Global population structure in MAST-4 unicellular marine predators

Marine heterotrophic flagellates (HFs) are key unicellular predators in marine food webs. Understanding their diversity and distributions is crucial for comprehending ocean ecosystems. MAST-4, an uncultured clade of Marine Stramenopiles, comprises a key group of bacterivorous heterotrophic flagellates (HFs) in the ocean microbiome. While we know that temperature is a major driver of MAST-4's biogeography, the population structure of MAST-4 species remains poorly known, limiting our ability to understand their ecology and adaptations. Here, we investigate the global population diversity and structure of MAST-4 species A, B, C, and E using metagenomics and single-cell genomics data from the Tara Oceans expedition. We find substantial population divergence in MAST-4A and C, with lower divergence in species B and E. Temperature and salinity are the primary factors structuring these populations. Analyses of positively selected genes reveal genomic regions likely involved in population adaptation to different environments. Our findings enhance the understanding of the population diversity and structure of these critical unicellular predators, providing insights into their ecological roles and adaptations in the global ocean. They also contribute to our general understanding of microbial populations, a largely unexplored dimension of biodiversity that plays a crucial role in grasping the impacts of global change.