New Insights into Methyl Bromide Cooxidation by Nitrosomonas europaea Obtained by Experimenting with Moderately Low Density Cell Suspensions

We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions (∼6 × 107 cells ml−1 ) of the NH3 -oxidizing bacteriumNitrosomonas europaea . In the presence of 10 mM NH4 + and 0.44, 0.22, and 0.11 mM MeBr, the initial rates of MeBr oxidation were sustained for 12, 12, and 24 h, respectively, despite the fact that only 10% of the NH4 + , 18% of the NH4 + , and 35% of the NH4 + , respectively, were consumed. Although the duration of active MeBr oxidation generally decreased as the MeBr concentration increased, similar amounts of MeBr were oxidized with a large number of the NH4 + -MeBr combinations examined (10 to 20 μmol mg [dry weight] of cells−1 ). Approximately 90% of the NH3 -dependent O2 uptake activity and the NO2 − -producing activity were lost afterN. europaea was exposed to 0.44 mM MeBr for 24 h. After MeBr was removed and the cells were resuspended in fresh growth medium, NO2 − production increased exponentially, and 48 to 60 h was required to reach the level of activity observed initially in control cells that were not exposed to MeBr. It is not clear what percentage of the cells were capable of cell division after MeBr oxidation because NO2 − accumulated more slowly in the exposed cells than in the unexposed cells despite the fact that the latter were diluted 10-fold to create inocula which exhibited equal initial activities. The decreases in NO2 − -producing and MeBr-oxidizing activities could not be attributed directly to NH4 + or NH3 limitation, to a decrease in the pH, to the composition of the incubation medium, or to toxic effects caused by accumulation of the end products of oxidation (NO2 − and formaldehyde) in the medium. Additional cooxidation-related studies ofN. europaea are needed to identify the mechanism(s) responsible for the MeBr-induced loss of cell activity and/or viability, to determine what percentages of cells damaged by cooxidative activities are culturable, and to determine if cooxidative activity interferes with the regulation of NH3 -oxidizing activity.