Heterotrophic nitrification in an acid forest soil: isolation and characterisation of a nitrifying bacterium

Bacteria and fungi were isolated from an acid forest soil in which nitrification occurred via a heterotrophic pathway. Enrichment of soil in a liquid inorganic salts medium, containing β-alanine as the sole source of C and N, led to formation of NO2− and NO3−. In pure culture, β-alanine was not a suitable substrate for nitrification by any of the isolates, but did support nitrification when one bacterium (BD) was co-cultured with a fungus. This bacterium was gram-positive and rod-shaped and identified provisionally as an Arthrobacter sp. The bacterium BD was able to nitrify ammonium acetate and, to a lesser extent peptone, in pure culture. Nitrification of ammonium acetate was greater in sterile soil solution than in defined media of inorganic salts. From a less reduced form of N, α-ketoglutaric oxime supported the highest rates of nitrification, but nitrification of pyruvic and ketobutyric oximes was at levels lower than from reduced N. The organism was able to survive and nitrify at pH 3, and despite its isolation from a very acidic environment, at pH≤10. It is proposed that a metabolic product of β-alanine produced by a non-nitrifying microorganism provided a suitable substrate for the nitrifying bacterium.