SUMMARYMethanol-limited cultures of Pseudomonas AM I were grown in chemostats (D, 0.1 h.-l; methanol 10 g./l.) at dissolved oxygen tensions between I and 690 mm. Hg. Decreasing the dissolved oxygen tension from I I O to 2 8 5 mm. Hg did not affect either respiration or carbon balance, but lowering it further caused a progressive increase in respiration which was maximal at 3 mm. Hg (the lowest dissolved oxygen tension at which a steady state could be maintained at the fixed dilution rate). Increasing the dissolved oxygen tension from 97 to 560 mm. Hg increased production of CO, and decreased yield; this effect was particularly evident between 420 and 560 mm. Hg. Yield varied little between 560 and 690 mm. Hg but CO, output decreased and exocellular products increased in concentration indicating a metabolic change. The possible mechanisms involved in metabolic regulation are discussed; it is concluded that dissolved oxygen tensions between 3 mm. and 690 mm. Hg exert a continuous quantitative control over respiration and metabolism of growing organisms. A stoichiometric equation is derived for the growth of Pseudomonas AM I on methanol.
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