Examination of an acid forest soil for ammonia- and nitrite-oxidizing autotrophic bacteria

Abstract

An acid forest soil (pH 3.9 – 4.4) from an undisturbed mixed oak stand in southern Indiana was examined for the occurrence of ammonia- and nitrite- oxidizing chemoautotrophs. Populations of both nitrifiers were detected in pH 7 most-probable-number (MPN) autotrophic media, and a Nitrosospira was isolated from highest dilution ammonia oxidizer MPN tubes. Populations of nitrite oxidizers were 10 to 1000 times higher than those of ammonia oxidizers. In pH 4.0 MPN media, ammonia oxidation was slight and unsustainable on 10% transfer to fresh medium, whereas nitrite oxidation was vigorous and sustainable. In pure culture the Nitrosospira isolate (Np IO1a) was completely inhibited by nitrapyrin at 5 μg mL−1, tolerant of 1.0 and 10.0 mM chlorate, and capable of growth only at pH 6.2 and above. Fluorescent antibodies raised against Np IO1a were used to confirm the predominance of Np IO1a in all MPN series examined. These results suggest that autotrophic ammonia oxidizers may be restricted to circumneutral microsites in this acid soil, whereas autotrophic nitrite oxidizers may not be limited to such sites.