Hydrogen cyanide (HCN) is a toxic compound, whose sources in natural aquatic systems are mostly of anthropogenic origin. Although a variety of organisms, most notoriously vascular plants, produces HCN, its release from natural sources is not considered to be an important source of HCN contamination of aquatic environments. Recent results of my research showed that concentrations of HCN, which are toxic to aquatic species, are released in the anoxic ferruginous and sulfidic sediments of the Delaware Great Marsh (DGM) from the roots of cord grass Spartina alterniflora. The main sinks for HCN in the DGM sediments are the formation of metallo-cyanide complexes and reaction with reactive zero-valent sulfur species, which lead to the formation of thiocyanate.I propose to conduct a study to clarify a) the prevalence of HCN release in various aquatic systems; b) which biogeochemical processes control the release and scavenging of HCN in natural aquatic systems; and c) the kinetics of chemical reactions underlying reactions of HCN in natural aquatic systems.In the first stage of the research representative examples of various types of oxygen-depleted aquatic systems (meromictic lakes, marine water bodies with restricted circulation, ocean upwelling zones and sedimentary pore-waters) will be assayed for the presence of relatively stable products of HCN transformations, i.e., thiocyanate and strong metallo-cyanide complexes. The next stage will include a detailed study of biogeochemical mechanisms of cyanide release and transformations in three selected systems. These studies will be accompanied by laboratory experiments to determine the rates of reactions between metallo-complexed and adsorbed cyanide with zero-valent sulfur species abundant in natural aquatic systems (e.g., solid sulfur, colloidal sulfur, dissolved sulfur, polysulfides, polythionates, thiosulfate and organic polysulfanes).