Effect of several nitrification inhibitors on carbon monoxide and methane oxidation by ammonium oxidizers

The effects of nine inhibitors of chemolithotrophic nitrification on carbon monoxide, methane, and ammonia oxidation by Nitrosomonas europaea, Nitrosococcus oceanus, and Nitrosomonas sp. 4W30 (a marine isolate) were examined. CH4 and [Formula: see text] oxidation were inhibited to a greater extent than the oxidation of CO. A concentration of 10 μg/mL of N-Serve was sufficient to completely inhibit [Formula: see text] oxidation in all three species, while a concentration of 100 μg/mL was necessary to completely inhibit CO oxidation. The three organisms used in this study showed widely differing susceptibilities to the inhibitors examined. CO and [Formula: see text] oxidation by Nitrosomonas europaea and Nitrosomonas sp. 4W30 were inhibited by >90% in the presence of 100 μg/mL amino-1,2,4-triazole, while Nitrosococcus oceanus was not inhibited. Terrazole at a concentration of 10 μg/mL inhibited carbon monoxide oxidation by Nitrosococcus oceanus 98.9%, while Nitrosomonas europaea and Nitrosomonas sp. 4W30 were only inhibited by 80.2 and 87.1%, respectively. At 100 μg/mL terrazole inhibition was complete. The results of this study emphasize the necessity of careful selection of nitrification inhibitors and concentrations to be used when examining ammonium oxidizers in natural samples. It also indicates that there arc subtle differences in the physiology of ammonium oxidizers that should be taken into consideration when studying the metabolism of these organisms.

Effects of various parameters on carbon monoxide oxidation by ammonium oxidizers

The effects of temperature, pH, nitrogen source, cell concentration, and the interactions between ammonium and carbon monoxide were examined with respect to the oxidation of carbon monoxide by several chemolithotrophic ammonium-oxidizing bacteria. The ammonium oxidizers Nitrosomonas europaea, Nitrosomonas sp. 4W30, a marine isolate, and Nitrosococcus oceanus were examined. All of the organisms were able to oxidize significant amounts of CO over wide ranges of temperature and pH. Ammonium at concentrations as low as 1 mg/L [Formula: see text] N initially inhibited CO oxidation in all three organisms; however, after 48 h, the presence of ammonium stimulated the CO-oxidizing ability of Nitrosomonas europaea and Nitrosomonas sp. 4W30, while Nitrosococcus oceanus remained inhibited. None of the other nitrogen sources examined had a significant effect on CO oxidation. No carbon from CO was incorporated into cellular material in the absence of ammonium, even in the presence of alternate nitrogen sources. Cells incubated in the presence of ammonium at concentrations as low as 1.0 mg/L [Formula: see text] N were able to incorporate CO carbon into cellular material. Increasing [Formula: see text] N concentrations to 50 mg/L stimulated the incorporation of CO carbon by Nitrosococcus oceanus. Concentrations of 10 mg/L gave the highest incorporation levels for Nitrosomonas europaea and Nitrosomonas sp. 4W30 and 50 mg/L inhibited the incorporation by these two organisms. The presence of CO inhibited the oxidation of ammonium by all of the organisms tested.

Carbon monoxide oxidation by chemolithotrophic ammonium oxidizers

Chemolithotrophic nitrifying bacteria were examined with respect to their ability to oxidize carbon monoxide in the presence and absence of ammonium. All of the ammonium oxidizers tested, including Nitrosomonas europaea and Nitrosococcus oceanus, were able to oxidize CO to CO2 in the presence and absence of ammonium. None of the organisms tested incorporated any of the carbon from CO into cellular components in the absence of ammonium. All were able to oxidize CO at trace concentrations of <0.5 nM. None of the nitrite oxidizers examined were capable of CO oxidation. CO oxidation by Nitrosomonas europaea, Nitrosococcus oceanus, and Nitrosomonas sp. 4S30, a marine ammonium oxidizer, were examined with respect to CO concentration and time course of CO oxidation, both in the presence and absence of 10 mg/L NH4 N. The CO oxidation rates at a CO concentration of 300 nL/L in the absence of ammonium ranged from 0.048 nmol/h per 106 cells for Nitrosomonas sp. 4W30 to 0.0025 nmol/h per 106 cells for N. europaea.