Main research focus of the Research Group for Marine Geochemistry (ICBM-MPI Bridging Group) is dissolved organic matter (DOM) in the ocean. The ocean is one of the largest carbon reservoirs on Earth. Dissolved organic matter alone contains a similar amount of carbon as all living biomass in the ocean and on land combined. Even though marine DOM is mainly of microbial origin, its turnover in the ocean is remarkably slow. Dissolved organic matter has accumulated in the ocean over thousands of years, and the controlling mechanisms behind its turnover and cycling are largely unknown. Advanced molecular techniques, in particular ultrahigh-resolution mass spectrometry, are used in the research group to obtain answers to the fundamental questions regarding the cycling of organic matter in the oceans.