The Polynucleobacter group (Betaproteobacteria, Burkholderiaceae) is of enormous environmental relevance in freshwater habitats. These free-living heterotrophic bacteria contribute up to 60% of total bacterial cell numbers in the pelagic zone of surface freshwater habitats and have also been detected in groundwater. Despite their enormous ecological importance, knowledge of the physiology and specific ecological function of numerically relevant freshwater bacteria is completely lacking. Genome sequencing of the free-living Polynucleobacter strain will provide deep insights into the ecological function of an enormously important group of freshwater bacteria. This knowledge will deepen the understanding of how freshwater ecosystems function and will facilitate better modeling of carbon fluxes in these ecosystems. This will be the first genome project on nonpathogenic heterotrophic freshwater bacteria of environmental relevance.
Endosymbiotic Polynucleobacter bacterium living in the cytoplasm of the ciliate Euplotes harpa. Two of the 6 to 10 electron-dense nucleoids are well visible in this thin section. The genus name of the bacteria refers to these multiple nucleoids. Scale bar, 1 µm. Courtesy of Claudia Vannini, University of Pisa, Italy.
Polynucleobacter species are also relevant to issues of climate change, which is causing thawing of Arctic permafrost. This is resulting in the release of organic carbon stored in the frozen Arctic peat and in the creation of extensive wetlands in many Arctic areas. Such habitats, typically inhabited by high numbers of free-living Polynucleaobacter bacteria, will be involved in the fate of the carbon released from the peat. Detailed knowledge of the ecological function of Polynucleobacter bacteria will enable a better understanding of microbial processes crucial for the fate of the released carbon.
Comparison of genomes of the closely related obligately free-living and obligately endosymbiotic Polynucleobacter strains will also provide unique insights into the evolutionary adaptations taking place during the early phase of endosymbiosis. Genome comparison with nonfreshwater Burkholderiaceae (Burkholderia spp., Ralstonia spp., Cupriavidus spp.) will provide first insights into evolutionary adaptations to planktonic life in freshwater.
CSP project participants: Martin W. Hahn (proposer, Austrian Academy of Sciences), Eric W. Triplett (Univ. of Florida), Giulio Petroni (Univ. of Pisa), Wolfgang R. Hess (Univ. Freiburg), and Katherine D. McMahon (Univ. of Wisconsin-Madison).