Characterization of the ammonia-oxidizing microbial community in Lake Constance

Abstract

Microbially driven ammonia oxidation to nitrite is the rate limiting step in nitrification and as such an important part of the global nitrogen cycle. Nitrification has a direct impact on primary producers in a wide variety of ecosystems such as soils, oceans and freshwater lakes. Lake Constance, located at the borders between Germany, Switzerland and Austria, is Central Europe’s third largest lake and the most important source of potable water for south-western Germany. This thesis followed potential nitrification rates and nitrifying organisms throughout the lake’s yearly cycle of spring and summer blooms and clear water phases. High nitrate and low ammonia concentrations suggest the presence of nitrifiers in Lake Constance throughout the year. Molecular methods employed (16S rRNA gene sequencing, phylogenetic analysis and terminal-restriction fragment length polymorphism (T-RFLP) analysis) further support this and give insight into the nature of the ammonia oxidising community. While complete ammonia oxidising (comammox) bacteria were not detectable in the water column of Lake Constance, both bacterial and thaumarchaeotal ammonia oxidisers were shown to be present in the lake water. The Thaumarchaeota were clearly the dominant group throughout the year. As has been shown in previous studies, the open water communities of ammonia oxidisers are not very diverse, with only two operational taxonomic units (OTU) within the Betaproteoacteria and one OTU within the Thaumarchaeota. In this study, the lifestyle of the lake nitrifiers was considered for the first time, by separating the molecular samples by size (5.0-30.0 micro meter (μm) and 0.1 to 5.0 μm groups). It could be clearly seen that the vast majority of both bacterial and thaumarchaeotal nitrifiers were in the smaller size group (0.1 to 5.0 μm) and therefore presumably free-living rather than particle-bound.