Consumption of Tropospheric Levels of Methyl Bromide by C 1 Compound-Utilizing Bacteria and Comparison to Saturation Kinetics

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Abstract
Pure cultures of methylotrophs and methanotrophs are known to oxidize methyl bromide (MeBr); however, their ability to oxidize tropospheric concentrations (parts per trillion by volume [pptv]) has not been tested. Methylotrophs and methanotrophs were able to consume MeBr provided at levels that mimicked the tropospheric mixing ratio of MeBr (12 pptv) at equilibrium with surface waters (≈2 pM). Kinetic investigations using picomolar concentrations of MeBr in a continuously stirred tank reactor (CSTR) were performed using strain IMB-1 and Leisingeria methylohalidivorans strain MB2 T — terrestrial and marine methylotrophs capable of halorespiration. First-order uptake of MeBr with no indication of threshold was observed for both strains. Strain MB2 T displayed saturation kinetics in batch experiments using micromolar MeBr concentrations, with an apparent K s of 2.4 μM MeBr and a V max of 1.6 nmol h −1 (10 6 cells) −1 . Apparent first-order degradation rate constants measured with the CSTR were consistent with kinetic parameters determined in batch experiments, which used 35- to 1 × 10 7 -fold-higher MeBr concentrations. Ruegeria algicola (a phylogenetic relative of strain MB2 T ), the common heterotrophs Escherichia coli and Bacillus pumilus , and a toluene oxidizer, Pseudomonas mendocina KR1, were also tested. These bacteria showed no significant consumption of 12 pptv MeBr; thus, the ability to consume ambient mixing ratios of MeBr was limited to C 1 compound-oxidizing bacteria in this study. Aerobic C 1 bacteria may provide model organisms for the biological oxidation of tropospheric MeBr in soils and waters.