Biogeography for bacteria

摘要

Almost all macroscopic animal and plant species have a limited distribution range on the surface of Earth-- not only because they require particular habitats or climates, but also because of historical contingencies. Over geological time, taxonomicgroups have evolved within continents or even within the confines of lakes, mountain ranges, or oceanic islands. Members of these groups have remained there as a result of natural barriers to migration. Microbes seem to be different. In 1913, the Dutch microbiologist Beijerinck concluded that any bacterial species can be found wherever its environmental requirements are met: The distribution of microbes requires no historical explanation, but can be understood solely in terms of habitat properties. Mosttaxonomic experts have implicitly assumed that this also applies to unicellular eukaryotes (protozoa, microalgae), but the topic has recently drawn renewed interest. For sexual, outbreeding eukaryotes, a theoretically based species concept exists, at least in principle, and so "cosmopolitan distribution" can be given a precise meaning--something that is less obvious in the case of bacteria. The explanation for cosmopolitan distribution of microbes is numbers: Microbial population sizes are enormous. Thus, the probability of dispersal is high and the probability of local extinction is extremely low. In the absence of effective migration barriers and local extinctions, every habitat will contain a pool of bacterial species that do not thrive locally, but may grow if the environment becomes more favorable. It has been calculated that about 10~(18) viable bacteria annually are transported through the atmosphere between continents. It is also possible to isolate bacteria from places where they "should notbe" such as thermophilic bacteria from cold seawater.