AMMONIA OXIDATION AT LOW PH BY ATTACHED POPULATIONS OF NITRIFYING BACTERIA

Biofilm populations of Nitrosomonas europaea were formed in continuous flow sand columns supplied with defined inorganic medium containing 50 mug NH4+-N ml-1. Steady-state nitrite concentrations in effluent from the columns decreased as the pH of the inflowing medium was reduced but ammonia oxidation occurred at a pH value of 6, even though the pH minimum for growth of the organism in liquid batch culture was 7. No evidence was obtained for NH3 limitation at low pH values, resulting from ionization to NH4+. The pH minimum for ammonia-oxidizing activity was also reduced in continuous flow vermiculite columns supplied with medium of pH 5.4, with an effluent pH of 6.3, but detailed assessment of the effects of low pH on activity was prevented by the strong buffering capacity of vermiculite. A multispecies, nitrifying biofilm, formed as wall growth in an ammonia-limited chemostat, was capable of ammonia-oxidizing activity, but not growth, at pH 5 while growth was possible at pH 5.5. The data provide evidence of the potential for autotrophic nitrification in acid soils.