Microaerophilic bacteria in continuous culture avidly utilize
O2 and grow when maintained at submicromolar DOT.
Such a capability requires alternative cytochrome oxidase
activities: either those which also use O2 but have substantially
greater O2 affinities (lower apparent (Km(O2) values),
or those which use alternative respiratory electron acceptors,
such as nitrate. Microbial physiologists have long understood
this necessity. As one example, consider symbiotic
legume nodules, which are formed with as endosymbiont
microaerophilic bacteria of the family Rhizobiaceae,
commonly called rhizobia. When fresh, active legume nodules are sectioned, the cortical tissue appears blood red. This
distinctive macroscopic and physiological property is due
to large amounts of hemoglobin (leghemoglobin) produced
by the legume host in nodule cortical tissue. O2 saturation
curves are typically calculated from spectroscopic measurements
of fresh nodule leghemoglobin preparations. By referencing
leghemoglobin O2 saturation curves, legume nodules
seem to operate with in vivo DOT levels in the range
of 10–20 nM! Nevertheless, in laboratory reconstruction
experiments, nodule-isolated rhizobia (“bacteroids”) show
high oxidative phosphorylation rates while maintained under
leghemoglobin-buffered (10–20 nM DOT) conditions, simulating
those of active nodules [30].