Diatom communities harbor cosmopolitan and geographically restricted species, and are shaped by both local environmental conditions and historical processes, yet the relative contributions of these patterns and processes likely differs depending on the geographic scale, diatom lineage, and the habitat that is being investigated. Consequently, communities can vary strongly in their share of cosmopolitan and endemic species. In this project I want to build on my earlier work and investigate why and where specific diatom species and clades are more prone to widespread or restricted distributions. Much of the early days of diatom biogeography were based on morphological studies, which compromised standardized global-scale meta-analyses and population-level research. Yet, with the advent of next-generation sequencing technologies, from whole-genome sequencing to environmental and ancient DNA surveys, it is now possible to integrate insights from multiple lines of evidence, and investigate diatom communities, species, and populations at an unprecedented scale and detail. I will hereby focus on understudied habitats and diatom lineages, study the impact of diatom traits on biogeographical patterns, perform targeted field and experimental work aimed at disentangling the processes that underlie diatom biogeographies, and contribute to the expansion of databases with sequence, trait, distributional, and phylogenetic information of diatoms. Given that a good understanding of species boundaries is central to unraveling diatom biogeography, an important component of this project focuses on life cycle dynamics in diatoms and in particular on the identification and functional characterization of reproductive barriers.