Abstract
We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions ( approximately 6 x 10(7) cells ml(-1)) of the NH(3)-oxidizing bacterium Nitrosomonas europaea. In the presence of 10 mM NH(4)(+) 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 NH(4)(+), 18% of the NH(4)(+), and 35% of the NH(4)(+), 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 NH(4)(+)-MeBr combinations examined (10 to 20 micromol mg [dry weight] of cells(-1)). Approximately 90% of the NH(3)-dependent O(2) uptake activity and the NO(2)(-)-producing activity were lost after 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, NO(2)(-) 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 NO(2)(-) 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 NO(2)(-)-producing and MeBr-oxidizing activities could not be attributed directly to NH(4)(+) or NH(3) 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 (NO(2)(-) and formaldehyde) in the medium. Additional cooxidation-related studies of N. 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 NH(3)-oxidizing activity.
Highlights
We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions (ϳ6 ؋ 107 cells ml؊1) of the NH3-oxidizing bacterium Nitrosomonas europaea
The MeBr-oxidizing ability of N. europaea was examined at three pH values representing the range of pH values likely to be encountered in many natural environments
Because the capacity of N. europaea to transform MeBr is between 10 to 20 mol of MeBr mg of cellsϪ1, we know that the quantities of cells used by our colleagues could transform approximately 10-fold more MeBr than the amounts used routinely in these types of studies (2 to 10 mol per assay mixture)
Summary
We examined the rates and sustainability of methyl bromide (MeBr) oxidation in moderately low density cell suspensions (ϳ6 ؋ 107 cells ml؊1) of the NH3-oxidizing bacterium Nitrosomonas europaea. Described studies of cooxidation of halogenated hydrocarbons by NH3-oxidizing bacteria have focused primarily on determining the range of compounds cooxidized by N. europaea [6, 7, 13, 16,17,18] and, to a lesser degree, on kinetic parameters [12] The majority of these studies were conducted by using short incubation periods (Յ1 h), high-density cell suspensions (109 to 1011 cells mlϪ1) exhibiting high rates of NO2Ϫ production (ϳ3 mol mlϪ1 hϪ1), and pH values considered to be optimal for NH3 oxidation (pH 7.8 to 8.0). The objective of this study was to examine cooxidation of MeBr by a moderately low-density suspension of N. europaea cells (ϳ6 ϫ 107 cells mlϪ1) that oxidized NH3 at a rate similar to the rates measured in nitrifying bioreactors [1, 2]
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