Ammonium-oxidation at low pH by a chemolithotrophic bacterium belonging to the genus Nitrosospira

Ammonium-oxidizing bacteria of the genus Nitrosospira have often been isolated from acid soils. However, the lower pH-limit for nitrifying activity of these bacteria in pure cultures or in mixed cultures with Nitrobacter sp. is much higher than ambient soil-pH. This study shows that the acid-sensitive, ammonium-oxidizing bacterium Nitrosospira strain AHB1, which has been isolated from an acid heathland soil, could be adapted to oxidize ammonium at pH 4 in the presence of the acid-tolerant nitrite-oxidizing bacterium Nitrobacter strain NHB1. Adaptation could be achieved in two different ways: (1) by immobilizing the bacteria in alginate beads; and (2) by exposing them to pH-fluctuations. Thus, Nitrosospira spp may be involved in the oxidation of ammonium at an ambient low soil-pH, even though they appear to be acid-sensitive after isolation.