Background 24The deep ocean is characterized by low temperatures, high hydrostatic pressures, and low 25 concentrations of organic matter. While these conditions likely select for distinct genomic 26 characteristics within prokaryotes, the attributes facilitating adaptation to the deep ocean are 27 relatively unexplored. In this study, we compared the genomes of seven strains within the genus 28Colwellia, including some of the most piezophilic microbes known, to identify genomic features 29 that enable life in the deep sea. 30 31 Results 32 Significant differences were found to exist between piezophilic and non-piezophilic strains of 33 Colwellia. Piezophilic Colwellia have a more basic and hydrophobic proteome. The piezophilic 34 abyssal and hadal isolates have more genes involved in replication/recombination/repair, cell 35 wall/membrane biogenesis, and cell motility. The characteristics of respiration, pilus generation, 36 and membrane fluidity adjustment vary between the strains, with operons for a nuo 37 dehydrogenase and a tad pilus only present in the piezophiles. In contrast, the piezosensitive 38 members are unique in having the capacity for dissimilatory nitrite and TMAO reduction. A 39 number of genes exist only within deep-sea adapted species, such as those encoding d-alanine-d-40 alanine ligase for peptidoglycan formation, alanine dehydrogenase for NADH/NAD + 41 homeostasis, and archaeal methyltransferase for tRNA modification. Many of these piezophile-42 specific genes are in variable regions of the genome near genomic islands, transposases, and 43 toxin-antitoxin systems. 44 45 Conclusions 46We identified a number of adaptations that may facilitate deep-sea radiation in members of the 47 genus Colwellia, as well as in other piezophilic bacteria. An enrichment in more basic and 48 hydrophobic amino acids could help piezophiles stabilize and limit water intrusion into proteins 49 as a result of high pressure. Variations in genes associated with the membrane, including those 50 involved in unsaturated fatty acid production and respiration, indicate that membrane-based 51 adaptations are critical for coping with high pressure. The presence of many piezophile-specific 52 genes near genomic islands highlights that adaptation to the deep ocean may be facilitated by 53 horizontal gene transfer through transposases or other mobile elements. Some of these genes are 54 amenable to further study in genetically tractable piezophilic and piezotolerant deep-sea 55 microorganisms.