The biosphere supports astronomical numbers of free-living microorganisms that belong to an indeterminate number of species. One view^{1, 2, 3} is that the abundance of microorganisms drives their dispersal, making them ubiquitous and resulting in a moderate global richness of species. But ubiquity is hard to demonstrate, not only because active species have a rapid turnover, but also because most species in a habitat at any moment in time are relatively rare or in some cryptic state⁴. Here we use microbes that leave traces of their recent population growth in the form of siliceous scale structures to show that all species in the chrysomonad flagellate genus Paraphysomonas are probably ubiquitous.

1. Institute of Freshwater Ecology, Windermere Laboratory, The Ferry House, Ambleside, Cumbria LA22 0LP, UK
   e-mail: Email: b.finlay@ife.ac.uk