"New developments in isotope ratio mass spectrometry have made possible the use of both traditional and rare sulfur isotopes in biogeochemical systems studies and open new ways to explore natural sulfur cycle. The project will focus on the detection of the abundances of four sulfur isotopes in reduced and intermediate sulfur compounds such as sulfide, dispersed particulate elemental sulfur, polysulfides, thiosulfate and polythionates in natural aquatic and sedimentary systems. Quantitative detection of these compounds together with determination of four sulfur isotopes abundances will allow development of the method, which answers the following long-standing questions: a) how to differentiate between the systems, where production of hydrogen sulfide from sulfate is due to sulfate reduction to sulfide, and the systems, where disproportionation of intermediate compounds like sulfur occurs; b) how to differentiate between the systems, where intermediate sulfur compounds, especially dissolved (polysulfidic) and non-soluble (elemental) zero-valent sulfur, are produced by oxidation of hydrogen sulfide by biotic and abiotic routes. Various systems with different rates (from minutes to thousands of years), microbial activities, pH and mechanisms of reduced and intermediate sulfur compounds cycling will be studied. Systems may include: Black Sea, North Sea, salt marshes, meromictic lakes, monomictic lakes, acidic pools (i.e. at Yellowstone National Park) and soda lakes. The samples from both water column and sediment will be analyzed in order to reconstruct the flow of sulfur through the complex biogeochemical system. Bacterial cultures will be studied to explain the role of microbial processes in sulfur isotopes fractionation."