The final step of aerobic respiration is carried out by a terminal oxidase transporting electrons to oxygen (O-2). Prokaryotes harbor diverse terminal oxidases that differ in phylogenetic origin, structure, biochemical function, and affinity for O-2. Here we report on the expression of high-affinity (cytochrome cbb(3) oxidase), low-affinity (cytochrome aa(3) oxidase), and putative low-affinity (cyanide-insensitive oxidase (CIO)) terminal oxidases in the marine bacteria Idiomarina loihiensis L-2-TR and Marinobacter daepoensis SW-156 upon transition to very low O-2 concentrations (