Summary: The effect of CO inhibition and O limitation on the growth of Pseudomonas fragi strain 72 was studied in carbon and/or energy limited continuous culture using citric acid as the growth substrate. The influence of different CO concentrations on the maximal growth capacity, estimated as the dilution rate at which the culture density begins to fall ( ) or the dilution rate at which biomass output rate is maximal ( ), was demonstrated together with the differences in influence of CO inhibition and O limitation, on the steady-state characteristics of the culture. The combined effect of CO inhibition and O limitation was measured. Comparative studies made in batch cultures suggested that continuous culture was more sensitive to change than the batch culture.

The maximum specific growth rate of the organism decreased when the CO concentration was increased. However, the results from the continuous cultures revealed a plateau in the curve of versus CO concentration (between 3-5% and 12% CO). This plateau could not be seen from the μ determinations (batch).

Unlike O limitation the CO inhibition did not disturb the regularity of the curves of the steady-state characteristics. Two levels of O limitation were indicated from the curve of biomass versus dilution rate, a minor one, slightly reducing the biomass concentration of the culture and a major one, significantly reducing the biomass. An O concentration of 5% of the incoming gas (dissolved oxygen tension in the culture: 3-5%) had no effect on the steady-state characteristics of the culture, while 0-4% O significantly affected the curve of biomass versus dilution rate (dissolved oxygen tension 0-0.4%).

The combined effect of oxygen limitation and 12% CO, further altered the steady-state characteristics and the biomass curve started to decrease at very low dilution rates. Thus, the two inhibitory agents co-operated and their joint growth restricting effect was considerably stronger than the strongest agent alone. The question whether the restricting effect of the two together should be regarded as “synergistic” or “additive”, is still open to discussion even though some evidence was obtained which suggested the former.


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