Illuminating the ecology of nitrite-oxidizing bacteria in soil and aquatic ecosystems
Nitrite-oxidizing bacteria (NOB) are key players of the biogeochemical nitrogen cycle. By catalyzing the second step of nitrification, the oxidation of nitrite to nitrate, NOB are the biological source of nitrate, which is a major form of fixed nitrogen in the biosphere and an important nitrogen source for many microorganisms and plants. Moreover, the activity of NOB determines whether fixed nitrogen is retained in ecosystems as nitrate or is lost to the atmosphere via other processes that convert nitrite to gaseous N-compounds. Despite the huge ecological importance of NOB, current knowledge of their microbiology and ecology is severely limited. Here we begin to bridge this knowledge gap by studying the structure and function of NOB communities in representative pristine or human-affected terrestrial and aquatic ecosystems: different types of soils, waters and sediments from freshwater streams and lakes, and moderately saline lakes in the East Austrian national park “Neusiedler See-Seewinkel”. The saline lakes are characterized by highly variable environmental conditions, but host extremely productive microbial communities that have barely been studied. To analyze the phylogenetically diverse NOB independently from cultivation, a novel and highly specific marker gene will be used. Sequences of this gene, nxrB coding for the beta subunit of nitrite oxidoreductase, have become available only recently with the first sequenced genomes from all known lineages of NOB.The NOB community structures in environmental samples will be analyzed by next-generation sequencing of nxrB genes, and the in situ nitrite-oxidizing activities of the different populations will be studied by monitoring nxrB transcription. In incubation experiments, the responses of NOB to changing environmental conditions and structure-function relationships of NOB communities will be investigated. NOB will also be enriched from selected samples for physiological analyses. The project will provide for the first time a broad picture of environmental NOB populations and their ecology in diverse habitats. This knowledge is urgently needed to better understand the effects of natural environmental fluctuations on the nitrogen cycle and to assess the consequences of human impact, such as an increasing nitrogen deposition, for vulnerable ecosystems.
This project is funded by the Austrian Science Fund (P25231-B21).