Knowledge on diversity and ecological function of freshwater bacteria is still superficial, what stands in strong contrast to their role in the metabolism of natural freshwater systems (Hahn, 2006). The research of the working group ‘Environmental Microbiology’ is addressing basic questions regarding ecology, diversity, and biogeography of planktonic freshwater bacteria. Our research approach is successfully combining cultivation methods and cultivation-independent (molecular) methods (e.g., PCR-driven methods, FISH). We developed a cultivation method enabling the isolation and cultivation of freshwater bacteria, which contribute up to 60% of cell numbers of natural bacterial communities (Hahn et al., 2005). This approach enables to study our model organisms in their natural environment (Fig. 1) by using molecular tools (e.g. Hahn et al., 2005), to study their ecological potential by analysis of genomes (see ‘Polynucleobacter Genome Project’ below), and to study their ecophysiology by performing experiments with cultivated strains (e.g., Hahn and Pöckl, 2005). Our major model organisms are Polynucleobacter spp. bacteria (Fig. 2), which represent, on the one hand, ubiquitous and abundant freshwater bacteria dwelling as free-living cells in virtually all standing freshwater habitats, and on the other hand, obligate endosymbionts of ciliates affiliated with the genus Euplotes (Hahn, 2003; Vannini et al, 2007). Other important model organisms are freshwater Actinobacteria (Hahn et al., 2003, Hahn and Pöckl, 2005; Hahn, in press), and filamentous bacteria (Fig. 3) affiliated with the phylum Bacteroidetes (Schauer and Hahn, 2005; Hahn and Schauer, 2007).

Current cooperation partners

Karel Šimek, Czech Republic
Jan Jezbera, Czech Republic
Adrian K. Sharma, USA
Qinglong L. Wu, P.R.China
Elke Lang, Germany
Antonio Camacho, Spain
Jakob Pernthaler, Switzerland  (ESF project FREDI)
Hans-Peter Grossart, Germany  (ESF project FREDI)
Stefan Bertilsson, Sweden  (ESF project FREDI)
Ed R.B. Moore, Sweden
Mitsunori Tarao, Japan
Claudia Vannini and Giulio Petroni, Italy
Ulrike Berninger, Austria
Sami Taipale, Finland

Fig. 3. Epifluorescencemicroscopic image of a water sample from oligotrophic Lake Attersee (Austria) stained with DAPI. The image shows a 46 mm long filament of "Candidatus Aquirestis calciphila" (Hahn and Schauer, 2007) surrounded by several much smaller cells of planktonic bacteria.

Maria Gadermaier (Laboratory technician)

Franziska Huber (Diploma / Masters and PhD Student)

Ulrike Koll (Laboratory technician) - on maternity leave

Maria Pichler (Laboratory technician)

Ecological diversification within the subspecies Polynucleobacter necessarius ssp. asymbioticus and environmental ecotype sorting (FWF I 482-B09), funded by the European Science Foundation (ESF), project FREDI, project leader Jakob Pernthaler, University of Zurich, Switzerland, ongoing Link

Ecological Diversity of Polynucleobacter Bacteria, funded by the Austrian Science Fund (FWF), Project no. P19853, finished

Polynucleobacter Genome Project Link, funded by the DOE Joint Genome Institute (USA), finished
Genome sequencing of a free-living Polynucleobacter necessarius strain Link and a obligately endosymbiotic P. necessarius strain Link

Mechanisms Controlling the Diversity of Bacterioplankton, funded by the Austrian Science Fund (FWF), Project no. P15655-Bio, finished

Interaction of Protists and Bacteria (Jubiläumsfondsprojekt #7767,) funded by the Austrian National Bank, finished

Biogeography and within-taxon ecological differentiation of Polynucleobacter necessarius subsp. asymbioticus and Limnohabitans planctonicus (Betaproteobacteria), Austrian-Czech cooperation project with Jan Jezbera (Czech Republic), funded by the OEAD, ongoing

Ecological and phylogenetic comparison of bacterial populations inhabiting Antarctic and European lakes, binational Acciones Integradas Projekt with Antonio Camacho (Spain) funded by the OEAD, finished

Eco-physiological characteristics of two important groups of Betaproteobacteria abundant in freshwater bacterioplankton, Austrian-Czech cooperation project with Karel Šimek (Czech Republic), funded by the OEAD, finished

Lecture courses and laboratory courses in Microbiology and Ecology at the University of Salzburg.

Lecturing in the IPGL International Training Programmes in Limnology Link

in press

Jezbera, J., Jezberová, J., Koll, U., Hornak, K., Simek, K., Hahn, M.W. (in press) Contrasting trends in distribution of four major planktonic betaproteobacterial groups along a pH gradient of epilimnia of 72 freshwater habitats. FEMS Microbiology Ecology.

2012

Hahn, M.W., Scheuerl, T., Jezberová, J., Koll, U., Jezbera, J., Šimek, K., Vannini, C., Petroni, G., and Q. L. Wu (2012) The passive yet successful way of planktonic life: Genomic and experimental analysis of the ecology of a free-living Polynucleobacter population. PLoS ONE 7(3): e32772. PDF

Meincke, L., A. Copeland, A. Lapidus, S. Lucas, K. W. Berry, T. Glavina Del Rio, N. Hammon, E. Dalin, H. Tice, S. Pitluck, P. Richardson, D. Bruce, L. Goodwin, C. Han, R. Tapia, J. C. Detter, J. Schmutz, T. Brettin, F. Larimer, M. Land, L. Hauser,, N. C. Kyrpides, N. Ivanova, M. Göker, T. Woyke, Q. L. Wu, M. Pöckl, M. W. Hahn, and H.-P. Klenk (2012). Complete genome sequence of Polynucleobacter necessarius subsp. asymbioticus type strain (QLW-P1DMWA-1T). Standards in Genomic Sciences 6:74-83 PDF

Hahn, M.W., Minasyan, A., Lang, E., Koll, U. and Spröer, C. (2012) Polynucleobacter difficilis sp. nov., a planktonic freshwater bacterium affiliated with subcluster B1 of the genus Polynucleobacter. Int. J. Syst. Evol. Microbiol. 62: 376 - 383.

2011

Wang, J., Yang, D., Zhang, Y., Shen, J., van der Gast, C., Hahn, M.W., and Q. Wu (2011) Do Patterns of Bacterial Diversity along Salinity Gradients Differ from Those Observed for Macroorganisms? PLoS ONE 6(11): e27597. LINK

Taipale, S., Kankaala, P., Hahn, M. W., Jones, R. I., and Tiirola, M. (2011). Methane oxidizing and photoautotrophic bacteria are major producers in a humic lake with large anoxic hypolimnion. Aquat Microb Ecol. 64: 81–95.

Hahn, M.W., Lang, E., Tarao, M., and Brandt, U. (2011). Polynucleobacter rarus sp. nov., a free-living planktonic bacterium isolated from an acidic lake . Int. J. Syst. Evol. Microbiol. 61: 781-787. PDF

Hahn, M.W., Lang, E., Brandt, U., and Spröer, C. (2011). Polynucleobacter acidiphobus sp. nov., a representative of an abundant group of planktonic freshwater bacteria. Int. J. Syst. Evol. Microbiol. 61: 788-794. PDF Suppl.Mat.

Jezbera, J., Jezberová, J., Brandt, U., and Hahn, M.W. (2011). Ubiquity of Polynucleobacter necessarius subspecies asymbioticus results from ecological diversification. Environ. Microbiol. 13: 922–931. PDF

2010

Kasalický, V, Jezbera, J., Šimek, K., and Hahn, M.W. (2010). Limnohabitans planktonicus sp. nov., and Limnohabitans parvus sp. nov., two novel planktonic Betaproteobacteria isolated from a freshwater reservoir and emended description of the genus Limnohabitans. Int. J. Syst. Evol. Microbiol. 60: 2710-2714.

Hahn, M.W., Kasalický, V., Jezbera, J., Brandt, U., and Šimek, K. (2010). Limnohabitans australis sp. nov., isolated from a freshwater pond, and emended description of the genus Limnohabitans. Int. J. Syst. Evol. Microbiol. 60: 2946-2950. PDF

Hahn, M.W., Kasalický, V., Jezbera, J., Brandt, U., Jezberová, J., and Šimek, K. (2010). Limnohabitans curvus gen. nov., sp. nov., a planktonic bacterium isolated from a freshwater lake. Int. J. Syst. Evol. Microbiol. 60: 1358-1365.

Jezberová, J., Jezbera, J., Brandt, U., Lindström, E.S., Langenheder, S., and Hahn, M.W. (2010). Ubiquity of Polynucleobacter necessarius ssp. asymbioticus in lentic freshwater habitats of a heterogenous 2000 km2 area. Environ. Microbiol. 12: 658-669.

Šimek, K., Kasalický, V, Horňák, K., Hahn M.W., and Weinbauer, M.G. (2010). Assessing niche separation among coexisting Limnohabitans strains through interactions with a competitor, viruses, and a bacterivore. Appl. Environ. Microbiol. 76: 1406–1416.

Šimek, K., Kasalický, V., Jezbera, J., Jezberová, J., Hejzlar, J., and Hahn, M.W. (2010). Broad habitat range of the phylogenetically narrow R-BT065 cluster, representing a core group of the betaproteobacterial genus Limnohabitans. Appl. Environ. Microbiol. 76: 631-639.

Hahn, M.W., Lang, E., Brandt, U., Lünsdorf, H., Wu, Q.L., and E. Stackebrandt. (2010): Polynucleobacter cosmopolitanus sp. nov., free-living planktonic bacteria inhabiting freshwater lakes and rivers. Int. J. Syst. Evol. Microbiol. 60: 166-173.

2009

Xing, P., Hahn, M.W., and Wu, Q.L. (2009): Low taxa richness of bacterioplankton in high-altitude lakes of the Eastern Tibetan Plateau with predominance of Bacteroidetes and Synechococcus. Appl. Environ. Microbiol. 75: 7017–7025

Jezbera, J., Sharma, A.K., Brandt, U., Doolittle, W.F., and M.W. Hahn. (2009): "Candidatus Planktophila limnetica", an actinobacterium representing one of the most numerically important taxa in freshwater bacterioplankton. Int. J. Syst. Evol. Microbiol. 59: 2864-2869.

Hahn, M.W., Lang, E., Brandt, U., Wu, Q.L., and Scheuerl, T. (2009): Emended description of the genus Polynucleobacter and the species P. necessarius and proposal of two subspecies, P. necessarius subspecies necessarius subsp. nov. and P. necessarius subsp. asymbioticus subsp. nov. Int. J. Syst. Evol. Microbiol. 59: 2002-2009.

Tarao, M., Jezbera, J., and Hahn, M.W. (2009): Involvement of cell surface structures in size-independent grazing resistance of freshwater Actinobacteria. Appl. Environ. Microbiol. 75: 4720-4726.

Hahn, M.W. (2009): Real-time evolution observatories in the wild (Crystal ball - 2009). Environ. Microbiol. Reports. 1: 8-9.

Sharma, A.K., Sommerfeld, K., Bullerjahn, G.S., Matteson, A.R., Wilhelm, S.W., Jezbera, J., Brandt, U., Doolittle, W.F., and Hahn, M.W. (2009): Actinorhodopsin genes discovered in diverse freshwater habitats and among cultivated freshwater Actinobacteria. The ISME Journal 3: 726–737.

Hahn, M.W. (2009): Description of seven Candidatus species affiliated with the phylum Actinobacteria, representing planktonic freshwater bacteria. Int. J. Syst. Evol. Microbiol. 59: 112-11.

2008

Hahn, W.W. and Wu, Q.L. (2008): Planktonic bacteria in Chinese and European lakes: Are they different? In: Microbes and the Environment: Perspective and Challenges, S.-J. Liu H. L. Drake (Eds.), Science Press, Beijing, pp. 73-78.

2007

Wu, Q.L., Zwart, G., Wu, J. , Kamst-van Agterveld, M.P., Liu, S., and Hahn, M.W. (2007): Submersed macrophytes play a key role in structuring bacterioplankton community composition in the large, shallow, subtropical Taihu Lake, China. Environ. Microbiol. 9:2765–2774.

Groben, R., Hahn, M.W., Horňák, K., Mergeay, J., Šimek, K., and Vrb, J.. (2007): Meeting Report: ALTER-Net Workshop about the “Application of Molecular Techniques to Study Biodiversity, Structure and Function of Planktonic Communities in Lakes” at Blanes, Spain, February 15-16, 2007. Protist 158:417-421.

Hahn, M.W. and Schauer, M. (2007): ‘Candidatus Aquirestis calciphila’ and ‘Candidatus Haliscomenobacter calcifugiens’, filamentous, planktonic bacteria inhabiting natural lakes. Int. J. Syst. Evol. Microbiol., 57:936-940.

Wu, Q.L., Chen, Y., Xu, K., Liu, Z., and Hahn, M.W. (2007): Intra-habitat heterogeneity of microbial food web structure under the regime of eutrophication and sediment resuspension in the large subtropical shallow Lake Taihu, China. Hydrobiologia 581:241–254.

Vannini, C., Pöckl, M., Petroni, G., Wu, Q.L., Lang, E., Stackebrandt, E., Schrallhammer, M., Richardson, P.M., and Hahn, M.W. (2007): Endosymbiosis in statu nascendi: Close phylogenetic relationship between obligately endosymbiotic and obligately free-living Polynucleobacter strains (Betaproteobacteria). Environ. Microbiol. 9:347-359.

2006

Wu, Q.L., Zwart, G., Schauer, M., Kamst-van Agterveld, M.P., and Hahn, M.W. (2006): Bacterioplankton community composition along a salinity gradient of sixteen high-mountain lakes located on the Tibetan Plateau, China. Appl. Environ. Microbiol. 72:5478–5485.

Wu, Q.L. and Hahn, M.W. (2006b): High predictability of the seasonal dynamics of a species-like Polynucleobacter population in a freshwater lake. Environ. Microbiol. 8:1660-1666.

Šimek, K., Horňák, K. , Jezbera, J. , Nedoma, J. , Vrba, J., Straškrábová, V. , Macek, M. , Dolan, J. R., and Hahn., M.W. (2006): Maximum growth rates and possible life strategies of different bacterioplankton groups in relation to phosphorus availability in a freshwater reservoir. Environ. Microbiol. 8:1613-1624.

Schauer, M., Jing, J., and Hahn, M.W. (2006): Recurrent seasonal variations in abundance and composition of filamentous SOL cluster bacteria (Saprospiraceae, Bacteroidetes) in oligomesotrophic Lake Mondsee (Austria). Appl. Environ. Microbiol. 72:4704-4712.

Hahn, M.W. (2006): The microbial diversity of inland waters. Current Opinion in Biotechnology 17: 256-261.

Wu, Q.L. and Hahn, M.W. (2006): Differences in structure and dynamics of Polynucleobacter communities in a temperate and a subtropical lake revealed at three phylogenetic levels. FEMS Microb. Ecol. 57(1):67.

2005

Schauer, M., Kamenik, C., and Hahn, M.W. (2005): Ecological differentiation within a cosmopolitan group of planktonic freshwater bacteria (SOL cluster, Saprospiraceae, Bacteroidetes). Appl. Environ. Microbiol. 71:5900–5907.

Camacho, A., Rochera, C., Silvestre, J.J., Vicente, E., and Hahn, M.W. (2005): Spatial Dominance and Inorganic Carbon Assimilation by Conspicuous Autotrophic Biofilms in a Physical and Chemical Gradient of a Cold Sulfurous Spring: The Role of Differential Ecological Strategies. Microb. Ecol. (50: 172 - 184.). Abstract

Hahn, M.W., Pöckl, M., and Wu, Q.L. (2005): Low intraspecific diversity in a Polynucleobacter subcluster population numerically dominating bacterioplankton of a freshwater pond. Appl. Environ. Microbiol. 71: 4539–4547. Abstract

Schauer, M. and Hahn, M.W. (2005): Diversity and phylogenetic affiliations of morphologically conspicuous large filamentous bacteria occurring in the pelagic zones of a broad spectrum of freshwater habitats. Appl. Environ. Microbiol. 71: 1931-1940. Abstract PDF

Hahn, M.W. and Pöckl, M. (2005): Ecotypes of planktonic Actinobacteria with identical 16S rRNA genes adapted to thermal niches in temperate, subtropical, and tropical freshwater habitats. Appl. Environ. Microbiol. 71: 766–773. Abstract PDF

2004

Boenigk, J., Stadler, P., Wiedlroither, A., and Hahn, M.W. (2004): Strain-specific differences in the grazing sensitivities of closely related ultramicrobacteria affiliated with the Polynucleobacter cluster . Appl. Environ. Microbiol. 70: 5787-5793.

Hahn, M.W. (2004): Broad diversity of viable bacteria in ‘sterile’ (0.2µm) filtered water. Research in Microbiology 155: 688-691. Abstract

Hahn, M.W., Stadler, P., Wu, Q.L., and Pöckl, M. (2004): The filtration–acclimatization method for isolation of an important fraction of the not readily cultivable bacteria. Journal of Microbiological Methods. 57: 379-390. Abstract

Hahn, M.W., Lünsdorf, H., and Janke, L. (2004): Exopolymer production and microcolony formation play an important role in protistan grazing defense of novel bacterial strains isolated from freshwater plankton. Aquat Microb Ecol. 35:297-308. Abstract

Wu Q.L., Boenigk J., and Hahn M.W. (2004): Successful predation by a nanoflagellate on filamentous bacteria challenges current models on flagellate bacterivory. Appl. Environ. Microbiol. 70: 332–339. Abstract PDF

2003

Hahn, M.W. (2003): Isolation of Strains Belonging to the Cosmopolitan Polynucleobacter necessarius Cluster from Freshwater Habitats Located in Three Climatic Zones. Appl. Environ. Microbiol. 69: 5248-5254. Abstract

Hahn, M.W., Lünsdorf, H., Wu, Q., Schauer, M., Höfle, M. G., Boenigk, J., and Stadler P. (2003): Isolation of novel ultramicrobacteria classified as Actinobacteria from five freshwater habitats in Europe and Asia. Appl. Environ. Microbiol. 69(3): 1442-1451. Abstract Full article: PDF

2001

Hahn, M.W. and Höfle, M.G.(2001): Grazing of protozoa and its effect on populations of aquatic bacteria (MiniReview). FEMS Microbiology Ecology 35: 113-121. Abstract Full article: PDF

2000

Hahn, M.W., Moore, E.R.B., and Höfle, M.G. (2000):. Role of microcolony formation in the protistan grazing defense of the aquatic bacterium Pseudomonas sp. MWH1. Microbial Ecology 39: 175-185. Abstract

Ambassador of the International Society for Microbial Ecology (ISME) Link (2003-present)

Frontiers in Aquatic Microbiology (2010 - present)

Applied and Environmental Microbiology (2001-2010)

Environmental Microbiology (2009-present)