Quantitative proteomics reveals oxygen-induced adaptations in Caldalkalibacillus thermarum TA2.A1 microaerobic chemostat cultures

The thermoalkaliphile <i>Caldalkalibacillus thermarum</i> possesses a highly branched respiratory chain. These primarily facilitate growth at a wide range of dissolved oxygen levels. The aim of this study was to investigate the regulation of <i>C. thermarum</i> respiratory chain. <i>C. thermarum</i> was cultivated in chemostat bioreactors with a range of oxygen levels (0.25% O₂-4.2% O₂). Proteomic analysis unexpectedly showed that both the type I and the type II NADH dehydrogenase present are constitutive. The two terminal oxidases detected were the cytochrome <i>c</i>:oxygen <i>aa</i> ₃ oxidase, whose abundance was highest at 4.2% O₂. The cytochrome <i>c</i>:oxygen <i>ba</i> ₃ oxidase was more abundant at most other O₂ levels, but its abundance started to decline below 0.42% O₂. We expected this would result in the emergence of the cytochrome <i>c</i>:oxygen <i>bb</i> ₃ complex or the menaquinol:oxygen <i>bd</i> complex, the other two terminal oxidases of <i>C. thermarum</i>; but neither was detected. Furthermore, the sodium-proton antiporter complex Mrp was downregulated under the lower oxygen levels. Normally, in alkaliphiles, this enzyme is considered crucial for sodium homeostasis. We propose that the existence of a sodium:acetate exporter decreases the requirement for Mrp under strong oxygen limitation.