Isotopic evidence for the origin of dimethylsulfide and dimethylsulfoniopropionate-like compounds in a warm, monomictic freshwater lake

Environmental context The volatile sulfur compound, dimethylsulfide (DMS), plays a major role in the global sulfur cycle by transferring sulfur from aquatic environments to the atmosphere. Compared to marine environments, freshwater environments are under studied with respect to DMS cycling. The goal of this study was to assess the formation pathways of DMS in a freshwater lake using natural stable isotopes of sulfur. Our results provide unique sulfur isotopic evidence for the multiple DMS sources and dynamics that are linked to the various biogeochemical processes that occur in freshwater lake water columns and sediments. Abstract The volatile methylated sulfur compound, dimethylsulfide (DMS), plays a major role in the global sulfur cycle by transferring sulfur from aquatic environments to the atmosphere. The main precursor of DMS in saline environments is dimethylsulfoniopropionate (DMSP), a common osmolyte in algae. The goal of this study was to assess the formation pathways of DMS in the water column and sediments of a monomictic freshwater lake based on seasonal profiles of the concentrations and isotopic signatures of DMS and DMSP. Profiles of DMS in the epilimnion during March and June 2014 in Lake Kinneret showed sulfur isotope (δ34S) values of +15.8±2.0 per mille (‰), which were enriched by up to 4.8 ‰ compared with DMSP δ34S values in the epilimnion at that time. During the stratified period, the δ34S values of DMS in the hypolimnion decreased to -7.0 ‰, close to the δ34S values of coexisting H2S derived from dissimilatory sulfate reduction in the reduced bottom water and sediments. This suggests that H2S was methylated by unknown microbial processes to form DMS. In the hypolimnion during the stratified period DMSP was significantly 34S enriched relative to DMS reflecting its different S source, which was mostly from sulfate assimilation. In the sediments, δ34S values of DMS were depleted by 2-4 ‰ relative to porewater (HCl-extracted) DMSP and enriched relative to H2S. This observation suggests two main formation pathways for DMS in the sediment, one from the degradation of DMSP and one from methylation of H2S. The present study provides isotopic evidence for multiple sources of DMS in stratified water bodies and complex DMSP-DMS dynamics that are linked to the various biogeochemical processes within the sulfur cycle.