Tyrosol degradation via the homogentisic acid pathway in a newly isolatedHalomonasstrain from olive processing effluents

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Abstract
To isolate a new Halomonas sp. strain capable of degrading tyrosol, a toxic compound present in olive mill wastewater, through the homogentisic acid (HGA) pathway. A moderately halophilic Gram-negative bacterium belonging to the Halomonas genus and designated strain TYRC17 was isolated from olive processing effluents. This strain was able to completely degrade tyrosol (2-(p-hydroxyphenyl)-ethanol), a toxic compound found in such effluent. Tyrosol degradation begins by an oxidation to 4-hydroxyphenylacetic acid (HPA), which is then converted into HGA by an HPA 1-monooxygenase, while closest Halomonas species degrade tyrosol through 3,4-dihydroxyphenylacetic acid (DHPA). In the presence of transition metals, HGA underwent a pH-dependent abiotic conversion into benzoquinone acetic acid, then into 2,5-dihydroxybenzaldehyde (gentisaldehyde) and pyomelanin, by oxidative decarboxylation and polymerization, respectively. Tyrosol degradation via HGA by the new Halomonas sp. strain TYRC17 was complete in the absence of trace elements. In their presence, HGA was abiotically converted into gentisaldehyde and pyomelanin. This is the first report on tyrosol degradation via the HGA pathway under hypersaline conditions and on the oxidative decarboxylation of HGA into gentisaldehyde. It underlines the importance of the Halomonas genus in the bioremediation of toxic-contaminated sites.

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