Gene expression of terminal oxidases in two marine bacterial strains exposed to nanomolar oxygen concentrations

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 (<200 nM), measured by RT-qPCR. In both strains, high-affinity cytochrome cbb(3) oxidase showed the highest expression levels and was significantly up-regulated upon transition to low O-2 concentrations. Low-affinity cytochrome aa(3) oxidase showed very low transcription levels throughout the incubation. Surprisingly, however, it was also up-regulated upon transition to low O-2 concentrations. In contrast, putative low-affinity CIO had much lower expression levels and markedly different regulation patterns between the two strains. These results demonstrate that exposure to low O-2 concentrations regulates the gene expression of different types of terminal oxidases, but also that the type and magnitude of transcriptional response is species-dependent. Therefore, in situ transcriptome data cannot, without detailed knowledge of the transcriptional regulation of the species involved, be translated into relative respiratory activity.