The effect of respiratory chain composition on the growth efficiencies of aerobic bacteria

The relationship between respiratory chain redox carrier composition and the efficiency of aerobic growth in continuous culture under carbon-limited conditions has been investigated for nine species of bacteria.True molar growth yields with respect to molecular oxygen () or dissimilated carbon source (Y_{substrate ox.}^max) were not significantly affected by the nature either of the major quinone component (ubiquinone or menaquinone), of the major cytochrome oxidase moiety (cytochrome aa₃ or o) or of the transhydrogenase segment (energy-dependent transhydrogenase, energy-independent transhydrogenase or no transhydrogenase), but were significantly influenced by the presence or absence of a high potential, membrane-bound cytochrome c. Under glycerol-limited conditions the presence of cytochrome c raised the average from 51.1 to 84.3 g cells · mole O₂^–1, and the average Y_{glycerol ox.}^maxfrom 154.6 to 233.2 g cells · mole glycerol oxidised^–1; the presence of this redox carrier also elicited increases in and Y_{substrate ox.}^maxof a similar order during growth under lactate, and glucose-limited conditions.The average efficiencies of aerobic energy conservation calculated from these true molar growth yields were 3.4 mole ATP equivalents · mole O₂^–1for organisms with respiratory chains which were deficient in cytochrome c and 5.9 mole ATP equivalents · mole O₂^–1for organisms with respiratory chains which contained cytochrome c.It is concluded from these data, and from parallel measurements of whole cell H⁺/O ratios, that bacterial respiratory systems invariably exhibit energy conservation at sites 1 and 2 but that the presence of a highpotential, membrane-bound cytochrome c is an obligatory prerequisite for energy conservation at site 3.