Abstract
Diatoms, a diverse and abundant group of microalgae, play a crucial role in the functioning of rivers, and are widely used as indicators of ecological quality. This microalgae group has an intraspecific genetic diversity that is poorly understood on a global scale. We examined their genetic diversity using metabarcoding data from Nordic to Equatorial rivers (n = 1103 samples). Notably, 61% of genetic variants were endemic to a single climate zone, including 33% from the Equatorial zone. Looking at the genetic diversity within species, one third of the species showed geographic pattern between climate zones and the phylogenetic structure of their communities indicated that they were shaped by environmental filtering. Another third showed no geographic pattern, and their communities were in majority shaped by neutral processes. A final group was between these two situations. Interestingly, no geographic pattern was observed within the same climate zones, even in regions over 10 000 km apart. We conclude that the numerous species showing allopatric diversification between climate zones, would deserve to be separated into new species to improve diatom-based biomonitoring tools. For future studies, expanding geographical sampling coverage, together with using multi-markers or metagenomes approaches would enable to go beyond these results.