Genome-streamlined SAR202 bacteria are widely present and active in the euphotic ocean.

SAR202 bacteria are a diverse group of bacteria in the ocean. The SAR202 lineages dominate the bacterial community and evolve specialized metabolisms for oxidizing recalcitrant organic compounds in the dark ocean. SAR202 bacteria are also present in the euphotic oceans; however, their ecological roles and metabolic potential remain poorly understood. In this study, we collected 392 non-redundant metagenome-assembled genomes from different oceans, with 18% of these SAR202 genomes characterized by small genome sizes (<2 Mbp), low GC content (<40%), and high gene density. The 70 genome-streamlined SAR202 bacteria constitute more than an average of 90% of SAR202 in the euphotic zone and exhibit streamlined metabolic features compared to the dark ocean SAR202. Genome-streamlined SAR202 is distributed in many major SAR202 lineages (i.e. I, II, III, and VI). Phylogenomic analysis shows that the genome-streamlined SAR202 clades diverged from the non-genome-streamlined SAR202 lineages and evolved independently within the same clades. Certain genes are enriched in genome-streamlined SAR202, such as proteorhodopsin genes and the coding genes of major facilitator superfamily transporters, nucleoside transporters, and deoxyribodipyrimidine photo-lyase, indicating their adaptation to sunlit oligotrophic water. A detailed comparison between genome-streamlined SAR202 and non-genome-streamlined SAR202 was made to illustrate their distinct niche distribution and metabolic buildup. In addition, the metatranscriptomic analysis supports that genome-streamlined SAR202 bacteria are active in the upper ocean. This study represents a systematic study of streamlined SAR202 bacteria that occupy the euphotic ocean and provides a comprehensive view of the ecological roles of SAR202 bacteria in the ocean.