A streamlined and abundant Nitrosopumilus population retains vitamin biosynthesis under high purifying selection in the Baltic Sea

Genome streamlining is expected to favor the loss of costly biosynthetic functions, raising the question of what benefit justifies the continued maintenance of complex pathways in small genomes. Here, we investigated this question in an abundant Nitrosopumilus population from the Baltic Sea that combined a streamlined genome (1.4 Mbp) with substantial biosynthetic capacity for vitamins B ₁ , B ₇ , and B ₁₂ . We found three lines of evidence that support the biological significance of this retention. First, phylogenomic analysis of 1,077 archaeal species representatives showed that B-vitamin biosynthesis is broadly conserved across ammonia-oxidizing archaea (AOA), indicating maintenance over deep evolutionary timescales. Second, genes associated with B-vitamin biosynthesis tended to be under stronger purifying selection than the genomic background across Baltic Sea metagenomes, consistent with selection preserving these functions in current environmental populations. Third, measurements in the cultured relative Nitrosopumilus maritimus showed B-vitamins and related precursors in both intracellular and extracellular fractions, providing a potential route through which these retained functions could influence metabolite availability beyond the producing cell. Together, our findings suggest that retained biosynthetic pathways in highly streamlined microorganisms reveal functions that are beneficial for the success of a natural population, while also pointing to their potential importance for other community members coexisting within the same environment.