Diversität und Aktivität Ammonium-oxidierender und denitrifizierender Bakterien in der Rhizosphäre aquatischer Makrophyten anhand der Beispiele Littorella uniflora, Juncus bulbosus und Elodea canadensis
Zusammenfassung der Projektergebnisse
Previous studies have shown that coupled nitrification-denitrification in freshwater sediments is strongly enhanced in the rhizosphere of aquatic macrophytes due to oxygen release from the plant roots. In this project, rhizosphere-associated communities of ammonia-oxidizers – including both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) – and nitrate reducers were studied in the rhizosphere of the macrophyte species Littorella uniflora, Juncus bulbosus and Myriophyllum alterniflorum, which differ with regard to the extent of oxygen release. The main objectives of this project were to investigate (1) whether the community composition of ammonia oxidizers and nitrate reducers was primarily influenced by the plant species or by lake type, and (2) whether AOA or AOB predominated in freshwater sediments. Samples were obtained from seven different lakes and pools, ranging from dys-oligotrophic to mesotrophic sites. To study the diversity of AOA and AOB, clone libraries were constructed targeting bacterial and archaeal amoA genes, which encode the alpha subunit of ammonia mono-oxygenase, the key enzyme of ammonia oxidation. The community composition of nitrate reducers was analyzed using T-RFLP based on the narG gene, encoding the alpha subunit of nitrate reductase. In addition, clone libraries targeting archaeal 16S rRNA genes were constructed for selected samples. Relative abundances of AOA and AOB were determined by quantitative PCR. Results showed that relative abundances of AOA and AOB were usually low in unvegetated sediment and in the rhizosphere of Myriophyllum alterniflorum at most of the sites investigated with AOA/AOB ratios ranging from <1 to 150. Relative abundances of AOA were much higher in the rhizosphere of the macrophyte species Littorella uniflora and Juncus bulbosus, where AOA outnumbered AOB by up to four orders of magnitude. On the whole, these results clearly point to a potential role of AOA in rhizosphere-associated coupled nitrification-denitrification. The diversity of AOA and AOB increased from oligotrophic to mesotrophic sites with communities in unvegetated sediment, rhizosphere sediment, and on root surfaces showing the same overall trend. Furthermore, comparison of AOA and AOB communities between samples revealed that the community composition of AOA was primarily determined by lake type, whereas there was no clear pattern of a lake type-dependent clustering of AOB communities. In general, community clustering did not support the idea of a site-independent association of a defined AOA or AOB community with a specific plant species. However, both AOA and AOB communities as well as nitrate-reducing communities showed a high similarity between communities in the rhizosphere and on the root surface of a given plant species at a given site, suggesting that there was at least a general effect of the plant on rhizosphereassociated communities.
