Spatial scales of soil bacterial diversity--the size of a clone.

The spatial distribution of the tremendous bacterial diversity in soil partially depends on the broad range of scales of soil physical structures and the size of bacteria. The aim of this article is to collect information on spatial distribution of bacteria, the genetic structure of bacterial populations and communities, and on spatial constraints that operate in soil. This has been addressed by studying the spatial pattern of micro-habitats for various bacterial types and the spatial spread of clones in soil environment. The clones were considered as the units of genetic population structure. Experimental findings from a number of studies provide evidence that in soils a clone and a micro-colony are not necessarily identical. For some bacterial types, members of the same clone have been found far apart. Besides, micro-colonies of a few cells have also been reported. Short-range cell movements seem to be common in soil, in agreement with the observation of high small-scale diversity (millimetre scale). The mechanisms for the spread of clones are complex and probably operate at different spatial scales, even for soil bacteria with no specific vectors. The hypothesis underlying the study of the spatial dimension of diversity is that it can reveal mechanisms of diversity maintenance and contribute to their evaluation, complementing available knowledge of genetic processes.