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Biodiversity of freshwater microorganisms - achievements, problems, and perspectives

Weisse Thomas

Polish Journal of Ecology

2006

Vol. 54, nr 4

633-652

The extent and significance of the diversity of freshwater microbes is at present controversially debated. Until 1980 it was assumed that there are no freshwater-specific bacteria and that the total number of bacterial species is low. The advent of molecular tools over the last ten years revealed that there is a bacterial freshwater assemblage which is phylogenetically different from soil and marine bacteria; secondly, it became obvious that the total number of cultured bacterial species ([similar to] 5900) underestimates bacterial diversity by several orders of magnitude. The current debate centres on 1) how to define a bacterial species and 2) if there is a microbial biogeography. The latter relates to the issue of ubiquity and cosmopolitanism, which is controversially discussed primarily in relation to eukaryotic microorganisms, namely ciliates. Although solid evidence is scarce, many microbial ecologists assume, in accordance with Baas Becking's famous 70-year old dictum - "everything is everywhere, the environment selects" - that freshwater microorganisms are easily dispersed and, therefore, potentially cosmopolitan. This review focuses on the often neglected second part of Baas Becking's metaphor. Evidence is accumulating rapidly that the environment does not simply act as a filter sensu Gleason's individualistic concept for widely dispersed microbes. Rather, prokaryotic and eukaryotic microorganisms have adapted to their specific habitat and perform better in this environment than newly invading congeners. There is an enormous ecophysiological diversity among closely related freshwater microbes which is neither obvious at the morphospecies level nor at the level of evolutionarily conserved genes, such as the small ribosomal RNA gene. Although this large diversity has been demonstrated for various groups of bacteria and protists, there is currently no measure available to compare microbial biodiversity across prokaryotic and eukaryotic domains. The current challenge is to link genetic divergence to ecophysiological diversity in the major taxa.