The genomic G+C content of ocean bacteria varies from below 30% to over 60%.This broad range of base composition is likely shaped by distinct mutational processes,recombination,effective population size,and selection driven by environmental factors.A number of studies have hypothesized that depletion of G/C in genomes of marine bacterioplankton cells is an adaptation to the nitrogen-poor pelagic oceans,but they failed to disentangle environmental factors from mutational biases and population history.Here,we reconstructed the evolutionary changes of bases at synonymous sites in genomes of two marine SAR11 populations and a freshwater counterpart with its evolutionary origin rooted in the marine lineage.Although they all have similar genome sizes,DNA repair gene repertoire,and base compositions,there is a stronger bias toward A/T changes,a reduced frequency of nitrogenous amino acids,and an exclusive occurrence of polyamine,opine,and taurine transport systems in the ocean populations,consistent with a greater nitrogen stressin surface oceans compared to freshwater lakes.Furthermore,the ratio of nonsynoymous to synonymous nucleotide diversity is not statistically distinguishable among these populations,suggesting that population history has a limited effect.Taken together,the ecological transition of SARll from ocean to freshwater habitats makes nitrogen more available to these organisms,and thus relaxation of purifying selection drove a genome-wide reduction in the frequency of G/C to A/T changes in the freshwater population.