Genomic streamlining is a theory in evolutionary biology and microbial ecology that suggests that there is a reproductive benefit to prokaryotes having a smaller genome size with less non-coding DNA and fewer non-essential genes.[1][2] There is a lot of variation in prokaryotic genome size, with the smallest free-living cell's genome being roughly ten times smaller than the largest prokaryote.[3] Two of the free-living bacterial taxa with the smallest genomes are Prochlorococcus and Pelagibacter ubique,[4][5] both highly abundant marine bacteria commonly found in oligotrophic regions. Similar reduced genomes have been found in uncultured marine bacteria, suggesting that genomic streamlining is a common feature of bacterioplankton.[6] This theory is typically used with reference to free-living organisms in oligotrophic environments.[1]