Availability of O(2) as a Substrate in the Cytoplasm of Bacteria under Aerobic and Microaerobic Conditions

The growth rates of Pseudomonas putida KT2442 and mt-2 on benzoate, 4-hydroxybenzoate, or 4-methylbenzoate showed an exponential decrease with decreasing oxygen tensions (partial O(2) tension [pO(2)] values). The oxygen tensions resulting in half-maximal growth rates were in the range of 7 to 8 mbar of O(2) (corresponding to 7 to 8 μM O(2)) (1 bar = 10(5) Pa) for aromatic compounds, compared to 1 to 2 mbar for nonaromatic compounds like glucose or succinate. The decrease in the growth rates coincided with excretion of catechol or protocatechuate, suggesting that the activity of the corresponding oxygenases became limiting. The experiments directly establish that under aerobic and microaerobic conditions (about 10 mbar of O(2)), the diffusion of O(2) into the cytoplasm occurs at high rates sufficient for catabolic processes. This is in agreement with calculated O(2) diffusion rates. Below 10 mbar of O(2), oxygen became limiting for the oxygenases, probably due to their high K(m) values, but the diffusion of O(2) into the cytoplasm presumably should be sufficiently rapid to maintain ambient oxygen concentrations at oxygen tensions as low as 1 mbar of O(2). The consequences of this finding for the availability of O(2) as a substrate or as a regulatory signal in the cytoplasm of bacterial cells are discussed.