Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Environmental Microbiology 14 (2012): 1210-1223, doi:10.1111/j.1462-2920.2012.02700.x. ; Dimethylsulfide (DMS) is a climatically relevant trace gas produced and cycled by the surface ocean food web. Mechanisms driving intraannual variability in DMS production and dimethylsulfoniopropionate (DMSP) degradation in open-ocean, oligotrophic regions were investigated during a 10 month time-series at the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Abundance and transcription of bacterial DMSP degradation genes, DMSP lyase enzyme activity, and DMS and DMSP concentrations, consumption rates, and production rates were quantified over time and depth. This interdisciplinary dataset was used to test current hypotheses of the role of light and carbon supply in regulating upper-ocean sulfur cycling. Findings supported UV-A dependent phytoplankton DMS production. Bacterial DMSP degraders may also contribute significantly to DMS production when temperatures are elevated and UV-A dose is moderate, but may favor DMSP demethylation under low UV-A doses. Three groups of bacterial DMSP degraders with distinct intraannual variability were identified and niche differentiation was indicated. The combination of genetic and biochemical data suggest a modified ‘bacterial switch’ hypothesis where the prevalence of different bacterial DMSP degradation pathways is regulated by a complex set of factors including carbon supply, temperature, and UV-A dose. ; This research was funded by National Science Foundation (NSF) grants OCE- 0525928, OCE-072417, and OCE-042516. Additional funding was provided by the NSF Center for Microbial Oceanography Research and Education (CMORE), the Gordon and Betty Moore Foundation, the Scurlock Fund, the Ocean Ventures Fund, a National Defense Science and Engineering Graduate Fellowship, and an Environmental Protection Agency STAR Graduate Fellowship.