Competition for oxygen and 3-chlorobenzoate between two aerobic bacteria using different degradation pathways

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In nature a significant part of the microbial activity is concentrated at or near oxic/anoxic interfaces, where oxygen concentrations are often low. Bacteria possessing different kinetic characteristics for oxygen and employing distinct metabolic pathways for the degradation of (halo)aromatic substrates for which oxygen is needed as co-substrate may have to compete with each other in such environments. In this study the competitiveness of Pseudomonas sp. strain A3 relative to Alcaligenes sp. strain L6 was tested in batch and in continuous cultures. While both of these strains are able to metabolise 3-chlorobenzoate (3CBA), the former was isolated under air saturating conditions and employs the catechol pathway, whereas the latter was isolated under reduced partial pressures of oxygen and was capable of metabolising 3CBA via the gentisate pathway. Competition experiments in batch culture resulted in pure cultures of Pseudomonas sp. strain A3 under air saturating conditions. However, if reduced partial pressures of oxygen (2%) were used, Alcaligenes sp. strain L6 remained present in substantial numbers after three transfers. Continuous culture experiments demonstrated that Alcaligenes sp. strain L6 was able to outcompete Pseudomonas sp. strain A3 under oxygen- as well as under carbon-limiting conditions as long as the dilution rate remained below 0.136 h−1 (low oxygen) and below 0.178 h−1 (high oxygen). These results support the hypothesis that organisms metabolising chlorobenzoate via the gentisate pathway may play a significant role in natural ecosystems where xenobiotic compounds and naturally produced aromatics occur at very low concentrations and in combination with limiting oxygen tensions.