Soil micro-organisms drive the biogeochemical cycles that underlie essential ecosystem functions. Yet, we are only beginning to grasp the drivers of terrestrial protist diversity and biogeography. This is especially true for soil protists, which have been little studied. In a new paper just published in Nature Communications, we used the diatom species complex Pinnularia borealis as a case study for the diversification and biogeography of terrestrial protists. Pinnularia borealis is an easily recognizable diatom that can be found living in soils and mosses on all continents. Although P. borealis is globally distributed, it is locally rare, meaning that it rarely generates large population sizes. Using a metabarcoding dataset, we found that its abundance, relative to other protists, is usually very low, indicating it is a rare biosphere taxon. To understand the global diversity of P. borealis, we built a global database comprising >1500 samples. Samples were collected all over the world, but the focus was directed towards temperate European regions, as well as the High Arctic and Antarctica. All samples were manually screened in a light microscope, and whenever P. borealis was observed, single cells were isolated, building a collection of >800 monoclonal cultures. The cultures were sequenced for two fast-evolving genes (28S and cox1). These fast evolving genes were then used to assess species boundaries in the complex. Using automated molecular species delimitation methods, we found evidence for at least 126 P. borealis species within our dataset. The majority of these are morphologically cryptic. Inter- and extrapolation of the species accumulation curve suggests that the global diversity of P. borealis is substantially higher than here uncovered. There might be hundreds of species hiding across the planet. Our study thus only shows the tip of the iceberg! Using a time-calibrated phylogeny, we showed that P. borealis started diversifying around the Eocene/Oligocene global cooling (ca. 25-36 Ma). This period was characterized by a profound change in climate, as the Earth shifted from a greenhouse to an icehouse state and known as a time of large-scale extinction and floral and faunal turnover. This period also marks the onset of a global expansion of open terrestrial landscapes, associated with colder and drier climates, in which P. borealis currently thrives. We found diversification in P. borealis to be largely driven by colonization of novel geographic areas and subsequent evolution in isolation. We found high levels of founder event speciation, and highlight the existence of intraspecific phylogeographic structuring. Altogether, these results illuminate our understanding of how protist diversity, biogeographical patterns, and members of the rare biosphere are generated, and suggest allopatric speciation to be a powerful mechanism for diversification of micro-organisms. Pinseel E., Janssens S.B., Verleyen E., Vanormelingen P., Kohler T.J., Biersma E.M., Sabbe K., Van de Vijver B. & Vyverman W. (2020) Global radiation in a rare biosphere soil diatom. Nature Communications 11: 2382. full text
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