Experts find self nitrification microbes
An international team of scientists has discovered microbes that perform complete nitrification individually, contrasting textbook knowledge.
The milestone finding was uncovered by University of Vienna microbiologists Holger Daims and Michael Wagner and part-funded by the European Research Council. The study included the participation of partners from Russia, Denmark and Germany.
Anthropogenic nitrogen deposition affects the natural nitrogen cycle where specific micro-organisms convert nitrogen compounds. Among these microbes are the nitrifiers, which carry out the nitrification process where ammonium (a common nitrogen fertiliser) is oxidised to nitrite and, subsequently, nitrite is oxidised to nitrate.
For over 100 years, the two nitrification steps were known to be always catalysed by different micro-organisms: the ammonia oxidisers and the nitrite oxidisers, whose co-operation is required for complete nitrification. However, no microbiologist ever understood why labour is divided in nitrification; a single microbe capable of catalysing both nitrification steps would actually benefit from conserving more energy. Microbiologists have coined a term to describe such a ‘complete’ nitrifier: ‘comammox’ (complete ammonia oxidizer). However, the existence of comammox has remained an unresolved question.
Yet Vienna-based scientists have now seemingly solved the comammox conundrum. The team analysed a bacterial culture from a 1,200m deep oil exploration well in Russia. Although this culture oxidised ammonia completely to nitrate, the only nitrifiers present were Nitrospira, so far known to be strict nitrite oxidisers; they could carry out the second step of nitrification, but any known ammonia oxidisers needed for the first step were absent.
A complete genomic analysis of all bacterial species in the culture revealed key findings. Wagner comments: “Nitrospira are well known nitrite oxidisers that occur almost everywhere. The function as comammox of some Nitrospira was overlooked for decades.
“The Nitrospira bacteria possessed all genes for oxidising both ammonia and nitrite, meaning complete nitrification. This seemed to be the long-sought comammox organism.”
The study’s results are published in full in Nature.
