In a former project we linked biogeochemical and microbial data in various ways. We showed e.g. that the glucosamine concentrations of the particulate organic matter correlate with chitinase activities. Highest chitinase activities were found in the zone of primary production and in the sediments. Bacterial community clusters correlated well with physico-chemical parameters.Studying the fate of the refractory biopolymer chitin, which is composed of the amino sugar glucosamine, we were able to depict different decomposition patterns for two lake systems distinguishing themselves by different nutrient and redox conditions: For the eutrophic Lake Zug the bulk of the chitinase activity was found in the sediments. In comparison, chitinase gene abundance distributed over the oligotrophic water column of Lake Brienz indicates a higher significance of chitin as carbon and nitrogen source as in the eutrophic water column of Lake Zug where more easily assimilable carbon and nitrogen sources are available.In order to improve our understanding of the sources and fates of the amino sugars and amino acids we will apply compound specific isotope measurements. These data will reveal bacterial alteration of the organic material and we can show whether new bacterial biomass is built up in the sediments. As terrestrial organic material buried in the sediments seems to be more refractory than the organic matter from autochthonous material, we want to estimate the fraction of terrestrial matter with the analysis of lipids. With the measurement of D-amino acids the fraction of the bacterial derived glucosamine will be estimated and we will be able to determine if bacteria are the main drivers in lacustrine organic nitrogen turnover.To study the diversity and distribution of bacterial chiA genes we are going to apply the high throughput 454 pyrosequencing technique on DNA extracted from the different lake compartments of the two lakes mentioned above. In order to analyze the proportion of bacterial groups carrying the chiA gene, we will perform fluorescence in-situ hybridization.The combination of isotopic, organic geochemical and molecular methods will significantly advance our understanding of how and by whom organic nitrogen compounds are transformed in lacustrine ecosystems.