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 (similar to 6 x 10(7) cells ml( -1)) of the NH3-oxidizing bacterium Nitrosomonas europaea. In the presence of 10 mM NH4+ and 0.44, 0.22, and 0.11 mM MeBr, the initial rates of MeBr o xidation were sustained for 12, 12, and 24 h, respectively, despite the fac t that only 10% of the NH4+, 18% of the NH4+, and 35% of the NH4+, respecti vely, were consumed. Although the duration of active MeBr oxidation general ly decreased as the MeBr concentration increased, similar amounts of MeBr w ere oxidized with a large number of the NH4+-MeBr combinations examined (10 to 20 mu mol mg [dry weight] of cells(-1)). Approximately 90% of the NH3-d ependent O-2 uptake activity and the NO2--producing activity were lost afte r N, 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 incr eased exponentially, and 48 to 60 h was required to reach the level of acti vity 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 aft er 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 dec reases in NO2--producing and MeBr-oxidizing activities could not be attribu ted directly to NH4+ or NH3 limitation, to a decrease in the pH, to the com position of the incubation medium, or to toxic effects caused by accumulati on of the end products of oxidation (NO2- and formaldehyde) in the medium. Additional cooxidation-related studies of N. europaea are needed to identif y the mechanism(s) responsible for the MeBr-induced loss of cell activity a nd/or viability, to determine what percentages of cells damaged by cooxidat ive activities are culturable, and to determine if cooxidative activity int erferes with the regulation of NH3-oxidizing activity.