BackgroundWhereas countless studies on the transformation of organic material were performed in the marine environment, studies in lakes on the same topic are rather rare. The here proposed study focuses on important research questions regarding the degradation/preservation of organic nitrogen compounds in two lake systems distinguishing themselves by different nutrient and redox conditions, i.e., an eutrophic and, below a certain depth, anoxic and an oligotrophic and fully oxic system.AimThe study will address the currently open question of how the patterns of formation and degradation of organic nitrogen compounds, specifically amino acids and amino sugars, are changing throughout the water column and in the sediment of these lakes. Likewise, we do not currently know the microorganisms involved in the degradation of biopolymers like chitin or peptidoglycan and their constituent amino sugars and amino acids in freshwater systems. How do these microbial communities impact the transformation of the organic material as it settles through the water column and accumulates in the sediment? The fate of amino sugars in lacustrine systems is particularly poorly studied, and we will for the first time answer questions about the sources, turnover, and ultimate fate of these quantitatively highly important compounds.MethodsTo answer these questions we will combine state of the art organic geochemistry, isotope geochemistry, and molecular ecology techniques. On the geochemistry side of the project we will focus on single amino acids and single amino sugars, molecules that comprise most of the particulate and dissolved organic matter in aquatic systems. Variations in concentration and isotopic signature will help to reveal sources, transformation, and preservation/ degradation behaviour of these compounds in the water column and underlying sediments. Methods of molecular microbiology such as Stable Isotope Probing (SIP) with 13C-labelled substrates will be applied to reveal the key bacterial/archaeal players responsible for transformation/degradation of amino sugars, amino acids, and important biopolymers of these compounds. Studies on a key gene for biopolymer degradation, chitinase, will help to reveal potential and active degraders in the unmanipulated natural system.SignificanceCombined, these approaches will significantly advance our understanding of how and by whom organic nitrogen compounds are transformed during sedimentation in lacustrine systems. This is especially interesting in the view of the high pressure aquatic systems are exposed to under global change and men made eutrophication.compounds are transformed during sedimentation in lacustrine systems.