The offered PhD position is part of an interdisciplinary research project funded by the Swiss National Science Foundation that aims to advance understanding of the global biogeochemical cycle of selenium. Selenium is an element that is naturally occurring in the environment and of key importance to human health. In the frame of this project two work programs have been defined that address important open questions concerning natural sources, emissions, speciation and fate of selenium in the environment. Each work program represents the topic of one doctoral thesis. will be part of the first work program, which targets to improve the understanding and quantification of biogenic production of methylated Se and S in the marine and terrestrial environment. Marine environments are presumed to be a major natural source of Se to the atmosphere. Also in terrestrial environments such as in peatbogs, selenium can be transformed into highly volatile methylated Se compounds that are emitted into the atmosphere. Nevertheless, the processes leading to formation of marine and terrestrial atmospheric selenium compounds are still largely unknown and need to be understood.