Biodegradation of hydroxylated and methoxylated benzoic, phenylacetic and phenylpropenoic acids present in olive mill wastewaters by two bacterial strains

Two aerobic bacterial strains, a chlorophenol-degrading bacterium characterized in this work as a Ralstonia sp. LD35 on the basis of the sequence of the gene encoding for 16S ribosomal RNA, and Pseudomonas putida DSM 1868, capable of metabolizing 4-methoxybenzoic acid, were tested for their capacity to degrade monocyclic aromatic acids responsible for the toxicity of olive mill wastewaters (OMWs). Both strains possess interesting and complementary degradation capabilities in resting cell conditions: Ralstonia sp. LD35 was found to metabolize 4-hydroxybenzoic, 4-hydroxyphenylacetic, 3,4-dihydroxycinnamic and cinnamic acid, whereas DSM 1868 was capable of metabolizing 4-hydroxy-3-methoxybenzoic, 3,4-dimethoxybenzoic and 4-hydroxy-3,5-dimethoxybenzoic acid, as well as 4-hydroxybenzoic and 4-hydroxyphenylacetic acid. The kinetic parameters describing the growth of the two strains on the same compounds were determined in growing-cell batch conditions, and showed that both strains presented high affinity and high specific growth rates towards all assayed substrates. In addition, the two strains were capable of growing on and extensively biodegrading a mixture of monocyclic aromatic acids commonly found at high concentrations in OMWs, and of growing on a 20% dilution of a natural OMW. All these features make the two strains attractive candidates for the development of a biotechnological process for the biodegradation of aromatic compounds found in OMWs.