I investigate the role and fate of different water column bacteria in freshwater (and marine) habitats in the context of food web structure and substrate availability. My research is targeted to two aspects of microbial ecology. Firstly I study the effects of predator-induced mortality on the composition of microbial assemblages, and in the potential adaptations of microbial species to compensate or to avoid such losses. In this context I have been specifically interested in the identity,activity, biogeography and population dynamics of filamentous aggregate-forming and of gram positive freshwater microbes. Secondly, I have been studying the life strategies of so-called "opportunistic" bacterial species in coastal and freshwater habitats, i.e. bacterial ecotypes that are superior competitors in steep environmental gradients or during rapid changes of growth conditions. These bacteria could potentially qualify as early indicator organisms of water pollution by organic carbon. I am moreover interested in the ecological role of facultatively anaerobic heterotrophic water column bacteria., the similarities and differences in the community composition of freshwater and coastal microbial assemblages, and in the distribution and activities of pelagic microbes in the deep Atlantic Ocean. An emerging subject of interest inspired by the facilities and expertise present at the Limnological Station is the potential role of infochemicals in the relationship between protistan predators and their prey.

I employ a suite of molecular biological approaches for the identification and staining of common environmental microbes without prior cultivation based upon comparative rRNA gene cloning and sequence analysis. In my laboratory methods have been improved for the determinative whole-cell fluorescence in situ hybridization (FISH) of water column microbes with rRNA-targeted probes. Specifically, the FISH technique was adapted for small slowly-growing bacteria and archaea in oligotrophic offshore or freshwater environments. In addition, we develop and apply image analysis strategies for the rapid quantification of population sizes and biovolumes of FISH-stained microorganisms by automated motorized microscopy. I am moreover interested in cytochemical and autoradiographic techniques to measure the DNA synthesis and specific substrate uptake of individual microbial populations in mixed communities at the single-cell level. This allows to establish a link between the phylogenetic identification of large, environmentally relevant populations of water-column microbes and their growth patterns in situ.

List of Publications