Nitrification

Nitrifier Diversity-Ecosystem Function | Biodiversity-Ecosystem Function | Archaeal Diversity

Nitrification, the oxidation of ammonia to nitrate, via nitrite, occupies a central position within the global nitrogen cycle. Nitrifiers are the only organisms capable of converting the most reduced form of nitrogen, ammonia, to the most oxidised form, nitrate. They also carry out a range of other important processes within the nitrogen cycle. These include denitrification, methane oxidation, degradation of xenobiotics and urea hydrolysis.

In terrestrial ecosystems, nitrification influences nitrogen supply to plants, increases loss of soil through leaching and increases nitrate pollution of groundwaters. It also leads to substantial losses of ammonia-based fertilisers. In aquatic ecosystems nitrifiers compete with primary producers for ammonia and in all ecosystems nitrification results in the generation of reactive gases, in particular nitrous oxide, which is involved in destruction of stratospheric ozone and global warming. In wastewater treatment processes, nitrification and denitrification are essential for the removal of high levels of nitrogen and the relatively slow growth and sensitivity of ammonia oxidising bacteria often limits throughput.

Despite the ubiquity of abundant and active populations of nitrifiers in natural environments, the organisms are very difficult to isolate, grow and maintain in laboratory culture and, consequently, little is known of their physiological diversity. Physiological studies have focussed on a restricted number of strains, particularly Nitrosomonas europaea, whose genome has now been sequenced, along with those of several other ammonia and nitrite oxidising bacteria and ammonia oxidising archaea.

We have used a combination of laboratory and field studies to elucidate the mechanisms and factors controlling the growth, activity and survival of nitrifiers in natural environments, and their ability to recover rapidly from periods of stress. These have included biofilm formation, effects of low pH, sensitivity to nitrification inhibitors and studies of physiological diversity. Attention is now focused on the relationships between the diversity and community structure of nitrifying bacteria, particularly ammonia oxidisers, and their ecosystem function in a range of environments.

page content last modified: 15th September 2009 16:54:50