Nitrate respiration in freshwater environments, microcosms and cultured bacteria.
Denitrification is the process by which bacteria reduce nitrate to dinitrogen gas. Most
denitrifying bacteria perform these reactions under anaerobic conditions only.
Thiosphaera pantotropha is one of a number of species capable of aerobic
denitrification. During aerobic growth T. pantotropha expresses a periplasmic nitrate
reductase but under anaerobic growth conditions nitrate is reduced by a membrane-bound
nitrate reductase. The periplasmic nitrate reductase is relatively insensitive to
azide and does not reduce chlorate. Aerobic denitrification provides a mechanism to
dispose of excess reducing equivalents during growth on reduced carbon sources.
Numbers of nitrate reducing bacteria, and nitrate and ammonia concentrations were
monitored in a Norfolk broad over a 12 month period. Several novel microorganisms
capable of aerobic nitrate respiration were isolated from the sediment of this broad.
All were shown to express a periplasmic nitrate reductase activity, and the effects of
growth rate and carbon substrate on the activity of this enzyme were studied. Of the
nine isolates studied, five were shown to be able to reduce nitrate at oxygen
concentrations up to 80% of air saturation. The remaining four were shown to be able
to reduce nitrate under anaerobic conditions. Analysis of 16S rDNA sequences was
used to identify the isolates, seven were assigned to the Genus Aeromonas, and two
to the Genus Pseudomonas. Sediment samples were used to establish a microcosm in
which changes in the concentration of nitrate, nitrite and ammoniacal nitrogen were