments (e.g., glacial ice, sea ice, and Lake Vostok accretion ice) also suggests that cosmopolitan groups of microorganisms adapted to life at subzero temperatures exist. Conversely, several Bacteria genera detected in each of the above mentioned studies also seem to be unique to the specific location under investigation (Tables 5.2 and 5.3). Taken together, these results indicate both cosmopolitan and endemic populations of microbes residing in geographically separated permafrost. However, one cannot conclusively prove an organism is not present in any given environment, due to the limitations of current technologies used in microbial ecology (Ramette and Tiedje 2007). It is also important to note that studies of the microbiology in permafrost are from a relatively small number of sites, and do not reflect a comprehensive survey of permafrost environments.
Work undertaken by Steven et al. (2008a) has also demonstrated the importance that comparisons between microbial communities in geographically separated permafrost should be made from similar horizons, as the composition of microbial communities varies with permafrost depth. For example, 55% of the Bacteria 16S rRNA gene sequences from a 1-m depth permafrost sample (Steven et al. 2008a) were most closely related to 16S rRNA gene sequences recovered from a ca. 1-m deep Spitsbergen Island permafrost sample (Hansen et al. 2007), compared to 15% of clones from a 2-m permafrost sample, while none of the clone sequences from a 9-m sample (Steven et al. 2007a) had closest relatives identified in the Spitsbergen Island permafrost sample.
The application of new techniques in biogeography theory, taxonomic level resolution and exhaustive sampling methods, and novel molecular approaches such as microarray and metagenomic technologies (Ramette and Tiedje 2007; Xu 2006) will lead to a greater understanding of microbial biogeography and the environmental factors in permafrost that control the abundance, distribution and diversity of the microbial populations.
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