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Description of Toluene Inhibition of Methyl Bromide Biodegradation in Seawater and Isolation of a Marine Toluene Oxidizer That Degrades Methyl Bromide

National Oceanographic and Atmospheric Administration, Atlantic Oceanographic and Meteorological Laboratories, Ocean Chemistry Division, 4301 Rickenbacker Cswy., Miami, Florida 33149,¹ Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H4J1,² Department of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Cswy., Miami, Florida 33149,³ Earth System Science, University of California at Irvine, Irvine, California 92697⁴

Received 6 October 2004/ Accepted 13 January 2005

Methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) are important precursors for destruction of stratospheric ozone, and oceanic uptake is an important component of the biogeochemical cycle of these methyl halides. In an effort to identify and characterize the organisms mediating halocarbon biodegradation, we surveyed the effect of potential cometabolic substrates on CH₃Br biodegradation using a ¹³CH₃Br incubation technique. Toluene (160 to 200 nM) clearly inhibited CH₃Br and CH₃Cl degradation in seawater samples from the North Atlantic, North Pacific, and Southern Oceans. Furthermore, a marine bacterium able to co-oxidize CH₃Br while growing on toluene was isolated from subtropical Western Atlantic seawater. The bacterium, Oxy6, was also able to oxidize o-xylene and the xylene monooxygenase (XMO) pathway intermediate 3-methylcatechol. Patterns of substrate oxidation, lack of acetylene inhibition, and the inability of the toluene 4-monooxygenase (T4MO)-containing bacterium Pseudomonas mendocina KR1 to degrade CH₃Br ruled out participation of the T4MO pathway in Oxy6. Oxy6 also oxidized a variety of toluene (TOL) pathway intermediates such as benzyl alcohol, benzylaldehyde, benzoate, and catechol, but the inability of Pseudomonas putida mt-2 to degrade CH₃Br suggested that the TOL pathway might not be responsible for CH₃Br biodegradation. Molecular phylogenetic analysis identified Oxy6 to be a member of the family Sphingomonadaceae related to species within the Porphyrobacter genus. Although some Sphingomonadaceae can degrade a variety of xenobiotic compounds, this appears to be the first report of CH₃Br degradation for this class of organism. The widespread inhibitory effect of toluene on natural seawater samples and the metabolic capabilities of Oxy6 indicate a possible link between aromatic hydrocarbon utilization and the biogeochemical cycle of methyl halides.

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