Patterns of purifying selection in marine microbes reveal optimization in the genetic code

Ocean microbes are responsible for about 50% of primary production on Earth, but selective forces governing their evolution are not well understood. We studied evolutionary conservation by examining single-nucleotide variants in the marine environment, and discovered strong purifying selection exerted across marine microbial genes. We show evidence demonstrating that this selection is driven by the environment, and especially by nitrogen availability. We further corroborate that nutrient availability drives this 9resource-driven9 selection by showing stronger selection on highly expressed and extracellular genes, that are more resource-consuming. Finally, we show that the standard genetic code, along with amino acid abundances, facilitate nutrient conservation through robustness to mutations that increase nitrogen and carbon consumption. Notably, this optimization also applies to multiple taxa across all domains of Life, including the Human genome, and it manifests in code structure. Overall, we uncover overwhelmingly strong purifying selective pressure across marine microbial life that may have contributed to the structure of our genetic code.