Yuko Omori

Enhancement of dimethylsulfide production by anoxic stress in natural seawater

Dimethylsulfide (DMS) is produced by phytoplankton in the ocean and plays an important role in biogeochemical cycles and climate system of the Earth. Previous field studies reported a possible relationship between DMS enhancement and anoxic condition, although the governing processes are still to be identified. Here we show the first direct evidence for the enhancement of DMS production by natural planktonic assemblages caused by anoxic stress. Under the anoxic condition, DMS production was considerably enhanced and DMS bacterial consumption was inhibited, resulting in an eightfold higher rate of gross DMS production than that under the oxic condition. Our results demonstrated that anoxic stress is one of important “environmental factors” in the marine DMS dynamics, suggesting the possible global importance due to ubiquity of anoxic conditions in the coastal oceans. This process would become more important in the future due to expansion of coastal hypoxic and anoxic zones by global warming.

Sea‐to‐air flux of dimethyl sulfide in the South and North Pacific Ocean as measured by proton transfer reaction‐mass spectrometry coupled with the gradient flux technique

Exchange of dimethylsulfide (DMS) between the surface ocean and the lower atmosphere was examined by using Proton Transfer Reaction-Mass Spectrometry coupled with the Gradient Flux (PTR-MS/GF) system. We deployed the PTR-MS/GF system and observed vertical gradients of atmospheric DMS just above the sea surface in the subtropical and transitional South Pacific Ocean and the subarctic North Pacific Ocean. In total, we obtained 370 in situ profiles, and of these we used 46 data sets to calculate the sea-to-air flux of DMS. The DMS flux determined was in the range from 1.9 to 31 μmol m–2 d–1 and increased with wind speed and biological activity, in reasonable accordance with previous observations in the open ocean. The gas transfer velocity of DMS derived from the PTR-MS/GF measurements was similar to either that of DMS determined by the eddy covariance technique or that of insoluble gases derived from the dual tracer experiments, depending on the observation sites located in different geographic regions. When atmospheric conditions were strongly stable during the daytime in the subtropical ocean, the PTR-MS/GF observations captured a daytime vs. nighttime difference in DMS mixing ratios in the surface air overlying the ocean surface. The difference was mainly due to the sea-to-air DMS emissions and stable atmospheric conditions, thus affecting the gradient of DMS. This indicates that the DMS gradient is strongly controlled by diurnal variations in the vertical structure of the lower atmosphere above the ocean surface.