We review the biogeography of microorganisms in light of the biogeography of macroorganisms. A large body of research supports the idea that free-living microbial taxa exhibit biogeographic patterns. Current evidence confirms that, as proposed by the Baas-Becking hypothesis, 'the environment selects' and is, in part, responsible for spatial variation in microbial diversity. However, recent studies also dispute the idea that 'everything is everywhere'. We also consider how the processes that generate and maintain biogeographic patterns in macroorganisms could operate in the microbial world.

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This review introduces the subjects of bacterial biodiversity and biogeography. Studies of biogeography are important for understanding biodiversity, the occurrence of threatened species, and the ecological role of free-living and symbiotic prokaryotes. A set of postulates is proposed for biogeography as a guide to determining whether prokaryotes are "cosmopolitan" (found in more than one geographic location on Earth) or candidate endemic species. The term "geovar" is coined to define a geographical variety of prokaryote that is restricted to one area on Earth or one host species. This review discusses sea ice bacteriology as a test case for examining bacterial diversity and biogeography. Approximately 7% of Earth's surface is covered by sea ice, which is colonized principally by psychrophilic microorganisms. This extensive community of microorganisms, referred to as the sea ice microbial community (SIMCO), contains algae (mostly diatoms), protozoa, and bacteria. Recent investigations indicate that the sea ice bacteria fall into four major phylogenetic groups: the proteobacteria, the Cytophaga-Flavobacterium-Bacteroides (CFB) group, and the high and low mol percent gram-positive bacteria. Archaea associated with sea ice communities have also been reported. Several novel bacterial genera and species have been discovered, including Polaromonas, Polaribacter, Psychroflexus, Gelidibacter, and Octadecabacter; many others await study. Some of the gram-negative sea ice bacteria have among the lowest maximum temperatures for growth known, < 10 degrees C for some strains. The polar sea ice environment is an ideal habitat for studying microbial biogeography because of the dispersal issues involved. Dispersal between poles is problematic because of the long distances and the difficulty of transporting psychrophilic bacteria across the equator. Studies to date indicate that members of some genera occur at both poles; however, cosmopolitan species have not yet been discovered. Additional research on polar sea ice bacteria is needed to resolve this issue and extend our understanding of its microbial diversity.

Polaribacter gen. nov., with three new species, P. irgensii sp. nov., P. franzmannii sp. nov. and P. filamentus sp. nov., gas vacuolate polar marine bacteria of the C'ophaga-Flavobacterium-Bacteroides group and reclassification of ' Flectobacillus glomeratus as Polaribacter glomeratus comb. nov.