COMPETITION FOR OXYGEN AND 3-CHLOROBENZOATE BETWEEN 2 AEROBIC-BACTERIA USING DIFFERENT DEGRADATION PATHWAYS

In nature a significant part of the microbial activity is concentrated at or near oxic/anoxic interfaces, where oxygen concentrations are of ten low. Bacteria possessing different kinetic characteristics for oxy gen and employing distinct metabolic pathways for the degradation of ( halo)aromatic substrates for which oxygen is needed as co-substrate ma y have to compete with each other in such environments. In this study the competitiveness of Pseudomonas sp. strain A3 relative to Alcaligen es 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 t he catechol pathway, whereas the latter was isolated under reduced par tial pressures of oxygen and was capable of metabolising 3CBA via the gentisate pathway. Competition experiments in batch culture resulted i n pure cultures of Pseudomonas sp. strain A3 under air saturating cond itions. However, if reduced partial pressures of oxygen (2%) were used , Alcaligenes sp. strain L6 remained present in substantial numbers af ter three transfers. Continuous culture experiments demonstrated that Alcaligenes sp. strain L6 was able to outcompete Pseudomonas sp. strai n 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 belo w 0.178 h(-1) (high oxygen). These results support the hypothesis that organisms metabolising chlorobenzoate via the gentisate pathway may p lay a significant role in natural ecosystems where xenobiotic compound s and naturally produced aromatics occur at very low concentrations an d in combination with limiting oxygen tensions. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V . All rights reserved.