Aquatic microbes, like all organisms, have biogeographies, but this subject has attracted relatively little attention. In this review, recent results exploiting techniques of molecular biology are summarized to place in perspective the studies of this Theme Section. The studies considered concern large-scale patterns of spatial distribution among heterotrophic planktonic prokaryotic and eukaryotic microbes. For freshwater bacterioplankton communities, reported patterns are inconsistent.

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... c richness may increase with system size, and composition may be related among neighboring bodies of water. However, inconsistencies in patterns may be due to differences in the temporal and spatial scales considered. Among planktonic marine prokaryotes, biogeographic patterns are known only in terms of high level groups, e.g. Archea are perhaps dominant in deep oceanic waters. However, studies of large-scale patterns have just begun and they suggest that some ribotypes or species may be restricted to certain oceanic areas. Eukaryotic microbes appear to characterised by high capacities for both dispersal and gene flow. Recent studies appear to conclude that we can form morphological, genetic and physiological groupings but their inter-relationships are obscure at this point in time. KEY WORDS: Bacteria · Protists · Latitudinal diversity gradient · Island biogeography · Biodiversity Resale or republication not permitted without written consent of the publisher