Ocean currents promote rare species diversity in protists

Oceans host communities of plankton composed of relatively few abundant species and many rare species. The number of rare protists species in these communities, as esti- mated in metagenomic studies, decays as a steep power law of their abundance. The ecological factors at the origin of this pattern remain elusive. We propose that oceanic currents affect biodiversity patterns of rare species. To test this hypothesis, we intro- duce a spatially-explicit coalescence model able to reconstruct the species diversity in a sample of water. Our model predicts, in the presence of oceanic currents, a steeper power law decay of the species abundance distribution and a steeper increase of the number of observed species with sample size. A comparison of two metagenomic stud- ies of planktonic protist communities in oceans and in lakes quantitatively confirms our prediction. Our results support that oceanic currents positively impact the diversity of rare aquatic microbes.