Award: OCE-1436576

The sub-arctic northeastern Pacific, a high-nutrient, low-chlorophyll region, has exceptionally high concentrations of the biogenic trace gas dimethylsulfide (DMS), which is related to the cycling of other sulfur compounds including total dimethylsulfoniopropionate (DMSPt). This region presents strong gradients in macronutrients, iron and phytoplankton community composition that likely drive variability in surface DMS levels. The focus of this study was to improve the understanding of spatial variability in oceanic DMS and DMSPt concentrations in surface waters of the sub-arctic Pacific by accurately measuring the cycling rates of these compounds. This was facilitated by two research cruises (July 10-27, 2016; August 11-27, 2017) aboard the Research Vessel Oceanus, which sailed from Seward, Alaska to Newport, Oregon. Chlorophyll a was low in the iron-limited central sub-arctic Pacific stations, while higher and more variable inshore along the British Columbia coast. Thus, the vessel tracks covered contrasting conditions of phytoplankton biomass and DMS, DMSP. Sulfur compound concentrations and flux rates were variable. DMSPt concentrations ranged from 20 to 100 nM with the highest concentrations (>300 nM) associated with late phytoplankton-bloom conditions. DMS concentrations were highly variable with several hot-spots along the cruise track having the highest concentrations (~20 nM). These data show a significant relationship between DMS and DMSPt. An additional sulfur compound, methanethiol (MeSH), was detected during both cruises; MeSH is the major sulfur gas produced from DMSP degradation, but its concentration is generally much lower than DMS because of its high biological and chemical reactivity. MeSH was detectable (> 0.07 nM) in all surface waters. The ratio of MeSH to DMS for the 5 meter samples ranged from 0.02 to 0.33; thus, the fluxes of MeSH-sulfur to the atmosphere should be significant relative to DMS and could impact estimates of global sulfur inputs to the atmosphere. The biological turnover rate of DMSP was rapid. The DMSP consumption varied by a factor of 30 with an average of ~80 nM per day. This consumption rate was ~160% of the average DMSPt standing stock; thus, DMSP turnover represents a substantial flux of sulfur and carbon in the microbial food web. Similarly, DMS consumption was also rapid with an average consumption rate was 126% of the DMS standing stock. Although highly variable, the consumption of DMS can be a major control on DMS concentrations and atmospheric flux. The PIs collaboration with the University of British Columbia group promoted increased understanding of dimethylsulfoxide (DMSO) cycling. DMSO exhibited a positive, and nonlinear, relationship with DMS, with the former being factors of 2-10 times higher. Within the broader region, these results also suggest that DMSO reduction can be an important DMS production term. Overall, this project quantified concentrations of multiple organosulfur compounds and suggested that the fluxes of MeSH-sulfur and DMS consumption by microbes could both impact estimates of global sulfur inputs to the atmosphere. Furthermore, these results demonstrated exceptional turnover of DMSP and DMS by the microbial food web, sustaining daily consumption rates which exceeded the standing stocks for these compounds by ~30-60%. Beyond increased sulfur-cycling understanding, this project also facilitated graduate student training and development by directly supporting two M.S. students at the University of South Alabama, and indirectly supporting (via cruise opportunity and data access) one M.S. student at the University of British Columbia). Last Modified: 06/07/2021 Submitted by: Jeffrey W Krause