Continuous culture enrichments of ammonia-oxidizing bacteria at low ammonium concentrations
Until now enrichments of ammonia-oxidizing bacteria from natural ammonium-limited environments have been performed mainly in the presence of much higher ammonia concentrations than those present in the natural environment and many have resulted in the enrichment and isolation of environmentally less important bacteria. Therefore, we used continuous cultures to enrich ammonia-oxidizing bacteria at growth-limiting ammonium concentrations of around 5 μM from the root zone of the macrophyte Glyceria maxima from the lake Drontermeer (The Netherlands). Molecular analysis at the end of the enrichment experiments showed that all continuous cultures consisted of Nitrosomonas cluster 6a, which comprises also Nitrosomonas ureae and Nitrosomonas oligotropha. This was independent of whether Nitrosomonas- or Nitrosospira-like bacteria were dominant in the inoculum. Thus all known ammonia-oxidizing bacteria belonging to Nitrosomonas cluster 6a are able to grow at very low ammonium concentrations.