| Popis: |
Nitrification, the transformation of nitrogen from its most reduced form (ammonia) to its most oxidized form (nitrate) is a component of the nitrogen cycle in aquatic habitats. The first step of this process, the oxidation of ammonia to nitrite, is performed by two distinct groups of microorganisms in the environment, the ammonia-oxidizing archaea (AOA) and bacteria (AOB). Here we present a study using cultivation techniques to investigate environmental factors that influence the niche differentiation of AOA and AOB from freshwater environments. In order to investigate factors that drive the diversity of freshwater AOB communities, we enriched AOB from lakes representing a range of trophic states. The resulting enrichments were not influenced by the ammonium concentrations used in enrichment medium, but were comprised of AOB species commonly detected in freshwater environments. The AOB communities of the sediment and enrichments were strongly influenced by the watershed land use of the lake, and ammonium and nitrate concentrations within the sediment. Further enrichment efforts yielded cultures of three species of AOA, two of which represent a previously undescribed genus. These AOA grew more slowly than a freshwater AOB enrichment in all conditions tested (ammonium concentration, oxygen concentration, pH, and light exposure). Data from these experiments indicated that ammonium and oxygen concentrations and light exposure, would be the strongest factors driving niche separation of AOA and AOB. Chemostat competition experiments were conducted using one representative AOA and AOB under ammonia-limiting, high oxygen (21%) and ammonia-limiting, low oxygen (1%) conditions. In all cases, the AOA outcompeted the AOB for ammonia, and the AOB was washed from the chemostat. AOA and AOB were also investigated with respect to their starvation tolerance; both AOA and AOB survived nearly two months of starvation while maintaining amoA mRNA and 16S rRNA, however the AOB was able to recover from starvation faster. The results from these experiments suggest that, within the ammonia-oxidizers, AOB represent a copiotrophic lifestyle and will thrive in conditions of pulses of high ammonia availability, while AOA represent an oligotrophic lifestyle and will thrive in conditions of very low but constant ammonia availability. |
