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 a ctivity of these bacteria in pure cultures or in mixed cultures with N itrobacter sp. is much higher than ambient soil-pH. This study shows t hat the acid-sensitive, ammonium-oxidizing bacterium Nitrosospira stra in 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-toler ant nitrite-oxidizing bacterium Nitrobacter strain NHB1. Adaptation co uld be achieved in two different ways: (1) by immobilizing the bacteri a 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 afte r isolation.