A mineral medium for studying the growth of Nitrosomonas europaea was developed and examined. The medium was defined in terms of chemical speciation by using chemical equilibrium computer models. The medium significantly increased the metabolic activity of the organisms compared with previously developed media, yielding a specific growth rate as high as 3.0 day-' (generation time, 5.5 h). The specific growth rate was enhanced by increasing the inoculum and was linearly correlated with the inoculum-to-total-culture volume ratio on a semilog scale. A reproducible growth rate for N. europaea was obtained with this medium under controlled experimental conditions.

The effects of copper, cadmium and nickel on Nitrosomonas europaea were characterized by growth response. Copper inhibition caused a decrease in the growth rate of N. europaea that occurred immediately after inoculation. Cadmium inhibition caused a sharp decrease in the growth rate that occurred after a certain time period, yielding two distinctive growth rates. Nickel inhibition was characterized by an extended lag phase. The combined effects of copper and nickel affected both growth rate and lag. Combined effects of cadmium and nickel were also observed. The combined effect of nickel and copper was antagonistic with respect to both the growth rate and the lag period. Nickel and cadmium acted synergistically to depress the specific growth rate, but the combined effect was either antagonistic or additive (depending upon substrate concentration) with respect to duration of the lag.

Long‐term water quality studies of surface waters and groundwaters in Iowa have shown increases in nitrate concentrations in surface supplies and shallow wells. Nitrate concentrations in deeper wells have remained low and have exhibited no appreciable increases since 1950. The application of nitrogenous fertilizers to agricultural land appears to be the primary source of nitrates in water supplies.

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The toxic effects of copper(II) and nickel(II) species on the growth of Nitrosomonas europuea were investigated. The initial conditions of media were defined in terms of chemical speciation using a chemical equilibrium model. The investigation measured the response of N. europaea (the oxidation of ammonium to nitrite) cultured in a simple medium containing five cations (Ca2+, Mg2+, Fe2+, K+, Na+ ) and five inorganic ligands (CO2−3, SO2−4, Cl−, PO3−4, OH−), to changing concentrations of heavy metals (Cu2+, Ni2+) and total ammonia (NH3 plus NH+4). Experimental conditions were controlled as to allow interpretation of the results in terms of chemical species and their toxic effects on the organisms. Decrease of the specific growth rate of N. europaea was linearly correlated (r≥ 0.90) with the logarithmic activities of Cu(II)‐ammine species (i.e., CuNH2+3, Cu(NH3)2+2, Cu(NH3)2+3, Cu(NH3)2+4,), regardless of the total copper(II) activity in the medium. Nickel(II) was far less toxic than copper(II) on the basis of specific growth rate, but nickel induced a lag or synchronized growth on the culture. The duration of lag increased with increasing total nickel concentration, while it decreased significantly as total ammonia concentration increased. The lag time was highly correlated (r>0.85) with total nickel and Ni2+ activities on log‐log scale at high ammonia concentrations, but no particular nickel species was implicated as the toxic agent. The toxicity of copper and nickel were markedly influenced by the substrate concentration.