Ammonia oxidation at pH 2.5 by a new ammonia-oxidizing bacterium

For long time ammonia oxidation was considered not possible under acidic conditions, because of decreased substrate availability and formation of toxic compounds. Nonetheless, recent studies described strains growing at pH as low as 4 and nitrification was widely observed at even lower pH values. In this study we report the discovery and cultivation of a novel species of an ammonia-oxidizing bacterium from an acidic air biofilter, able to grow and oxidize ammonia even at pH 2.5. We investigated the physiology, the genome sequence and performed transcriptomics analysis to study the gene expression. This microorganism possesses a chemolithoautotrophic lifestyle, using ammonia or urea as energy source. A high affinity for ammonia (Km(app)= 147 ± 14 nM) and tolerance to toxic nitric oxide could represent an advantage in acidic environments. Electron microscopic analysis showed a coccoid cell morphology with a large amount of membrane stacks, typical of ammonia oxidizers. Furthermore, phylogenetic analysis revealed that this strain belonged to a new bacterial genus, for which we proposed the name “Candidatus Nitrosacidococcus tergens RJ19”.