Widely-distributed freshwater microorganisms with streamlined genomes co-occur in cohorts with high abundance.

Genome size is known to reflect the eco-evolutionary history of prokaryotic species, including their lifestyle, environmental preferences, and habitat breadth. However, it remains uncertain how strongly genome size is linked to prokaryotic prevalence, relative abundance and co-occurrence. To address this gap, we present a systematic and global-scale evaluation of the relationship between genome size, relative abundance and prevalence in freshwater ecosystems. Our study includes 80,561 medium-to-high quality genomes, from which we identified 9,028 species (ANI > 95%) present in a manually curated dataset of 636 freshwater metagenomes. Our results show that prokaryotes with reduced genomes exhibited higher prevalence and relative abundance, suggesting that genome streamlining may promote cosmopolitanism. Furthermore, network analyses revealed that the most prevalent prokaryotes have streamlined genomes that are found in co-occurrent cohorts potentially sustained by metabolic dependencies. Overall, species in these groups possess a diminished capacity for synthesizing different essential metabolites such as vitamins, amino acids and nucleotides, potentially fostering metabolic complementarities within the community. Moreover, we found the presence of the essential biosynthetic functions to be usage-dependent: nucleotide and amino acids biosynthesis are the most complete, whereas vitamin biosynthesis is most incomplete. Our results underscore genome streamlining as a central eco-evolutionary strategy that both shapes and is shaped by community dynamics, ultimately fostering interdependences among prokaryotes.