Nitrification in Marine Systems

This chapter presents new information on the NH₃^-oxidizing Archaea, which were unknown at the times of the earlier reviews. The most important organisms in aerobic nitrification are nitrifying bacteria and the NH₃ oxidizing Archaea (AOA). Until very recently, the known nitrifiers were restricted to the Proteobacterial phylum. There are two functionally distinct groups of nitrifiers: those that oxidize NH₄⁺ to NO₂^- (NH₃-oxidizing bacteria and Archaea, NH₃-oxidizing bacteria (AOB) and AOA) and those that oxidize nitrite to NO₃^- (NO₂^- -oxidizing bacteria, NOB). No organism is known to carry out both reactions. These unique metabolic traits are not without costs; the chemolithoautotrophic lifestyle enables nitrifiers to exploit a unique niche in natural systems, but also confers constraints such as slow growth and inflexible nutritional requirements. A new group of aerobic NO₃^- oxidizing nitrifiers was recently discovered, first by detection of NH₃ oxidizing genes of apparent Archaeal origin in environmental metagenomic libraries and subsequently verified with the cultivation of a strain of Archaea (NH3-oxidizing Archaea, AOA) that oxidizes NH₃ to NO₂^-, apparently using a pathway very much like that known in AOB. It now seems likely that AOA are more abundant than AOB in marine systems, and are also prevalent in soils. © 2008