Magnimaribacterales marine bacteria genetically partition across the nearshore to open-ocean in the tropical Pacific Ocean

The bacterial order Magnimaribacterales, previously known as the SAR86 lineage, is among the most abundant groups of planktonic bacteria inhabiting the global surface ocean. Despite their prevalence, our understanding of how this genetically diverse lineage partitions into units with coherent ecology and evolution remains limited. Here we surveyed multiple stations in the tropical Pacific Ocean using shotgun metagenomes and 16S rRNA gene amplicons to resolve distinct habitat preferences for Magnimaribacterales lineages across nearshore, offshore, and open-ocean environments. The comprehensive collection of genomes that captured a large fraction of the known evolutionary breadth of Magnimaribacterales, revealed patterns of ecotypic differentiation manifested primarily among genus-level clusters with specific clear preferences for distinct marine habitats. Enrichment analyses identified several functional genes associated with genomes from genera abundant in the nearshore environment, including those associated with sugar metabolism, peptide transport, and glycerophospholipid biosynthesis. Such metabolic adaptations likely facilitate the predominance of specific Magnimaribacterales genera in nearshore environments, promoting ecological partitioning across marine habitats. ImportanceUnderstanding the nature by which abundant, genetically-diverse planktonic marine bacteria organize into evolutionarily related and functionally coherent units remains an important question for scientists interested in the ecology of the global ocean. The bacterial order Magnimaribacterales (formerly SAR86) is one of the most prevalent clades in the global surface ocean, yet its ecological differentiation remains poorly resolved. By integrating metagenomic and amplicon analyses across a nearshore-to-open-ocean gradient in the tropical Pacific, this study reveals distinct habitat preferences within this lineage. The study also highlights how differences in distribution appear are primarily found between genus-level groupings. Our findings also highlight key functional traits, including sugar metabolism and glycerophospholipid biosynthesis, that were associated with the partitioning of nearshore and open-ocean Magnimaribacterales. This work enhances our understanding of the evolutionary processes shaping the diversity of one of the oceans most abundant bacterial clades.