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Alteromonas macleodii is a marine heterotrophic bacterium with widespread distribution − from temperate to tropical oceans, and from surface to deep waters. Strains of A. macleodii exhibit considerable genomic and metabolic variability, and can grow rapidly on diverse organic compounds. A. macleodii is a model organism for the study of population genomics, physiological adaptations and microbial interactions, with individual genomes encoding diverse phenotypic traits influenced by recombination and horizontal gene transfer. 

Evolutionary adaptations of prokaryotes to the environment sometimes result in genome reduction. Our knowledge of this phenomenon among free‐living bacteria remains scarce. We address the dynamics and limits of genome reduction by examining one of the most abundant bacteria in the ocean, the SAR86 clade. Despite its abundance, comparative genomics has been limited by the absence of pure cultures and the poor representation in metagenome‐assembled genomes. We co‐assembled multiple previously available single‐amplified genomes to obtain the first complete genomes from members of the four families. All families showed a convergent evolutionary trajectory with characteristic features of streamlined genomes, most pronounced in the TMED112 family. This family has a genome size of ca. 1 Mb and only 1 bp as median intergenic distance, exceeding values found in other abundant microbes such as SAR11, OM43 andProchlorococcus. This genomic simplification led to a reduction in the biosynthesis of essential molecules, DNA repair‐related genes, and the ability to sense and respond to environmental factors, which could suggest an evolutionary dependence on other co‐occurring microbes for survival (Black Queen hypothesis). Therefore, these reconstructed genomes within the SAR86 clade provide new insights into the limits of genome reduction in free‐living marine bacteria.