A comparison of ammonia-oxidiser populations in eutrophic and oligotrophicbasins of a large freshwater lake

A combination of PCR amplification and oligonucleotide probing was used to investigate the populations of ammonia-oxidisers of the beta -Proteobacteri a in the eutrophic and oligotrophic basins of Lake Windermere, a large temp erate lake in the English Lake District. Numbers of ammonia-oxidisers (MPN) in the Windermere lakewater were low (< 100 cells ml(-1)) throughout the y ear with the exception of peaks in August, which coincided with stratificat ion, and November in the South Basin where overturn may have introduced amm onia-oxidising bacteria into the water column. Sediment samples contained l arger populations of ammonia oxidisers, usually ca. 10(4) per g. dry weight , which remained relatively constant throughout the seasonal cycle in both Basins. DNA was recovered from lakewater and sediment samples and Nitrososp ira and N. europaea-eutropha lineage16S rRNA genes amplified in a nested PC R reaction, with confirmation of identity by oligonucleotide hybridisation. Nitrosospira 16S rDNA was readily detected in all samples and therefore fo und to be ubiquitous. In contrast, nitrosomonad DNA of the N. europaea-eutr opha lineage could only be detected in the oligotrophic North Basin. Enrich ment cultures of lakewater samples only exhibited nitrification at low (0.6 7 mM) and medium (5 mM) ammonium concentrations, whilst sediment enrichment s nitrified at all concentrations tested including high (12.5 mM) ammonium medium. These data suggest that ammonia-oxidiser populations may be physiol ogically distinguished between lakewater and sediment, and that species dis tribution in a single lake is non-uniform.