Global distribution of free‐living microbial species

Species of large animals and plants have geographically restricted distributions, but it is unclear if this also applies to free‐living micro‐organisms. We have attempted to clarify this, by investigating the ciliated protozoa living in a habitat that is separated from northern Europe by geographical barriers and great distance ‐ the sediment of a Holocene volcanic crater‐lake with brackish water, in Australia. Of the 85 ciliate species recorded, none was ‘new’, and all (apart from one species previously described only from tropical Africa) are known from northern Europe. All species appear to have reached the crater by passive dispersal from other freshwater and marine environments. The significance of this finding lies in the fact that ciliates are among the largest and most fragile of microbes. If ciliate species have global distributions, it is likely that the same is true for the many smaller, more abundant and more easily dispersed microbial species, including bacteria. There is some support for this in the literature, and most species smaller than ca 1 mm may have global distributions. Biodiversity at the microbial level is fundamentally different from that of macroscopic animals and plants, and it may be difficult to make realistic extrapolations from the attributes of microbial communities, to biodiversity in general.