Bicarbonate Uptake by Nitrifiers: Effects of Growth Rate, pH, Substrate Concentration, and Metabolic Inhibitors

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Abstract
The ratios of bicarbonate uptake to substrate oxidation were measured for three genera of nitrifying bacteria. The ratios for the two ammonium oxidizers tested were essentially the same; 0.0863 ± 0.0055 and 0.0868 ± 0.0091 μmol of bicarbonate were taken up per umol ammonium oxidized for Nitrosomonas europaea and a Nitrosospira strain, respectively. For Nitrobacter sp., a ratio of 0.0236 ± 0.0013 μmol of bicarbonate taken up per umol of nitrite oxidized was obtained. Cells were grown in substrate-limited continuous culture and in batch culture, with generation times ranging between 16 and 189 h for the ammonium oxidizers and 18 and 69 h for Nitrobacter sp. All ratios appeared to be independent of growth rates and pH. However, short-term changes in substrate concentration and certain metabolic inhibitors significantly changed the efficiency of bicarbonate uptake. The significance of these results to the application of the nitrapyrin-sensitive bicarbonate uptake method for measuring nitrification rates in natural samples is discussed.