Genome-resolved biogeography of Phaeocystales, cosmopolitan bloom-forming algae

Phaeocystis is a genus of nanoplanktonic haptophytes, prevalent in all of the world’s oceans. At least three Phaeocystis species form large mucilaginous colonies that contribute to high-biomass blooms, making them major players in biogeochemical cycles, especially of carbon and sulfur. To investigate the ecological success of these primary producers, we assembled genomic data for thirteen Phaeocystis strains and annotated three with the highest contiguity. Using these data and additional metagenome-assembled genomes, we present a robust phylogeny for Phaeocystis and their sister clade, identifying several undescribed but abundant lineages. We distinguish two major ecological distribution patterns, cosmopolitan and polar, with fine-tuned preferences for nutrients, temperature, and motility. The gene repertoires of three annotated Phaeocystis reference genomes reflect a unique strategy of gene expansion among algae: repetitive elements, horizontal gene transfer, and full-length endogenous virus insertions. Phaeocystales thus emerge as an ecologically versatile group with diverse adaptations to biotic and abiotic stressors.