The proposed research will investigate the biogeochemical cycle of dimethylsulfide (DMS) in seawater. The ocean is a major source of biogenic reduced sulfur to the global atmosphere, at a level roughly equivalent to anthropogenic SO2 emissions. DMS accounts for some 90% of the marine biogenic sulfur flux. The objective of this investigation is to elucidate the rates, pathways, and processes involved in the formation, transformation, and consumption of DMS in the ocean. This biogeochemical information is vital to understanding the DMS concentration profiles which have been observed in the field and may influence future sampling strategies. One a larger scale, a fuller understanding of the marine biogeochemistry of DMS and the processes involved in DMS cycling in seawater will aid in defining the relationship between oceanic biogeochemical processes and the production of the volatile biogenic sulfur compound which plays a major role in the global atmospheric sulfur cycle. Four working hypotheses will be addressed: 1) Degradation of algal dimethylsulfoniopropionate (DMSP, the DMS precursor) and release of DMS is the result of normal metabolic processes by phytoplankton; 2) release of DMS results form physiological stress on the phytoplankton; 3) microbial degradation of DMSF- containing algal material is a source of DMS in seawater; 4) microbial degradation is a sink of DMS in seawater. The approach to assessing these hypotheses will combine laboratory and field incubation experiments. Laboratory studies will be conducted under controlled environmental conditions and using selected individual and mixed culture algal and bacterial species. Experimental conditions will be varied to examine specific factors which may be important in DMS cycling. Field studies will then be used to examine the processes under natural conditions.