This project proposal focuses on the organophosphonate compound methylphosphonate (MPn) and its contribution as potential precursor to oxic microbial formation of the greenhouse gas methane. Our study area is the Baltic Sea environment for which the relevance of oxic methane formation was reported. Quantitative data for MPn in the aquatic environment are scarce and detailed knowledge on its specific contribution to oxic methane formation as well as drivers promoting this pathway are lacking. However, this information is necessary to assess the magnitude of this potential pathway to oxic methane formation. This projects aims a better understanding of the potential contribution of MPn to oxic methane formation in the Baltic Sea, and will build on further knowledge of i) the spatial distribution and temporal variability of the occurrence of MPn, ii) the function of MPn as precursor for methane producing C-P-lyase cleavage and iii) oxic methane production in the field. The work comprises methodological amendments for MPn and methane stable isotope ratio analyses (WP 1), collection of quantitative data for MPn in water samples from the Baltic Sea environment (WP 2) and traceable incubation studies with 13C labeled MPn in the laboratory as well as in the field (WP 3).