EFFECTS OF SOIL ON AMMONIA, ETHYLENE, CHLOROETHANE, AND 1,1,1-TRICHLOROETHANE OXIDATION BY NITROSOMONAS-EUROPAEA

Ammonia monooxygenase (AMO) from Nitrosomonas europaea catalyzes the o xidation of ammonia to hydroxylamine and has been shown to oxidize a v ariety of halogenated and nonhalogenated hydrocarbons. As part of a pr ogram focused upon extending these observations to natural systems, a study was conducted to examine the influence: of soil upon the cooxida tive abilities of N. europaea. Small quantities of Willamette silt loa m (organic carbon content, 1.8%; cation-exchange capacity, 15 cmol/kg of soil) were suspended with N. europaea cells in a soil-slurry-type r eaction mixture. The oxidations of ammonia and three different hydroca rbons (ethylene, chloroethane, and 1,1,1-trichloroethane) were compare d to results for controls in which no soil was added, The soil signifi cantly inhibited nitrite production from 10 mM ammonium by N. europaea . Inhibition resulted from a combination of ammonium adsorption onto s oil colloids and the exchangeable acidity the soil lowering the pH of the reaction mixture, These phenomena resulted in a substantial drop i n the concentrations of NH4+ in solution (10 to 4.5 mM) and, depending upon the pH, in a reduction in the amount of available NH3, to concen trations (8 to 80 mu M) similar to the k(s) value of AMO for NH3 (simi lar to 29 mu M). At a fixed initial pH (7.8), the presence of soil als o modified the rates of oxidation of ethylene and chloroethane and cha nged the concentrations at which their maximal rates of oxidation occu rred. The modifying effects of soil on nitrite production and on the c ooxidation of ethylene and chloroethane could be circumvented by raisi ng the ammonium concentration in the reaction mixture from 10 to 50 mM . Soil had virtually no effect on the oxidation of 1,1,1 trichloroetha ne.