Nitrobacteraceae

Abstract: Publisher Summary This chapter highlights the physiology of autotrophic ammonia- and nitrite-oxidizing bacteria. Several aspects of the physiology of nitrifiers of relevance to their growth and activity in natural environments are considered. The chapter describes the basic features of the biochemistry of ammonia and nitrite oxidation and discusses the growth limiting factors and activity of these organisms. The influence of oxygen concentration, pH value, and inhibitors on their physiology is also described. Nitrification plays a central role in the nitrogen cycle of terrestrial and aquatic ecosystems, converting the most reduced form of nitrogen, NH3, to the most oxidized form, NO3. Nitrifying bacteria occupy niches in many ecosystems and compete successfully with faster and more efficiently growing organisms for oxygen, ammonia, and carbon dioxide. Nitrifiers are capable of reversing the nitrification process, carrying out denitrification and producing nitrite, ammonia, nitrous and nitric oxides, and gaseous nitrogen. Ammonia oxidizers can metabolize urea and can assimilate carbon from methane while nitrite oxidizers can grow anaerobically in the presence of organic compounds and nitrate.

Abstract: Ammonia oxidation in laboratory liquid batch cultures of autotrophic ammonia oxidizers rarely occurs at pH values less than 7, due to ionization of ammonia and the requirement for ammonium transport rather than diffusion of ammonia. Nevertheless, there is strong evidence for autotrophic nitrification in acid soils, which may be carried out by ammonia oxidizers capable of using urea as a source of ammonia. To determine the mechanism of urea-linked ammonia oxidation, a ureolytic autotrophic ammonia oxidizer, Nitrosospira sp. strain NPAV, was grown in liquid batch culture at a range of pH values with either ammonium or urea as the sole nitrogen source. Growth and nitrite production from ammonium did not occur at pH values below 7. Growth on urea occurred at pH values in the range 4 to 7.5 but ceased when urea hydrolysis was complete, even though ammonia, released during urea hydrolysis, remained in the medium. The results support a mechanism whereby urea enters the cells by diffusion and intracellular urea hydrolysis and ammonia oxidation occur independently of extracellular pH in the range 4 to 7.5. A proportion of the ammonia produced during this process diffuses from the cell and is not subsequently available for growth if the extracellular pH is less than 7. Ureolysis therefore provides a mechanism for nitrification in acid soils, but a proportion of the ammonium produced is likely to be released from the cell and may be used by other soil organisms.

Abstract: Nitrosomonas europaea, an obligate ammonia-oxidizing bacterium, lost an increasing amount of ammonia oxidation activity upon exposure to increasing concentrations of nitrite, the primary product of ammonia-oxidizing metabolism. The loss of activity was specific to the ammonia monooxygenase (AMO) enzyme, as confirmed by a decreased rate of NH4+-dependent O2 consumption, some loss of active AMO molecules observed by polypeptide labeling with 14C2H2, the protection of activity by substrates of AMO, and the requirement for copper. The loss of AMO activity via nitrite occurred under both aerobic and anaerobic conditions, and more activity was lost under alkaline than under acidic conditions except in the presence of large concentrations (20 mM) of nitrite. These results indicate that nitrite toxicity in N. europaea is mediated by a unique mechanism that is specific for AMO.